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Home | Alpha Telephone | Domain Names | Web Hosting | Get Traffic | xrEvidence | xrSoccer United States Patent
Method and apparatus for radiation image processing and x-ray image processing, including spatial frequency filtering to improve graininess In a radiation image processing method, unsharp mask signals Sus.k are calculated by averaging original image signals detected within predetermined ranges surrounding each scanning point on an image-recorded stimulable phosphor, and at least a single attenuation coefficient .beta.l among attenuation coefficients .beta.k corresponding to the unsharp mask signals Sus.k is adjusted to be a constant within the range of 0<.beta.l wherein .beta.l.noteq.1. An operation represented by a formula ##EQU1## where Sb1 and Sb2 each denote the original image signal or an image signal obtained by intermediate processing of the original image signal, and S' denotes an image signal obtained by the operation processing is carried out by use of the attenuation coefficient .beta.l, whereby spatial frequency components above the spatial frequency component which the unsharp mask signal Sus.l corresponding to the attenuation coefficient .beta.l has are attenuated.
Attorney, Agent or Firm: I claim: 1. A radiation image processing method which, in the course of performing a read-out operation by scanning a stimulable phosphor, carrying a radiation image stored thereon, with stimulating rays which cause said stimulable phosphor to emit light in proportion to the stored radiation energy, obtaining an original image signal by photoelectrically detecting the light emitted by each scanning point on said stimulable phosphor, and reproducing said radiation image as a visible image on a recording medium, comprises the steps of: i) obtaining a single unsharp mask signal Sus.k by averaging original image signals within a predetermined range surrounding each scanning point or averaging image signals obtained by carrying out intermediate processing of the original image signals, or obtaining a plurality of unsharp mask signals Sus.k, where k=1, 2, . . . , n, wherein n denotes an integer representing the number of said unsharp mask signals, by changing said predetermined range, ii) denoting a single attenuation coefficient corresponding to single said unsharp mask signal or denoting a plurality of attenuation coefficients corresponding to a plurality of said unsharp mask signals by .beta.k where k=1, 2, . . . , n, and iii) adjusting at least a single attenuation coefficient .beta.l, where l denotes an integer within the range of 1 to n, among step ii's said single attenuation coefficient or attenuation coefficients .beta.k, where k'1, 2, . . . , n, to be a constant within the range of iv) carrying out an operation represented by a formula ##EQU40## where Sb1 and Sb2 each denote said original image signal or an image signal obtained by carrying out intermediate processing of said original image signal, and S' denotes an image signal obtained by the operation processing, by use of said attenuation coefficient .beta.l, whereby there is performed an attenuating of spatial frequency components above the spatial frequency component which the unsharp mask signal Sus.l corresponding to said attenuation coefficient .beta.l has. 2. A radiation image processing method as defined in claim 1 wherein said attenuation coefficient .beta.l is a constant within the range of 3. A radiation image processing method as defined in claim 1 or 2 wherein both Sb1 and Sb2 each denoting said original image signal or an image signal obtained by carrying out intermediate processing of said original image signal are same said original image signal. 4. A radiation image processing method as defined in claim 1 or 2 wherein both Sb1 and Sb2 each denoting said original image signal or an image signal obtained by carrying out intermediate processing of said original image signal are the same image signal obtained by carrying out the same intermediate processing of said original image signal. 5. A radiation image processing method as defined in claim 1 or 2 wherein one of Sb1 and Sb2 each denoting said original image signal or an image signal obtained by carrying out intermediate processing of said original image signal is said original image signal or an image signal obtained by carrying out first intermediate processing of said original image signal, and the other of Sb1 and Sb2 is an image signal obtained by carrying out second intermediate processing of said original image signal. 6. A radiation image processing apparatus in a radiation image recording and reproducing system for scanning a stimulable phosphor, carrying a radiation image stored thereon, with stimulating rays which cause said stimulable phosphor to emit light in proportion to the stored radiation energy, obtaining an original image signal by photoelectrically detecting the light emitted by each scanning point on said stimulable phosphor, processing the original image signal by an operation device, and reproducing said radiation image as a visible image on a recording medium by use of the processed image signal, wherein said operation device comprises: 1) means for obtaining a single unsharp mask signal Sus.k by averaging original image signals within a predetermined range surrounding each scanning point or averaging image signals obtained by carrying out intermediate processing of the original image signals, or obtaining a plurality of unsharp mask signals Sus.k, where k=1, 2, . . . , n, wherein n denotes an integer representing the number of said unsharp mask signals, by changing said predetermined range, ii) means for denoting a single attenuation coefficient corresponding to single said unsharp mask signal or denoting a plurality of attenuation coefficients corresponding to a plurality of said unsharp mask signals by .beta.k where k=1, 2, . . . , n, iii) means for adjusting at least a single attenuation coefficient .beta.l, where l denotes an integer within the range of 1 to n, among step ii's said single attenuation coefficient or attenuation coefficients .beta.k, where k=1, 2, . . . , n, to be a constant within the range of and iv) means for carrying out an operation represented by a formula ##EQU41## where Sb1 and Sb2 each denote said original image signal or an image signal obtained by carrying out intermediate processing of said original image signal, and S' denotes an image signal obtained by the operation processing, by use of said attenuation coefficient .beta.l. 7. A radiation image processing method which, in the course of performing a read-=out operation by scanning a stimulable phosphor, carrying a radiation image stored thereon, with stimulating rays which cause said stimulable phosphor to emit light in proportion to the stored radiation energy, obtaining an original image signal by photoelectrically detecting the light emitted by each scanning point on said stimulable phosphor, and reproducing said radiation image as a visible image on a recording medium, comprises the steps of: i) obtaining a single unsharp mask signal Sus.k by averaging original image signals within a predetermined range surrounding each scanning point or averaging image signals obtained by carrying out intermediate processing of the original image signals, or obtaining a plurality of unsharp mask signals Sus.k, where k=1, 2, . . . , n, wherein n denotes an integer representing the number of said unsharp mask signals, by changing said predetermined range, ii) denoting a single attenuation coefficient corresponding to single said unsharp mask signal or denoting a plurality of attenuation coefficients corresponding to a plurality of said unsharp mask signals by .beta.k where k=1, 2, . . . , n, iii) adjusting at least a single attenuation coefficient .beta.l, where l denotes an integer within the range of 1 to n, among step ii's said single attenuation coefficient or attenuation coefficients .beta.k, where k=1, 2, . . . , n, to be a variable which is always within the range of and which varies in each said radiation image, and iv) carrying out an operation represented by a formula ##EQU42## wherein Sb1 and Sb2 each denote said original image signal or an image signal obtained by carrying out intermediate processing of said original image signal, and S' denotes an image signal obtained by the operation processing, by use of said attenuation coefficient .beta.l, whereby there is performed an attenuating of spatial frequency components above the spatial frequency component which the unsharp mask signal Sus.l corresponding to said attenuation coefficient .beta.l has. 8. A radiation image processing method as defined in claim 7 wherein said attenuation coefficient .beta.l is a variable which is always within the range of 9. A radiation image processing method as defined in claim 8 wherein said attenuation coefficient .beta.l is a function of said original image signal or of the image signal obtained by carrying out intermediate processing of said original image signal. 10. A radiation image processing method as defined in claim 7 wherein said attenuation coefficient .beta.l is a function of said original image signal or of the image signal obtained by carrying out intermediate processing of said original image signal. 11. A radiation image processing method as defined in any one of claims 7 to 10 wherein both Sb1 and Sb2 each denoting said original image signal or an image signal obtained by carrying out intermediate processing of said original image signal are the same said original image signal. 12. A radiation image processing method as defined in any one of claims 7 to 10 wherein both Sb1 and Sb2 each denoting said original image signal or an image signal obtained by carrying out intermediate processing of said original image signal are the same image signal obtained by carrying out the same intermediate processing of said original image signal. 13. A radiation image processing method as defined in any one of claims 7 to 10 wherein one of Sb1 and Sb2 each denoting said original image signal or an image signal obtained by carrying out intermediate processing of said original image signal is said original image signal or an image signal obtained by carrying out first intermediate processing of said original image signal, and the other of Sb1 and Sb2 is an image signal obtained by carrying out second intermediate processing of said original image signal. 14. A radiation image processing apparatus in a radiation image recording and reproducing system for scanning a stimulable phosphor, carrying a radiation image stored thereon, with stimulating rays which cause said stimulable phosphor to emit light in proportion to the stored radiation energy, obtaining an original image signal by photoelectrically detecting the light emitted by each scanning point on said stimulable phosphor, processing the original image signal by an operation device, and reproducing said radiation image as a visible image on a recording medium by use of the processed image signal, wherein said operation device comprises: 1) means for obtaining a single unsharp mask signal Sus.k by averaging original image signals within a predetermined range surrounding each scanning point or averaging image signals obtained by carrying out intermediate processing of the original image signals, or obtaining a plurality of unsharp mask signals Sus.k, where k=1, 2, . . . , n, wherein n denotes an integer representing the number of said unsharp mask signals, by changing said predetermined range, ii) means for denoting a single attenuation coefficient corresponding to single said unsharp mask signal or denoting a plurality of attenuation coefficients corresponding to a plurality of said unsharp mask signals by .beta.k where k=1, 2, . . . , n, iii) means for adjusting at least a single attenuation coefficient .beta.l, where l denotes an integer within the range of 1 to n, among step ii's said single attenuation coefficient or attenuation coefficients .beta.k, where k=1, 2, . . . , n, to be a variable which is always within the range of and which varies in each said radiation image, and iv) means for carrying out an operation represented by a formula ##EQU43## where Sb1 and Sb2 each denote said original image signal or an image signal obtained by carrying out intermediate processing of said original image signal, and S' denotes an image signal obtained by the operation processing, by use of said attenuation coefficient .beta.l. 15. A radiation image processing method which, in the course of performing a read-out operation by scanning a stimulable phosphor, carrying a radiation image stored thereon, by exposure to radiation with stimulating rays which cause said stimulable phosphor to emit light in proportion to the stored radiation energy, obtaining an original image signal by photoelectrically detecting the light emitted by each scanning point on said stimulable phosphor, and reproducing said radiation image as a visible image on a recording medium, comprises the steps of: i) obtaining a single unsharp mask signal Sus by averaging original image signals within a predetermined range surrounding each scanning point, ii) denoting a coefficient corresponding to said unsharp mask signal Sus by .beta., iii) adjusting said coefficient .beta. to be a function shifting from .beta.<0 to .beta.>0 as a dose of said radiation irradiated to each point on said stimulable phosphor increases, and iv) carrying out an operation represented by a formula where Sorg denotes said original image signal, and S' denotes an image signal obtained by the operation processing, by use of said coefficient .beta., whereby there is performed an attenuating of spatial frequency components above the spatial frequency component which said unsharp mask signal Sus has in a region of a low radiation dose inside of single said radiation image, and emphasizing the spatial frequency components above the spatial frequency component which said unsharp mask signal Sus has in a region of a high radiation dose inside of single said radiation image. 16. A radiation image processing method which, in the course of performing a read-out operation by scanning a stimulable phosphor, carrying a radiation image stored thereon, by exposure to radiation with stimulating rays which cause said stimulable phosphor to emit light in proportion to the stored radiation energy, obtaining an original image signal by photoelectrically detecting the light emitted by each scanning point on said stimulable phosphor, and reproducing said radiation image as a visible image on a recording medium, comprises the steps of: i) obtaining a single unsharp mask signal Sus.k by averaging original image signals within a predetermined range surrounding each scanning point or averaging image signals obtained by carrying out intermediate processing of the original image signals, or obtaining a plurality of unsharp mask signals Sus.k, where k=1, 2, . . . , n, wherein n denotes an integer representing the number of said unsharp mask signals, by changing said predetermined range, ii) denoting a single coefficient corresponding to single said unsharp mask signal or denoting a plurality of coefficients corresponding to a plurality of said unsharp mask signals by .beta.k where k=1, 2, . . . , n, iii) adjusting at least a single coefficient .beta.l, where l denotes an integer within the range of 1 to n, among step ii's said single coefficient or coefficients .beta.k, where k=1, 2, . . . , n, to be a function shifting from .beta.l<0 to .beta.l>0 as a dose of said radiation irradiated to each point on said stimulable phosphor increases, and iv) carrying out an operation represented by a formula ##EQU44## where Sb1 and Sb2 each denote said original image signal or an image signal obtained by carrying out intermediate processing of said original image signal, and S' denotes an image signal obtained by the operation processing, by use of said coefficient .beta.l, whereby there is performed an attenuating of spatial frequency components above the spatial frequency component which the unsharp mask signal Sus.l corresponding to said coefficient .beta.l has in a region of a low radiation dose inside of single said radiation image, and emphasizing the spatial frequency components above the spatial frequency component which the unsharp mask signal Sus.l corresponding to said coefficient .beta.l has in a region of a high radiation dose inside of single said radiation image. 17. A radiation image processing apparatus in a radiation image recording and reproducing system for scanning a stimulable phosphor carrying a radiation image stored thereon by exposure to radiation therefor stimulating rays which cause said stimulable phosphor to emit light in proportion to the stored radiation energy, obtaining an original image signal by photoelectrically detecting the light emitted by each scanning point on said stimulable phosphor, processing the original image signal by an operation device, and reproducing said radiation image as a visible image on a recording medium by use of the processed image signal, wherein the improvement comprises constituting said operation device for: i) obtaining an unsharp mask signal Sus by averaging original image signals within a predetermined range surrounding each scanning point, ii) denoting a coefficient corresponding to said unsharp mask signal Sus by .beta., iii) adjusting said coefficient .beta. to be a function-shifting from .beta.<0 to .beta.>0 as a dose of said radiation irradiated to each point on said stimulable phosphor increases, and iv) carrying out an operation represented by a formula where Sorg denotes said original image signal, and S' denotes an image signal obtained by the operation processing, by use of said coefficient .beta.. 18. A radiation image processing apparatus in a radiation image recording and reproducing system for scanning a stimulable phosphor, carrying a radiation image stored thereon by exposure to radiation, with stimulating rays which cause said stimulable phosphor to emit light in proportion to the stored radiation energy, obtaining an original image signal by photoelectrically detecting the light emitted by each scanning point on said stimulable phosphor, processing the original image signal by an operation device, and reproducing said radiation image as a visible image on a recording medium by use of the processed image signal, wherein said operation device comprises: i) means for obtaining a single unsharp mask signal Sus.k by averaging original image signals within a predetermined range surrounding each scanning point or averaging image signals obtained by carrying out intermediate processing of the original image signals, or obtaining a plurality of unsharp mask signals Sus.k, where k=1, 2, . . . , n, wherein n denotes an integer representing the number of said unsharp mask signals, by changing said predetermined range, ii) means for denoting a single attenuation coefficient corresponding to single said unsharp mask signal or denoting a plurality of coefficients corresponding to a plurality of said unsharp mask signals by .beta.k where k=1, 2, . . . , n, iii) means for adjusting at least a single coefficient .beta.l, where l denotes an integer within the range of 1 to n, among step ii's said single attenuation coefficient or coefficients .beta.k, where k=1, 2, . . . , n, to be a function shifting from .beta.l<0 to .beta.l>0 as a dose of said radiation irradiated to each point on said stimulable phosphor increases, and iv) means for carrying out an operation represented by a formula ##EQU45## where Sb1 and Sb2 each denote said original image signal or an image signal obtained by carrying out intermediate processing of said original image signal, and S' denotes an image signal obtained by the operation processing, by use of said coefficient .beta.l. 19. An X-ray image processing method which comprises the steps of: in the course of scanning an original photograph carrying an X-ray image recorded thereon, reading out an original image density at each scanning point on said original photograph, and reproducing said X-ray image as a visible image on a copy photograph or the like, i) obtaining a single unsharp mask by averaging original image densities within a predetermined range surrounding each scanning point or image densities obtained by carrying out intermediate processing of signals representing the original image densities, or obtaining a plurality of unsharp masks by changing said predetermined range, and denoting densities of said unsharp masks by Dus.k, where k=1, 2, . . . , n wherein n denotes an integer representing the number of said unsharp masks, ii) denoting a single attenuation coefficient corresponding to single said unsharp mask or a plurality of attenuation coefficients corresponding to a plurality of said unsharp masks by .beta.k where k=1, 2, . . . , n, iii) adjusting at least a single attenuation coefficient .beta.l, where l denotes an integer within the range of 1 to n, among said attenuation coefficients .beta.k, where k=1, 2, . . . , n, to be a constant within the range of iv) carrying out an operation represented by a formula ##EQU46## where Db1 and Db2 each denote said original image density or an image density obtained by carrying out intermediate processing of a signal representing said original image density, and D' denotes an image density obtained by the operation processing, by use of said attenuation coefficient .beta.l, and v) attenuating spatial frequency components above the spatial frequency component which the density Dus.l of the unsharp mask corresponding to said attenuation coefficient .beta.l has. 20. An X-ray image processing method as defined in claim 19 wherein said attenuation coefficient .beta.l is a constant within the range of 21. An X-ray image processing method as defined in claim 19 or 20 wherein both Db1 and Db2 each denoting said original image density or an image density obtained by carrying out intermediate processing of a signal representing said original image density are same said original image density. 22. An X-ray image processing method as defined in claim 19 or 20 wherein both Db1 and Db2 each denoting said original image density or an image density obtained by carrying out intermediate processing of a signal representing said original image density are the same image density obtained by carrying out the same intermediate processing of the signal representing said original image density. 23. An X-ray image processing method as defined in claim 19 or 20 wherein one of Db1 and Db2 each denoting said original image density or an image density obtained by carrying out intermediate processing of a signal representing said original image density is said original image density or an image density obtained by carrying out first intermediate processing of the signal representing said original image density, and the other of Db1 and Db2 is an image density obtained by carrying out second intermediate processing of the signal representing said original image density. 24. An X-ray image processing apparatus for processing a signal representing an original image density, which has been read out at each scanning point on an original photograph carrying an X-ray image recorded thereon, by an operation device, and reproducing said X-ray image as a visible image on a copy photograph or the like by use of the signal representing the processed image density, wherein said operation device comprises: i) means for obtaining a single unsharp mask by averaging original image densities within a predetermined range surrounding each scanning point or averaging image densities obtained by carrying out intermediate processing of signals representing the original image densities, or obtaining a plurality of unsharp masks by changing said predetermined range, and denoting densities of said unsharp masks by Dus.k, where k=1, 2, . . . , n, wherein n denotes an integer representing the number of said unsharp masks, ii) means for denoting a single attenuation coefficient corresponding to single said unsharp mask or denoting a plurality of attenuation coefficients corresponding to a plurality of said unsharp masks by .beta.k where k=1, 2, . . . , n, iii) means for adjusting at least a single attenuation coefficient .beta.l, where l denotes an integer within the range of 1 to n, among step ii's said single attenuation coefficient or attenuation coefficients .beta.k, where k=1, 2, . . . , n, to be a constant within the range of and iv) means for carrying out an operation represented by a formula ##EQU47## where Db1 and Db2 each denote said original image density or an image density obtained by carrying out intermediate processing of a signal representing said original image density, and D' denotes an image density obtained by the operation processing, by use of said attenuation coefficient .beta.l. 25. An X-ray image processing method which, in the course of performing a read out operation by scanning an original photograph carrying an X-ray image recorded thereon, reading out an original image density at each scanning point on said original photograph, and reproducing said X-ray image as a visible image on a copy photograph or the like, comprises the steps of: i) obtaining a single unsharp mask by averaging original image densities within a predetermined range surrounding each scanning point or averaging image densities obtained by carrying out intermediate processing of signals representing the original image densities, or obtaining a plurality of unsharp masks by changing said predetermined range, and denoting densities of said unsharp masks by Dus.k, where k=1, 2, . . . , n, wherein n denotes an integer representing the number of said unsharp masks, ii) denoting a single attenuation coefficient corresponding to single said unsharp mask or denoting a plurality of attenuation coefficients corresponding to a plurality of said unsharp masks by .beta.k where k=1, 2, . . . , n, iii) adjusting at least a single attenuation coefficient .beta.l, where l denotes an integer within the range of 1 to n, among step ii's said single attenuation coefficient or attenuation coefficients .beta.k, where k=1, 2, . . . , n, to be a variable which is always within the range of and which varies in each said X-ray image, and iv) carrying out an operation represented by a formula ##EQU48## where Db1 and Db2 each denote said original image density or an image density obtained by carrying out intermediate processing of a signal representing said original image density, and D' denotes an image density obtained by the operation processing, by use of said attenuation coefficient .beta.l, whereby there is performed an attenuating of spatial frequency components above the spatial frequency component which the density Dus.l of the unsharp mask corresponding to said attenuation coefficient .beta.l has. 26. An X-ray image processing method as defined in claim 25 wherein said attenuation coefficient .beta.l is a variable which is always within the range of 27. An X-ray image processing method as defined in claim 25 wherein said attenuation coefficient .beta.l is a function of said original image density or of the image density obtained by carrying out intermediate processing of the signal representing said original image density. 28. An X-ray image processing method as defined in claim 26 wherein said attenuation coefficient .beta.l is a function of said original image density or of the image density obtained by carrying out intermediate processing of the signal representing said original image density. 29. An X-ray image processing method as defined in any one of claims 25 to 28 wherein both Db1 and Db2 each denoting said original image density or an image density obtained by carrying out intermediate processing of a signal representing said original image density are same said original image density. 30. An X-ray image processing method as defined in any one of claims 25 to 28 wherein both Db1 and Db2 each denoting said original image density or an image density obtained by carrying out intermediate processing of a signal representing said original image density are the same image density obtained by carrying out the same intermediate processing of the signal representing said original image density. 31. An X-ray image processing method as defined in any one of claims 25 to 28 wherein one of Db1 and Db2 each denoting said original image density or an image density obtained by carrying out intermediate processing of a signal representing said original image density is said original image density or an image density obtained by carrying out first intermediate processing of the signal representing said original image density, and the other of Db1 and Db2 is an image density obtained by carrying out second intermediate processing of the signal representing said original image density. 32. An X-ray image processing apparatus for processing a signal representing an original image density, which has been read out at each scanning point on an original photograph carrying an X-ray image recorded thereon, by an operation device, and reproducing said X-ray image as a visible image on a copy photograph or the like by use of the signal representing the processed image density, wherein said operation device comprises: i) means for obtaining a single unsharp mask by averaging original image densities within a predetermined range surrounding each scanning point or averaging image densities obtained by carrying out intermediate processing of signals representing the original image densities, or obtaining a plurality of unsharp masks by changing said predetermined range, and denoting densities of said unsharp masks by Dus.k, where k=1, 2, . . . , n, wherein n denotes an integer representing the number of said unsharp masks, ii) means for denoting a single attenuation coefficient corresponding to single said unsharp mask or denoting a plurality of attenuation coefficients corresponding to a plurality of said unsharp masks by .beta.k where k=1, 2, . . . , n, iii) means for adjusting at leas a single attenuation coefficient .beta.l, where l denotes an integer within the range of 1 to n, among step ii's said single attenuation coefficient or attenuation coefficients .beta.k, where k=1, 2, . . . , n, to be a variable which is always within the range of and which varies in each said X-ray image, and iv) means for carrying out an operation represented by a formula ##EQU49## where Db1 and Db2 each denote said original image density or an image density obtained by carrying out intermediate processing of a signal representing said original image density, and D' denotes an image density obtained by the operation processing, by use of said attenuation coefficient .beta.l. 33. An X-ray image processing method which comprises the steps of: in the course of scanning an original photograph carrying an X-ray image recorded thereon and obtained by exposing a photographic film to X-rays, reading out an original image density at each scanning point on said original photograph, and reproducing said X-ray image as a visible image on a copy photograph or the like, i) obtaining an unsharp mask by averaging original image densities within a predetermined range surrounding each scanning point, and denoting a density of said unsharp mask by Dus, ii) denoting a coefficient corresponding to said unsharp mask by .beta., iii) adjusting said coefficient .beta. to be a function shifting from .beta.<0 to .beta.>0 as a dose of said X-rays irradiated to each point on said photographic film increases, iv) carrying out an operation represented by a formula where Dorg denotes said original image density, and D' denotes an image density obtained by the operation processing, by use of said coefficient .beta., and v) attenuating spatial frequency components above the spatial frequency component which said unsharp mask density Dus has in a region of a low X-ray dose inside of signal said X-ray image, and emphasizing the spatial frequency components above the spatial frequency component which said unsharp mask density Dus has in a region of a high X-ray dose inside of single said X-ray image. 34. An X-ray image processing method which in the course of performing a read out operation by scanning an original photograph carrying an X-ray image recorded thereon as obtained by exposing a photographic film to X-rays, reading out an original image density at each scanning point on said original photograph, and reproducing said X-ray image as a visible image on a copy photograph or the like, comprises the steps of: i) obtaining a single unsharp mask by averaging original image densities within a predetermined range surrounding each scanning point or averaging image densities obtained by carrying out intermediate processing of signals representing the original image densities, or obtaining a plurality of unsharp masks by changing said predetermined range, and denoting densities of said unsharp masks by Dus.k, where k=1, 2, . . . , n, wherein n denotes an integer representing the number of said unsharp masks, ii) denoting a single coefficient corresponding to single said unsharp mask or denoting a plurality of coefficients corresponding to a plurality of said unsharp masks by .beta.k where k=1, 2, . . . , n, iii) adjusting at least a single attenuation coefficient .beta.l, where l denotes an integer within the range of 1 to n, among step ii's said single coefficient or coefficients .beta.k, where k=1, 2, . . . , n, to be a function shifting from .beta.l<0 to .beta.l>0 as a dose of said X-rays irradiated to each point on said photographic film increases, and iv) carrying out an operation represented by a formula ##EQU50## where Db1 and Db2 each denote said original image density or an image density obtained by carrying out intermediate processing of a signal representing said original image density, and D' denotes an image density obtained by the operation processing, by use of said coefficient .beta.l, whereby there is performed an attenuating of spatial frequency components above the spatial frequency component which the unsharp mask density Dus.l corresponding to said coefficient .beta.l has in a region of a low X-ray dose inside of single said X-ray image, and emphasizing the spatial frequency components above the spatial frequency component which the unsharp mask density Dus.l corresponding to said coefficient .beta.l has in a region of a high X-ray dose inside of single said X-ray image. 35. An X-ray image processing apparatus for processing a signal representing an original image density, which has been read out at each scanning point on an original photograph obtained by exposure of a photographic film to X-rays, by an operation device, and reproducing said X-ray image as a visible image on a copy photograph or the like by use of the signal representing the processed image density, wherein the improvement comprises constituting said operation device for: i) obtaining an unsharp mask by averaging original image densities within a predetermined range surrounding each scanning point, and denoting a density of said unsharp mask by Dus, ii) denoting a coefficient corresponding to said unsharp mask by .beta., iii) adjusting said coefficient .beta. to be a function shifting from .beta.<0 to .beta.>0 as a dose of said X rays irradiated to each point on said coefficient .beta. to be a function shifting from .beta.<0 to .beta.>0 as a dose of said X-rays irradiated to each point on said photographic film increases, and iv) carrying out an operation represented by a formula where Dorg denotes said original image density, and D' denotes an image density obtained by the operation processing, by use of said coefficient .beta.. 36. An X-ray image processing apparatus for processing a signal representing an original image density, which has been read out at each scanning point on an original photograph obtained by exposure of a photographic film to X-rays, by an operation device, and reproducing said X-ray image as a visible image on a copy photograph or the like by use of the signal representing the processed image density, wherein said operation device comprises: i) means for obtaining a single unsharp mask by averaging original image densities within a predetermined range surrounding each scanning point or averaging image densities obtained by carrying out intermediate processing of signals representing the original image densities, or obtaining a plurality of unsharp masks by changing said predetermined range, and denoting densities of said unsharp masks by Dus.k, where k=1, 2, . . . , n, wherein n denotes an integer representing the number of said unsharp masks, ii) means for denoting a single coefficient corresponding to single said unsharp mask or denoting a plurality of coefficients corresponding to a plurality of said unsharp masks by .beta.k where k=1, 2, . . . , n, iii) means for adjusting at least a single coefficient .beta.l, where l denotes an integer within the range of 1 to n, among step ii's said single coefficient or coefficients .beta.k, where k=1, 2, . . . , n, to be a function shifting from .beta.l<0 to .beta.l>0 as a dose of said X-rays irradiated to each point on said photographic film increases, and iv) means for carrying out an operation represented by a formula ##EQU51## where Db1 and Db2 each denote said original image density or an image density obtained by carrying out intermediate processing of a signal representing said original image density, and D' denotes an image density obtained by the operation processing, by use of said coefficient .beta.l. BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to frequency response processing for a radiation image, particularly a radiation image processing method in a radiation image recording and reproducing system for recording a radiation image on a stimulable phosphor as an intermediate medium, obtaining image signals from the radiation image, and reproducing the radiation image as a visible image on a recording medium by use of the image signals, and an apparatus for carrying out the method. This invention also relates to an X-ray image processing method for processing the signals representing original image densities detected from an X-ray image, which has been recorded on an original photograph by the irradiation of X-rays to an object, at the time the X-ray image is to be copied, and an apparatus for carrying out the method. 2. Description of the Prior Art When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then two-dimensionally scanned by stimulating rays such as a laser beam which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected by a photodetector and converted to electric image signals, and the radiation image of the object is reproduced as a visible image by use of the image signals on a recording medium such as a photographic film, a display device such as a cathode ray tube (CRT), or the like. The radiation image recording and reproducing system using a stimulable phosphor sheet is advantageous over conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range (latitude) of radiation exposure. More specifically, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor varies over a wide range in proportion to the amount of said stored energy, it is possible to obtain an image having desirable density regardless of the amount of exposure of the stimulable phosphor sheet to the radiation, by reading out the emitted light with an appropriate read-out gain and converting it into electric signals to reproduce a visible image on a recording medium or a display device. In the case where the aforesaid radiation image recording and reproducing system is used for diagnosis of the human body, the radiation dose to the human body can be decreased markedly as compared with the conventional X-ray image recording diagnosis system. However, as the dose of radiation irradiated to the object at the time of the image recording is decreased, adverse effects of quantum noise of radiation or the like on the radiation image increase. As a result, graininess of the image deteriorates, and the reproduced visible image becomes rough. In order to improve the graininess, the apparatus may be devised as described below. For example, a blur image may be stored on the stimulable phosphor sheet at the time of the image recording by making the stimulable phosphor sheet thicker or by making larger the grains of the stimulable phosphor used in the stimulable phosphor sheet. Alternatively, the image may be blurred at the time of the image read-out by increasing the beam diameter of stimulating rays used for the scanning, or the read-out image may be blurred by feeding the read-out analog image signals into an analog filter. Fine control is necessary in order to improve the graininess while deterioration of the other image quality factors such as sharpness are being minimized. However, with the aforesaid approaches to the improvement of the graininess, the kind of the stimulable phosphor sheet must be increased, and the degree of freedom of the control is limited even though the kind of the stimulable phosphor sheet is increased. Also, the degree of freedom of the control is very low though the mechanism becomes complicated, and the control is possible only in the direction of flow of the sequential image signals (the direction of main scanning). On the other hand, in order to improve the graininess by image processing, frequency response processing may be carried out by use of FFT (fast Fourier transform), or the image may be digitally blurred by calculating a mean value of the image signals around each scanning point. With the method using FFT, the degree of freedom of the control is very high. However, with this method, the processing speed is too low to process large numbers of the image signals, and a high cost is required to increase the processing speed. With the method wherein the image is digitally blurred by use of the mean value, fine control cannot be achieved and the image is generally blurred excessively even though the processing can be carried out quickly. SUMMARY OF THE INVENTION The primary object of the present invention is to provide a radiation image processing method which improves graininess of a radiation image while deterioration of other image quality factors is being minimized, and which is carried out without an apparatus being caused to become complicated and in an operation time within a substantially allowable range, and an apparatus for carrying out the method. Another object of the present invention is to provide a radiation image processing method which improves the overall image quality by improving the sharpness, contrast and the like while grain noise of the radiation image is being restricted, and which is carried out without an apparatus being caused to become complicated and in an operation time within a substantially allowable range, and an apparatus for carrying out the method. A further object of the present invention is to provide an X-ray image processing method which improves graininess of an X-ray image while deterioration of other image quality factors is being minimized, and which is carried out without an apparatus being caused to become complicated and in an operation time within a substantially allowable range, and an apparatus for carrying out the method. A still further object of the present invention is to provide an X-ray image processing method which improves the overall image quality by improving the sharpness, contrast and the like while grain noise of the X-ray image is being restricted, and which is carried out without an apparatus being caused to become complicated and in an operation time within a substantially allowable range, and an apparatus for carrying out the method. The present invention provides a first radiation image processing method which comprises the steps of: in the course of scanning a stimulable phosphor carrying a radiation image stored thereon by stimulating rays which cause said stimulable phosphor to emit light in proportion to the stored radiation energy, obtaining an original image signal by photoelectrically detecting the light emitted by each scanning point of said stimulating rays on said stimulable phosphor, and reproducing said radiation image as a visible image on a recording medium, i) obtaining a single unsharp mask signal Sus.k by averaging original image signals within a predetermined range surrounding each scanning point or image signals obtained by carrying out intermediate processing of the original image signals, or obtaining a plurality of unsharp mask signals Sus.k, where k=1, 2, . . . , n wherein n denotes an integer representing the number of said unsharp mask signals, by changing said predetermined range, ii) denoting a single attenuation coefficient corresponding to single said unsharp mask signal or a plurality of attenuation coefficients corresponding to a plurality of said unsharp mask signals by .beta.k where k=1, 2, . . . , n, iii) adjusting at least a single attenuation coefficient .beta.l, where l denotes an integer within the range of 1 to n, among said attenuation coefficients .beta.k, where k=1, 2, . . . , n, to be a constant within the range of iv) carrying out an operation represented by a formula ##EQU2## where Sb1 and Sb2 each denote said original image signal or an image signal obtained by carrying out intermediate processing of said original image signal, and S' denotes an image signal obtained by the operation processing, by use of said attenuation coefficient .beta.l, and v) attenuating spatial frequency components above the spatial frequency component which the unsharp mask signal Sus.l corresponding to said attenuation coefficient .beta.l has. The first radiation image processing method in accordance with the present invention is carried out by a first radiation image processing apparatus in a radiation image recording and reproducing system for scanning a stimulable phosphor carrying a radiation image stored thereon by stimulating rays which cause said stimulable phosphor to emit light in proportion to the stored radiation energy, obtaining an original image signal by photoelectrically detecting the light emitted by each scanning point of said stimulating rays on said stimulable phosphor, processing the original image signal by an operation device, and reproducing said radiation image as a visible image on a recording medium by use of the processed image signal, wherein the improvement comprises constituting said operation device for: i) obtaining a single unsharp mask signal Sus.k by averaging original image signals within a predetermined range surrounding each scanning point or image signals obtained by carrying out intermediate processing of the original image signals, or obtaining a plurality of unsharp mask signals Sus.k, where k=1, 2, . . . , n wherein n denotes an integer representing the number of said unsharp mask signals, by changing said predetermined range, ii) denoting a single attenuation coefficient corresponding to single said unsharp mask signal or a plurality of attenuation coefficients corresponding to a plurality of said unsharp mask signals by .beta.k where k=1, 2, . . . , n, iii) adjusting at least a single attenuation coefficient .beta.l, where l denotes an integer within the range of 1 to n, among said attenuation coefficients .beta.k, where k=1, 2, . . . , n, to be a constant within the range of and iv) carrying out an operation represented by a formula ##EQU3## where Sb1 and Sb2 each denote said original image signal or an image signal obtained by carrying out intermediate processing of said original image signal, and S' denotes an image signal obtained by the operation processing, by use of said attenuation coefficient .beta.l. With the first radiation image processing method in accordance with the present invention, at least a single attenuation coefficient .beta.l among the attenuation coefficients .beta.k, where k=1, 2, . . . , n, is adjusted to be a constant within the range of and the operation represented by the formula ##EQU4## is carried out. Formula (1) can be rewritten into the form of ##EQU5## As for the second term .beta.l(Sb2-Sus.l) of Formula (2), the unsharp mask signal Sus.l is subtracted from Sb2 which is, by way of example, the original image signal as represented by Sb2-Sus.l in the parentheses of the second term, whereby the low spatial frequency component which the unsharp mask signal Sus.l has is subtracted from Sb2. Also, Sb2-Sus.l is multiplied by the attenuation coefficient .beta.l satisfying the condition of 0<.beta.l wherein .beta.l.noteq.1 as represented by .beta.l(Sb2-Sus.l), and .beta.l(Sb2-Sus.l) is subtracted from Sb1 which is, by way of example, the original image signal. In this manner, the high spatial frequency component which Sb2-Sus.l has can be attenuated from the signal Sb1. In the case where the high spatial frequency component is made to coincide with grainy noise of the image and the attenuation coefficient .beta.l is adjusted to be an appropriate value satisfying the condition of 0<.beta.l wherein .beta.l.noteq.1, grainy noise of the image can be attenuated, and deterioration of other image quality factors such as sharpness can be minimized. Also, the first radiation image processing apparatus for carrying out the first radiation image processing method is not complicated as compared with the radiation image processing apparatuses in the radiation image recording and reproducing system proposed by the applicant in, for example, U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, and can achieve the operation in a time within a substantially allowable range. The original image signal obtained by the photoelectric detection may be used as the image signals Sb1 and Sb2, or an image signal obtained by carrying out intermediate processing of the original image signal may be used as one or both of the image signals Sb1 and Sb2. The third and fourth terms of Formula (2) will now be described below. Grainy noise has a wide range of spatial frequency components. Therefore, in the case where grainy noise cannot be substantially restricted by the combination of the first term with the second term of Formula (2), the same operation as the second term is carried out in the third term or the fourth term by changing the spatial frequency region from the frequency region in the second term. Also, an attenuation coefficient .beta.m where m.noteq.l may be adjusted so that .beta.m<0 in the third and fourth term and, for example, an operation for emphasizing specific spatial frequency components as proposed by the applicant in U.S. Pat. No. 4,315,318 may be used in combination. Basic differences between the first radiation image processing method in accordance with the present invention and the method as proposed by the applicant in, for example, U.S. Pat. No. 4,315,318 will now be described below. In the proposed method, an operation represented by a formula where Sus denotes an unsharp mask signal, Sorg denotes an original image signal, .beta. denotes an emphasis coefficient, and S' denotes a signal obtained by processing, is carried out for emphasizing specific spatial frequency components. The simplest formula of the first radiation image processing method in accordance with the present invention comprises only the first term and the second term of Formula (2), i.e. is expressed as As mentioned above, Formula (4) indicates that the spatial frequency components which grainy noise has are attenuated positively. However, it was found by the inventors of U.S. Pat. No. 4,315,318 that the spatial frequency that grain noise has overlaps the spatial frequency affecting other image quality factors such as sharpness. Therefore, it is considered that in the case where the spatial frequency that grain noise has is attenuated positively, other image quality factors will deteriorate to an unrestorable extent. Accordingly, the image quality has heretofore been improved by emphasizing the spatial frequency components having a comparatively high degree of contribution to other image quality factors such as sharpness, instead of the graininess, without positively attenuating the spatial frequency components which grain noise has. The inventors of the present invention studied the properties of grainy noise and found that grainy noise can be rendered imperceptible while deterioration of other image quality factors such as sharpness is being minimized by accurately selecting the spatial frequency which is to be attenuated and the extent of attenuation of said spatial frequency, and positively restricting the spatial frequency components which grainy noise has. The optimal value of the attenuation coefficient .beta.l employed for carrying out the attenuation is generally present in the range of 0<.beta.l<1, depending on the kind of the radiation image or the like. As mentioned above, with the first radiation image processing method in accordance with the present invention, after the original image signal is obtained by scanning the stimulable phosphor carrying a radiation image stored thereon by stimulating rays which cause the stimulable phosphor to emit light in proportion to the stored radiation energy, and photoelectrically detecting the light emitted by each scanning point of the stimulating rays on the stimulable phosphor, at least a single attenuation coefficient .beta.l among the attenuation coefficients .beta.k where k=1, 2, . . . , n is adjusted to be a constant within the range of and the operation represented by the formula ##EQU6## is carried out. Therefore, the spatial frequency components above the spatial frequency component which the unsharp mask signal Sus.l has can be attenuated, grainy noise of the radiation image can be attenuated efficiently, and deterioration of other image quality factors can be minimized. Also, the apparatus for carrying out the first radiation image processing method in accordance with the present invention is not so complicated and can achieve the operation in a time within a substantially allowable range. The present invention also provides a second radiation image processing method which comprises the steps of: in the course of scanning a stimulable phosphor carrying a radiation image stored thereon by stimulating rays which cause said stimulable phosphor to emit light in proportion to the stored radiation energy, obtaining an original image signal by photoelectrically detecting the light emitted by each scanning point of said stimulating rays on said stimulable phosphor, and reproducing said radiation image as a visible image on a recording medium, i) obtaining a single unsharp mask signal Sus.k by averaging original image signals within a predetermined range surrounding each scanning point or image signals obtained by carrying out intermediate processing of the original image signals, or obtaining a plurality of unsharp mask signals Sus.k, where k=1, 2, . . . , n wherein n denotes an integer representing the number of said unsharp mask signals, by changing said predetermined range, ii) denoting a single attenuation coefficient corresponding to single said unsharp mask signal or a plurality of attenuation coefficients corresponding to a plurality of said unsharp mask signals by .beta.k where k=1, 2, . . . , n, iii) adjusting at least a single attenuation coefficient .beta.l, where l denotes an integer within the range of 1 to n, among said attenuation coefficients .beta.k, where k=1, 2, . . . , n, to be a variable which is always within the range of and which varies in each said radiation image, iv) carrying out an operation represented by a formula ##EQU7## where Sb1 and Sb2 each denote said original image signal or an image signal obtained by carrying out intermediate processing of said original image signal, and S' denotes an image signal obtained by the operation processing, by use of said attenuation coefficient .beta.l, and v) attenuating spatial frequency components above the spatial frequency component which the unsharp mask signal Sus.l corresponding to said attenuation coefficient .beta.l has. The second radiation image processing method in accordance with the present invention is carried cut by a second radiation image processing apparatus in a radiation image recording and reproducing system for scanning a stimulable phosphor carrying a radiation image stored thereon by stimulating rays which cause said stimulable phosphor to emit light in proportion to the stored radiation energy, obtaining an original image signal by photoelectrically detecting the light emitted by each scanning point of said stimulating rays on said stimulable phosphor, processing the original image signal by an operation device, and reproducing said radiation image as a visible image on a recording medium by use of the processed image signal, wherein the improvement comprises constituting said operation device for: i) obtaining a single unsharp mask signal Sus.k by averaging original image signals within a predetermined range surrounding each scanning point or image signals obtained by carrying out intermediate processing of the original image signals, or obtaining a plurality of unsharp mask signals Sus.k, where k=1, 2, . . . , n wherein n denotes an integer representing the number of said unsharp mask signals, by changing said predetermined range, ii) denoting a single attenuation coefficient corresponding to single said unsharp mask signal or a plurality of attenuation coefficients corresponding to a plurality of said unsharp mask signals by .beta.k where k=1, 2, . . . , n, iii) adjusting at least a single attenuation coefficient .beta.l, where l denotes an integer within the range of 1 to n, among said attenuation coefficients .beta.k, where k=1, 2, . . . , n, to be a variable which is always within the range of and which varies in each said radiation image, and iv) carrying out an operation represented b) a formula ##EQU8## where Sb1 and Sb2 each denote said original image signal or an image signal obtained by carrying out intermediate processing of said original image signal, and S' denotes an image signal obtained by the operation processing, by use of said attenuation coefficient .beta.l. With the second radiation image processing method in accordance with the present invention, at least a single attenuation coefficient .beta.l among the attenuation coefficients .beta.k, where k=1, 2, . . . , n, is adjusted to be a variable always having a value within the range of and the operation represented by the formula ##EQU9## is carried out. Formula (5) can be rewritten into the form of ##EQU10## As for the second term .beta.l(Sb2-Sus.l) of Formula (6), the unsharp mask signal Sus.l is subtracted from Sb2 which is, by way of example, the original image signal as represented by Sb2-Sus.l in the parentheses of the second term, whereby the low spatial frequency component which the unsharp mask signal Sus.l has is subtracted from Sb2. Also, Sb2-Sus.l is multiplied by the attenuation coefficient .beta.l satisfying the condition of 0.ltoreq..beta.l as represented by .beta.l(Sb2-Sus.l), and .beta.l(Sb2-Sus.l) is subtracted from Sb1 which is, by way of example, the original image signal. In this manner, in the region of .beta.l.ltoreq.0 (where .beta.l is the variable varying in the radiation image) inside of the radiation image, the high spatial frequency component which Sb2-Sus.l has can be attenuated from the signal Sb1. In the case where the high spatial frequency component is made to coincide with grainy noise of the image and the attenuation coefficient .beta.l is adjusted to be an appropriate value as the variable varying within the range of 0.ltoreq..beta.l, grainy noise of the image can be attenuated, and deterioration of other image quality factors such as sharpness can be minimized in accordance with the condition of each region inside of a single image. Also, the second radiation image processing apparatus for carrying out the second radiation image processing method is not complicated as compared with the radiation image processing apparatuses in the radiation image recording and reproducing system proposed by the applicant in, for example, U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, and can achieve the operation in a time within a substantially allowable range. The original image signal obtained by the photoelectric detection may be used as the image signals Sb1 and Sb2, or an image signal obtained by carrying out intermediate processing of the original image signal may be used as one or both of the image signals Sb1 and Sb2. The third and fourth terms of Formula (6) will now be described below. Grainy noise has a wide range of spatial frequency components. Therefore, in the case where grainy noise cannot be substantially restricted by the combination of the first term with the second term of Formula (6) or finer image processing is to be carried out by changing the spatial frequency region for each region inside of a single image area, the same operation as the second term is carried out in the third term or the fourth term by changing the spatial frequency region from the frequency region in the second term. Also, an attenuation coefficient .beta.m where m<l may be adjusted so that .beta.m<0 in the third and fourth term and, for example, an operation for emphasizing specific spatial frequency components as proposed by the applicant in U.S. Pat. No. 4,315,318 may be used in combination. The simplest formula of the second radiation image processing method in accordance with the present invention comprises only the first term and the second term of Formula (6), i.e. is expressed as As mentioned above, Formula (7) indicates that the spatial frequency components which grainy noise has are attenuated positively. In the case where the attenuation coefficient .beta.l for carrying out the attenuation is varied within the range of 0.ltoreq..beta.<1, it can optimize each region inside of the image for almost every image. As for the attenuation coefficient .beta.l, various function forms may be selected in accordance with the purpose of image processing or the like. For example, the attenuation coefficient .beta.l may be adjusted to be a function of the image signals such that a portion of a low image density in the radiation image where grainy noise is comparatively perceptible is blurred by increasing the extent of the attenuation, and the extent of the attenuation is decreased for a portion of a high image density where grainy noise is comparatively imperceptible to make the detailed structure sharper. Alternatively, the attenuation coefficient .beta.l may be varied in accordance with the object portion inside of a single image such as a bone portion, a lung portion or a heart portion in the radiation image of the chest of the human body so that image processing is carried out to be suitable for each object portion. As mentioned above, with the second radiation image processing method in accordance with the present invention, after the original image signal is obtained by scanning the stimulable phosphor carrying a radiation image stored thereon by stimulating rays which cause the stimulable phosphor to emit light in proportion to the stored radiation energy, and photoelectrically detecting the light emitted by each scanning point of the stimulating rays on the stimulable phosphor, at least a single attenuation coefficient .beta.l among the attenuation coefficients .beta.k where k=1, 2, . . . , n is adjusted to be a variable which is always within the range of and which varies in each radiation image, and the operation represented by the formula ##EQU11## is carried out. Therefore, the spatial frequency components above the spatial frequency component which the unsharp mask signal Sus.l has can be attenuated. Also, grainy noise of the radiation image can be attenuated efficiently, and deterioration of other image quality factors can be minimized in accordance with each region inside of the radiation image. Moreover, the apparatus for carrying out the second radiation image processing method in accordance with the present invention is not so complicated and can achieve the operation in a time within a substantially allowable range. The present invention further provides a third radiation image processing method which comprises the steps of: in the course of scanning a stimulable phosphor carrying a radiation image stored thereon by exposure to radiation by stimulating rays which cause said stimulable phosphor to emit light in proportion to the stored radiation energy, obtaining an original image signal by photoelectrically detecting the light emitted by each scanning point of said stimulating rays on said stimulable phosphor, and reproducing said radiation image as a visible image on a recording medium, i) obtaining an unsharp mask signal Sus by averaging original image signals within a predetermined range surrounding each scanning point, ii) denoting a coefficient corresponding to said unsharp mask signal Sus by .beta., iii) adjusting said coefficient .beta. to be a function shifting from .beta.<0 to .beta.>0 as a dose of said radiation irradiated to each point on said stimulable phosphor increases, iv) carrying out an operation represented by a formula where Sorg denotes said original image signal, and S' denotes an image signal obtained by the operation processing, by use of said coefficient .beta., and v) attenuating spatial frequency components above the spatial frequency component which said unsharp mask signal Sus has in a region of a low radiation dose inside of single said radiation image, and emphasizing the spatial frequency components above the spatial frequency component which said unsharp mask signal Sus has in a region of a high radiation dose inside of single said radiation image. Other operations as well as the operation corresponding to Formula (8) may also be contained in the third radiation image processing method in accordance with the present invention. Specifically, the present invention also provides a fourth radiation image processing method which comprises the steps of: in the course of scanning a stimulable phosphor carrying a radiation image stored thereon by exposure to radiation by stimulating rays which cause said stimulable phosphor to emit light in proportion to the stored radiation energy, obtaining an original image signal by photoelectrically detecting the light emitted by each scanning point of said stimulating rays on said stimulable phosphor, and reproducing said radiation image as a visible image on a recording medium, i) obtaining a single unsharp mask signal Sus.k by averaging original image signals within a predetermined range surrounding each scanning point or image signals obtained by carrying out intermediate processing of the original image signals, or obtaining a plurality of unsharp mask signals Sus.k, where k=1, 2, . . . , n wherein n denotes an integer representing the number of said unsharp mask signals, by changing said predetermined range, ii) denoting a single coefficient corresponding to single said unsharp mask signal or a plurality of coefficients corresponding to a plurality of said unsharp mask signals by .beta.k where k=1, 2, . . . , n, iii) adjusting at least a single coefficient .beta.l, where l denotes an integer within the range of 1 to n, among said coefficients .beta.k, where k=1, 2, . . . , n, to be a function shifting from .beta.l<0 to .beta.l>0 as a dose of said radiation irradiated to each point on said stimulable phosphor increases, iv) carrying out an operation represented by a formula ##EQU12## where Sb1 and Sb2 each denote said original image signal or an image signal obtained by carrying out intermediate processing of said original image signal, and S' denotes an image signal obtained by the operation processing, by use of said coefficient .beta.l, and v) attenuating spatial frequency components above the spatial frequency component which the unsharp mask signal Sus.l corresponding to said coefficient .beta.l has in a region of a low radiation dose inside of single said radiation image, and emphasizing the spatial frequency components above the spatial frequency component which the unsharp mask signal Sus.l corresponding to said coefficient .beta.l has in a region of a high radiation dose inside of single said radiation image. The third radiation image processing method in accordance with the present invention is carried out by a third radiation image processing apparatus in a radiation image recording and reproducing system for scanning a stimulable phosphor carrying a radiation image stored thereon by exposure to radiation by stimulating rays which cause said stimulable phosphor to emit light in proportion to the stored radiation energy, obtaining an original image signal by photoelectrically detecting the light emitted by each scanning point of said stimulating rays on said stimulable phosphor, processing the original image signal by an operation device, and reproducing said radiation image as a visible image on a recording medium by use of the processed image signal, wherein the improvement comprises constituting said operation device for: i) obtaining an unsharp mask signal Sus by averaging original image signals within a predetermined range surrounding each scanning point, ii) denoting a coefficient corresponding to said unsharp mask signal Sus by .beta., iii) adjusting said coefficient .beta. to be a function shifting from .beta.<0 to .beta.>0 as a dose of said radiation irradiated to each point on said stimulable phosphor increases, and iv) carrying out an operation represented by a formula where Sorg denotes said original image signal, and S' denotes an image signal obtained by the operation processing, by use of said coefficient .beta.. The third radiation image processing apparatus for carrying out the third radiation image processing method in accordance with the present invention may be provided with a function of carrying out other operations as well as the operation corresponding to Formula (10). Specifically, the present invention further provides a fourth radiation image processing apparatus in a radiation image recording and reproducing system for scanning a stimulable phosphor carrying a radiation image stored thereon by exposure to radiation by stimulating rays which cause said stimulable phosphor to emit light in proportion to the stored radiation energy, obtaining an original image signal by photoelectrically detecting the light emitted by each scanning point of said stimulating rays on said stimulable phosphor, processing the original image signal by an operation device, and reproducing said radiation image as a visible image on a recording medium by use of the processed image signal, wherein the improvement comprises constituting said operation device for: i) obtaining a single unsharp mask signal Sus.k by averaging original image signals within a predetermined range surrounding each scanning point or image signals obtained by carrying out intermediate processing of the original image signals, or obtaining a plurality of unsharp mask signals Sus.k, where k=1, 2, . . . , n wherein n denotes an integer representing the number of said unsharp mask signals, by changing said predetermined range, ii) denoting a single coefficient corresponding to single said unsharp mask signal or a plurality of coefficients corresponding to a plurality of said unsharp mask signals by .beta.k where k=1, 2, . . . , n, iii) adjusting at least a single coefficient .beta.l, where l denotes an integer within the range of 1 to n, among said coefficients .beta.k, where k=1, 2, . . . , n, to be a function shifting from .beta.l<0 to .beta.l>0 as a dose of said radiation irradiated to each point on said stimulable phosphor increases, and iv) carrying out an operation represented by a formula ##EQU13## where Sb1 and Sb2 each denote said original image signal or an image signal obtained by carrying out intermediate processing of said original image signal, and S' denotes an image signal obtained by the operation processing, by use of said coefficient .beta.l. The term "shifting from .beta.<0 to .beta.>0" as used herein for the third radiation image processing method and apparatus and the term "shifting from .beta.l<0 to .beta.l>0" as used herein for the fourth radiation image processing method and apparatus embrace the case where, as shown in FIG. 5C by way of example, a region of .beta.=0 or .beta.l=0 is present at an intermediate region. In the course of the operations expressed as Formulas (8) to (11), a signal (Sorg=k.multidot.E where k is a constant) proportional to the optical amount E of the light emitted by the stimulable phosphor may be used as the original image signal Sorg, and the operations may be carried out by use of Sus, .beta., Sus.k, .beta.k, Sb1 and Sb2 corresponding to said signal. Alternatively, from the viewpoint of signal amount compression or the like, the original image signal (Sorg=k'.multidot.log E where k' is a constant) proportional to a logarithmic value of the optical amount E of the light emitted by the stimulable phosphor may be used, and the operations may be carried out by use of Sus, .beta., Sus.k, .beta.k, Sb1 and Sb2 corresponding to said original image signal. In general, both a region of a high radiation dose and a region of a low radiation dose are present in a single radiation image in accordance with the distribution of various tissues constituting the object, a difference in thickness of the object, and the like. In the case where the image signals obtained by the image read-out are uniformly subjected to an operation for emphasizing the contrast, the sharpness and the like by use of the method disclosed in, for example, U.S. Pat. No. 4,315,318, grain noise is emphasized and the image becomes rough in the region of a low radiation dose including more grain noise even though the image quality is improved in the region of a high radiation dose originally including less grain noise. On the other hand, in the case where grain noise is positively reduced by use of the aforesaid second radiation image processing method in accordance with the present invention in order to restrict grain noise in the region of a low radiation dose, the sharpness and the contrast are deteriorated slightly in the region of a high radiation dose. With both of these methods, it is necessary for image processing to be carried out by ascertaining the balance among the image quality factors of the overall image. In the third radiation image processing method in accordance with the present invention, by considering that both the region of a high radiation dose and the region of a low radiation dose are present in a single radiation image, the coefficient .beta. is shifted from .beta.<0 to .beta.>0 as the radiation dose increases in the course of carrying out image processing represented by Formula (8). In this manner, grain noise can be reduced positively in the region of a low radiation dose including more grain noise in the radiation image, and the image quality factors such as the sharpness and the contrast can be improved positively in the region of a high radiation dose originally including less grain noise in the radiation image. Therefore, the image quality of a reproduced visible image can be improved markedly over the case where image processing is carried out uniformly for the overall image. The radiation dose in each region of the radiation image is approximately proportional to the light emitted by the stimulable phosphor when the stimulable phosphor is scanned by stimulating rays. Therefore, the radiation dose in each region of the radiation image can be detected by investigating the image signal obtained by photoelectrically detecting the emitted light. Also, in order to carry out image processing suitable for each region of the radiation image as mentioned above, the method as disclosed in U.S. Pat. No. 4,315,318 and the second radiation image processing method in accordance with the present invention may be combined with each other, and an operation may be carried out as represented by a formula where Sorg denotes the original image signal, Sus' and Sus" denote unsharp mask signals subjected to appropriate frequency response processing, .beta.' and .beta." (.beta.', .beta.">0) denote coefficients each having an appropriate function form as the function of the image signal (the function of the radiation dose), and S' denotes the image signal obtained by processing. However, with this method, both the operation of the second term .beta.'(Sorg-Sus') and the operation of the third term .beta."(Sorg-Sus") must at least be carried out for each scanning point on the radiation image. On the other hand, in the case where Formula (8) which is the most basic formula in the third radiation image processing method in accordance with the present invention is used, only a single term of .beta.(Sorg-Sus) may be calculated, and the operation car be completed in a time approximately half the operation time of Formula (12). Also, in the case where the apparatus is constituted to carry out the operation by hardware, the apparatus configuration is simplified markedly. As indicated by Formula (9), the fourth radiation image processing method in accordance with the present invention includes other operations as well as the operation represented by Formula (8). Formula (9) can be rewritten into the form of ##EQU14## When the first term Sb1 and Sb2 of the second term in Formula (13) are expressed as the original image signal Sorg, the combination Sb1+.beta.l(Sb2-Sus.l) of the first term with the second term becomes identical with Formula (8). Specifically, for a single radiation image, various kinds of image processing such as various kinds of noise reducing processing and window processing for taking up only the necessary spatial frequency components are often carried out as well as the processing in accordance with the present invention. Therefore, in the course of using the fourth radiation image processing method in accordance with the present invention, the original image signal Sorg obtained by reading out the radiation image need not necessarily be used directly, and an image signal obtained by subjecting the original image signal Sorg to intermediate processing, for example, of the type as mentioned above may be used. Also, in this case, nearly the same effects as Formula (8) can be obtained, and the operation can be combined efficiently with other operation processing. The image signal obtained by intermediate processing may also be the image signal generated in the course of carrying out the fourth radiation image processing method in accordance with the present invention. The third and fourth terms of Formula (13) will now be described below. Spatial frequency components of grainy noise and spatial frequency components carrying the image quality factors such as the sharpness and the contrast are present over wide ranges. Therefore, in the case where image processing is to be carried out more finely than image processing using the combination of the first term with the second term of Formula (13), the same operation as the second term is carried out in the third term or the fourth term by changing the spatial frequency region from the frequency region in the second term. Also, a coefficient .beta.m where m.noteq.l may be adjusted so that .beta.m>0 in the third and fourth terms and, for example, an operation for emphasizing specific spatial frequency components as proposed by the applicant in U.S. Pat. No. 4,315,318 may be carried out over the overall image in order to compensate the operation of the first and second terms. Also, a coefficient .beta.n where n.noteq.l may be adjusted so that .beta.n<0, and the operation for reducing grain noise in accordance with the second radiation image processing method of the present invention may be carried out over the overall image. As mentioned above, the operation time of the operation of the first and second terms of Formula (13), i.e. the operation corresponding to the operation represented by Formula (8), is markedly shortened as compared with the operation represented by Formula (12) or the like. Therefore, finer image processing can be achieved by carrying out the operation of the third and fourth terms as mentioned above by the utilization of the margin time. With the third and fourth radiation image processing apparatuses in accordance with the present invention wherein the operation device is provided with the function of the aforesaid operation processing, the software execution time can be shortened in the case where the function is achieved by the software, or the apparatus configuration can be simplified in the case where the function is achieved by the hardware. As mentioned above, with the third radiation image processing method in accordance with the present invention, after the original image signal is obtained by scanning the stimulable phosphor carrying a radiation image stored thereon by exposure to radiation by stimulating rays which cause the stimulable phosphor to emit light in proportion to the stored radiation energy, and photoelectrically detecting the light emitted by each scanning point of the stimulating rays on the stimulable phosphor, the operation represented by the formula is carried out by using the coefficient .beta. shifting from .beta.<0 to .beta.>0 as the dose of radiation irradiated to each point on the stimulable phosphor increases. Therefore, grain noise can be reduced in the region of a low radiation dose including more grain noise in the radiation image, and the image quality factors such as the sharpness and the contrast can be improved positively in the region of a high radiation dose originally including less grain noise in the radiation image. As a result, the image quality of the overall image can be improved. Also, as the operation represented by Formula (8) can be completed in a short operation time, finer image processing can be carried out as represented by Formula (9) ##EQU15## by the utilization of the margin time in accordance with the fourth radiation image processing method of the present invention. Furthermore, the apparatuses for carrying out the third and fourth radiation image processing methods in accordance with the present invention are not so complicated and can achieve the operation in a time within a substantially allowable range. The present invention still further provides a first X-ray image processing method which comprises the steps of: in the course of scanning an original photograph carrying an X-ray image recorded thereon, reading out an original image density at each scanning point on said original photograph, and reproducing said X-ray image as a visible image on a copy photograph or the like, i) obtaining a single unsharp mask by averaging original image densities within a predetermined range surrounding each scanning point or image densities obtained by carrying out intermediate processing of signals representing the original image densities, or obtaining a plurality of unsharp masks by changing said predetermined range, and denoting densities of said unsharp masks by Dus.k, where k=1, 2, . . . , n wherein n denotes an integer representing the number of said unsharp masks, ii) denoting a single attenuation coefficient corresponding to single said unsharp mask or a plurality of attenuation coefficients corresponding to a plurality of said unsharp masks by .beta.k where k=1, 2, . . . , n, iii) adjusting at least a single attenuation coefficient .beta.l, where l denotes an integer within the range of 1 to n, among said attenuation coefficients .beta.k, where k=1, 2, . . . , n, to be a constant within the range of iv) carrying out an operation represented by a formula ##EQU16## where Db1 and Db2 each denote said original image density or an image density obtained by carrying out intermediate processing of a signal representing said original image density, and D' denotes an image density obtained by the operation processing, by use of said attenuation coefficient .beta.l, and v) attenuating spatial frequency components above the spatial frequency component which the density Dus.l of the unsharp mask corresponding to said attenuation coefficient .beta.l has. The first X-ray image processing method in accordance with the present invention is carried out by a first X-ray image processing apparatus for processing a signal representing an original image density, which has been read out at each scanning point on an original photograph carrying an X-ray image recorded thereon by scanning said original photograph, by an operation device, and reproducing said X-ray image as a visible image on a copy photograph or the like by use of the signal representing the processed image density, wherein the improvement comprises constituting said operation device for: i) obtaining a single unsharp mask by averaging original image densities within a predetermined range surrounding each scanning point or image densities obtained by carrying out intermediate processing of signals representing the original image densities, or obtaining a plurality of unsharp masks by changing said predetermined range, and denoting densities of said unsharp masks by Dus.k, where k=1, 2, . . . , n wherein n denotes an integer representing the number of said unsharp masks, ii) denoting a single attenuation coefficient corresponding to single said unsharp mask or a plurality of attenuation coefficients corresponding to a plurality of said unsharp masks by .beta.k where k=1, 2, . . . , n, iii) adjusting at least a single attenuation coefficient .beta.l, where l denotes an integer within the range of 1 to n, among said attenuation coefficients .beta.k, where k=1, 2, . . . , n, to be a constant within the range of and iv) carrying out an operation represented by a formula ##EQU17## where Db1 and Db2 each denote said original image density or an image density obtained by carrying out intermediate processing of a signal representing said original image density, and D' denotes an image density obtained by the operation processing, by use of said attenuation coefficient .beta.l. With the first X-ray image processing method in accordance with the present invention, at least a single attenuation coefficient .beta.l among the attenuation coefficients .beta.k, where k=1, 2, . . . , n, is adjusted to be a constant within the range of and the operation represented by the formula ##EQU18## is carried out. Formula (14) can be rewritten into the form of ##EQU19## As for the second term .beta.l(Db2-Dus.l) of Formula (15), the unsharp mask density Dus.l is subtracted from Db2 which is, by way of example, the original image density as represented by Db2-Dus.l in the parentheses of the second term, whereby the low spatial frequency component which the unsharp mask density Dus.l has is subtracted from Db2. Also, Db2-Dus.l is multiplied by the attenuation coefficient .beta.l satisfying the condition of 0<.beta.l wherein .beta.l.noteq.1 as represented by .beta.l(Db2-Dus.l), and .beta.l(Db2-Dus.l) is subtracted from Db1 which is, by way of example, the original image density. In this manner, the high spatial frequency component which Db2-Dus.l has can be attenuated from the density Db1. In the case where the high spatial frequency component is made to coincide with grainy noise of the image and the attenuation coefficient .beta.l is adjusted to be an appropriate value satisfying the condition of 0<.beta.l wherein .beta.l.noteq.1, grainy noise of the image can be attenuated, and deterioration of other image quality factors such as sharpness can be minimized. Also, the first X-ray image processing apparatus for carrying out the first X-ray image processing method is not complicated as compared with the conventional X-ray image processing apparatus, and can achieve the operation in a time within a substantially allowable range. Both the image densities Db1 and Db2 may be the original image density, or one or both of the image densities Db1 and Db2 may be the image density obtained by carrying out intermediate image processing of the signal representing the original image density. The third and fourth terms of Formula (15) will now be described below. Grainy noise has a wide range of spatial frequency components. Therefore, in the case where grainy noise cannot be substantially restricted by the combination of the first term with the second term of Formula (15), the same operation as the second term is carried out in the third term or the fourth term by changing the spatial frequency region from the frequency region in the second term. Also, an attenuation coefficient .beta.m where m.noteq.l may be adjusted so that .beta.m<0 in the third and fourth term and, for example, an operation for emphasizing specific spatial frequency components as proposed by the applicant in U.S. Pat. No. 4,317,179 may be used in combination. Basic differences between the first X-ray image processing method in accordance with the present invention and the method as proposed by the applicant in, for example, U.S. Pat. No. 4,317,179 will now be described below. In the proposed method, an operation represented by a formula where Dus denotes an unsharp mask density, Dorg denotes a density of an original photograph, .beta. denotes an emphasis coefficient, and D' denotes a density reproduced on a copy photograph or the like, is carried out for emphasizing specific spatial frequency components. The simplest formula of the first X-ray image processing method in accordance with the present invention comprises only the first term and the second term of Formula (15), i.e. is expressed as As mentioned above, Formula (17) indicates that the spatial frequency components which grainy noise has are attenuated positively. However, it was found by the inventors of U.S. Pat. No. 4,317,179 that the spatial frequency that grain noise has overlaps the spatial frequency affecting other image quality factors such as sharpness. Therefore, it is considered that in the case where the spatial frequency that grain noise has is attenuated positively, other image quality factors will deteriorate to an unrestorable extent. Accordingly, the image quality has heretofore been improved by emphasizing the spatial frequency components having a comparatively high degree of contribution to other image quality factors such as sharpness, instead of the graininess, without positively attenuating the spatial frequency components which grain noise has. The inventors of the present invention studied the properties of grainy noise and found that grainy noise can be rendered imperceptible while deterioration of other image quality factors such as sharpness is being minimized by accurately selecting the spatial frequency which is to be attenuated and the extent of attenuation of said spatial frequency, and positively restricting the spatial frequency components which grainy noise has. The optimal value of the attenuation coefficient .beta.l employed for carrying out the attenuation is generally present in the range of 0<.beta.l<1, depending on the kind of the X-ray image or the like. As mentioned above, with the first X-ray image processing method in accordance with the present invention, after the original image density at each scanning point is read out by scanning the original photograph carrying an X-ray image recorded thereon, at least a single attenuation coefficient .beta.l among the attenuation coefficients .beta.k where k=1, 2, . . . , n is adjusted to be a constant within the range of and the operation represented by the formula ##EQU20## is carried out. Therefore, the spatial frequency components above the spatial frequency component which the unsharp mask density Dus.l has can be attenuated, grainy noise of the X-ray image can be attenuated efficiently, and deterioration of other image quality factors can be minimized. Also, the apparatus for carrying out the first X-ray image processing method in accordance with the present invention is not so complicated and can achieve the operation in a time within a substantially allowable range. The present invention also provides a second X-ray image processing method which comprises the steps of: in the course of scanning an original photograph carrying an X-ray image recorded thereon, reading out an original image density at each scanning point on said original photograph, and reproducing said X-ray image as a visible image on a copy photograph or the like, i) obtaining a single unsharp mask by averaging original image densities within a predetermined range surrounding each scanning point or image densities obtained by carrying out intermediate processing of signals representing the original image densities, or obtaining a plurality of unsharp masks by changing said predetermined range, and denoting densities of said unsharp masks by Dus.k, where k=1, 2, . . . , n wherein n denotes an integer representing the number of said unsharp masks, ii) denoting a single attenuation coefficient corresponding to single said unsharp mask or a plurality of attenuation coefficients corresponding to a plurality of said unsharp masks by .beta.k where k=1, 2, . . . , n, iii) adjusting at least a single attenuation coefficient .beta.l, where l denotes an integer within the range of 1 to n, among said attenuation coefficients .beta.k, where k=1, 2, . . . , n, to be a variable which is always within the range of and which varies in each said X-ray image, iv) carrying out an operation represented by a formula ##EQU21## where Db1 and Db2 each denote said original image density or an image density obtained by carrying out intermediate processing of a signal representing said original image density, and D' denotes an image density obtained by the operation processing, by use of said attenuation coefficient .beta.l, and v) attenuating spatial frequency components above the spatial frequency component which the density Dus.l of the unsharp mask corresponding to said attenuation coefficient .beta.l has. The second X-ray image processing method in accordance with the present invention is carried out by a second X-ray image processing apparatus for processing a signal representing an original image density, which has been read out at each scanning point on an original photograph carrying an X-ray image recorded thereon by scanning said original photograph, by an operation device, and reproducing said X-ray image as a visible image on a copy photograph or the like by use of the signal representing the processed image density, wherein the improvement comprises constituting said operation device for: i) obtaining a single unsharp mask by averaging original image densities within a predetermined range surrounding each scanning point or image densities obtained by carrying out intermediate processing of signals representing the original image densities, or obtaining a plurality of unsharp masks by changing said predetermined range, and denoting densities of said unsharp masks by Dus.k, where k=1, 2, . . . , n wherein n denotes an integer representing the number of said unsharp masks, ii) denoting a single attenuation coefficient corresponding to single said unsharp mask or a plurality of attenuation coefficients corresponding to a plurality of said unsharp masks by .beta.k where k=1, 2, . . . , n, iii) adjusting at least a single attenuation coefficient .beta.l, where l denotes an integer within the range of 1 to n, among said attenuation coefficients .beta.k, where k=1, 2, . . . , n, to be a variable which is always within the range of and which varies in each said X-ray image, and iv) carrying out an operation represented by a formula ##EQU22## where Db1 and Db2 each denote said original image density or an image density obtained by carrying out intermediate processing of a signal representing said original image density, and D' denotes an image density obtained by the operation processing, by use of said attenuation coefficient .beta.l. With the second X-ray image processing method in accordance with the present invention, at least a single attenuation coefficient .beta.l among the attenuation coefficients .beta.k, where k=1, 2, . . . , n, is adjusted to be a variable always having a value within the range of and the operation represented by the formula ##EQU23## is carried out. Formula (18) can be rewritten into the form of ##EQU24## As for the second term .beta.l(Db2-Dus.l) of Formula (19), the unsharp mask density Dus.l is subtracted from Db2 which is, by way of example, the original image density as represented by Db2-Dus.l in the parentheses of the second term, whereby the low spatial frequency component which the unsharp mask density Dus.l has is subtracted from Db2. Also, Db2-Dus.l is multiplied by the attenuation coefficient .beta.l satisfying the condition of 0.ltoreq..beta.l as represented by .beta.l(Db2-Dus.l), and .beta.l(Db2-Dus.l) is subtracted from Db1 which is, by way of example, the original image density. In this manner, in the region of .beta.l.noteq.0 (where .beta.l is the variable varying in the X-ray image) inside of the X-ray image, the high spatial frequency component which Db2-Dus.l has can be attenuated from the density Db1. In the case where the high spatial frequency component is made to coincide with grainy noise of the image and the attenuation coefficient .beta.l is adjusted to be an appropriate value as the variable varying within the range of 0.ltoreq..beta.l, grainy noise of the image can be attenuated, and deterioration of other image quality factors such as sharpness can be minimized in accordance with the condition of each region inside of a single image. Also, the second X-ray image processing apparatus for carrying out the second X-ray image processing method is not complicated as compared with the conventional X-ray image processing apparatus, and can achieve the operation in a time within a substantially allowable range. Both the image densities Db1 and Db2 may be the original image density, or one or both of the image densities Db1 and Db2 may be the image density obtained by carrying out intermediate image processing of the signal representing the original image density. The third and fourth terms of Formula (19) will now be described below. Grainy noise has a wide range of spatial frequency components. Therefore, in the case where grainy noise cannot be substantially restricted by the combination of the first term with the second term of Formula (19) or finer image processing is to be carried out by changing the spatial frequency region for each region inside of a single image area, the same operation as the second term is carried out in the third term or the fourth term by changing the spatial frequency region from the frequency region in the second term. Also, an attenuation coefficient .beta.m where m.noteq.l may be adjusted so that .beta.m<0 in the third and fourth term and, for example, an operation for emphasizing specific spatial frequency components as proposed by the applicant in U.S. Pat. No. 4,317,179 may be used in combination. The simplest formula of the second X-ray image processing method in accordance with the present invention comprises only the first term and the second term of Formula (19), i.e. is expressed as As mentioned above, Formula (20) indicates that the spatial frequency components which grainy noise has are attenuated positively. In the case where the attenuation coefficient .beta.l for carrying out the attenuation is varied within the range of 0.ltoreq..beta.<1, it can optimize each region inside of the image for almost every image. As for the attenuation coefficient .beta.l, various function forms may be selected in accordance with the purpose of image processing or the like. For example, the attenuation coefficient .beta.l may be adjusted to be a function of the image signals such that a portion of a low image density in the X-ray image where grainy noise is comparatively perceptible is blurred by increasing the extent of the attenuation, and the extent of the attenuation is decreased for a portion of a high image density where grainy noise is comparatively imperceptible to make the detailed structure sharper. Alternatively, the attenuation coefficient .beta.l may be varied in accordance with the object portion inside of a single image such as a bone portion, a lung portion or a heart portion in the X-ray image of the chest of the human body so that image processing is carried out to be suitable for each object portion. As mentioned above, with the second X-ray image processing method in accordance with the present invention, after the original image density at each scanning point is read out by scanning the original photograph carrying an X-ray image recorded thereon, at least a single attenuation coefficient .beta.l among the attenuation coefficients .beta.k where k=1, 2, . . . , n is adjusted to be a variable which is always within the range of and which varies in each X-ray image, and the operation represented by the formula ##EQU25## is carried out. Therefore, the spatial frequency components above the spatial frequency component which the unsharp mask density Dus.l has can be attenuated. Also, grainy noise of the X-ray image can be attenuated efficiently, and deterioration of other image quality factors can be minimized in accordance with each region inside of the X-ray image. Moreover, the apparatus for carrying out the second X-ray image processing method in accordance with the present invention is not so complicated and can achieve the operation in a time within a substantially allowable range. The present invention further provides a third X-ray image processing method which comprises the steps of: in the course of scanning an original photograph carrying an X-ray image recorded thereon and obtained by exposing a photographic film to X-rays, reading out an original image density at each scanning point on said original photograph, and reproducing said X-ray image as a visible image on a copy photograph or the like, i) obtaining an unsharp mask by averaging original image densities within a predetermined range surrounding each scanning point, and denoting a density of said unsharp mask by Dus, ii) denoting a coefficient corresponding to said unsharp mask by .beta., iii) adjusting said coefficient .beta. to be a function shifting from .beta.<0 to .beta.>0 as a dose of said X-rays irradiated to each point on said photographic film increases, iv) carrying out an operation represented by a formula where Dorg denotes said original image density, and D' denotes an image density obtained by the operation processing, by use of said coefficient .beta., and v) attenuating spatial frequency components above the spatial frequency component which said unsharp mask density Dus has in a region of a low X-ray dose inside of single said X-ray image, and emphasizing the spatial frequency components above the spatial frequency component which said unsharp mask density Dus has in a region of a high X-ray dose inside of single said X-ray image. Other operations as well as the operation corresponding to Formula (21) may also be contained in the third X-ray image processing method in accordance with the present invention. Specifically, the present invention also provides a fourth X-ray image processing method which comprises the steps of: in the course of scanning an original photograph carrying an X-ray image recorded thereon and obtained by exposing a photographic film to X-rays, reading out an original image density at each scanning point on said original photograph, and reproducing said X-ray image as a visible image on a copy photograph or the like, i) obtaining a single unsharp mask by averaging original image densities within a predetermined range surrounding each scanning point or image densities obtained by carrying out intermediate processing of signals representing the original image densities, or obtaining a plurality of unsharp masks by changing said predetermined range, and denoting densities of said unsharp masks by Dus.k, where k=1, 2, . . . , n wherein n denotes an integer representing the number of said unsharp masks, ii) denoting a single coefficient corresponding to single said unsharp mask or a plurality of coefficients corresponding to a plurality of said unsharp masks by .beta.k where k=1, 2, . . . , n, iii) adjusting at least a single coefficient .beta.l, where l denotes an integer within the range of 1 to n, among said coefficients .beta.k, where k=1, 2, . . . , n, to be a function shifting from .beta.l<0 to .beta.l>0 as a dose of said X-rays irradiated to each point on said photographic film increases, iv) carrying out an operation represented by a formula ##EQU26## where Db1 and Db2 each denote said original image density or an image density obtained by carrying out intermediate processing of a signal representing said original image density, and D' denotes an image density obtained by the operation processing, by use of said coefficient .beta.l, and v) attenuating spatial frequency components above the spatial frequency component which the unsharp mask density Dus.l corresponding to said coefficient .beta.l has in a region of a low X-ray dose inside of single said X-ray image, and emphasizing the spatial frequency components above the spatial frequency component which the unsharp mask density Dus.l corresponding to said coefficient .beta.l has in a region of a high X-ray dose inside of single said X-ray image. The third X-ray image processing method in accordance with the present invention is carried out by a third X-ray image processing apparatus for processing a signal representing an original image density, which has been read out at each scanning point on an original photograph carrying an X-ray image recorded thereon obtained by exposure of a photographic film to X-rays by scanning said original photograph, by an operation device, and reproducing said X-ray image as a visible image on a copy photograph or the like by use of the signal representing the processed image density, wherein the improvement comprises constituting said operation device for: i) obtaining an unsharp mask by averaging original image densities within a predetermined range surrounding each scanning point, and denoting a density of said unsharp mask by Dus, ii) denoting a coefficient corresponding to said unsharp mask by .beta., iii) adjusting said coefficient .beta. to be a function shifting from .beta.<0 to .beta.>0 as a dose of said X-rays irradiated to each point on said photographic film increases, and iv) carrying out an operation represented by a formula where Dorg denotes said original image density, and D' denotes an image density obtained by the operation processing, by use of said coefficient .beta.. The third X-ray image processing apparatus for carrying out the third X-ray image processing method in accordance with the present invention may be provided with a function of carrying out other operations as well as the operation corresponding to Formula (23). Specifically, the present invention further provides a fourth X-ray image processing apparatus for processing a signal representing an original image density, which has been read out at each scanning point on an original photograph carrying an X-ray image recorded thereon obtained by exposure of a photographic film to X-rays by scanning said original photograph, by an operation device, and reproducing said X-ray image as a visible image on a copy photograph or the like by use of the signal representing the processed image density, wherein the improvement comprises constituting said operation device for: i) obtaining a single unsharp mask by averaging original image densities within a predetermined range surrounding each scanning point or image densities obtained by carrying out intermediate processing of signals representing the original image densities, or obtaining a plurality of unsharp masks by changing said predetermined range, and denoting densities of said unsharp masks by Dus.k, where k=1, 2, . . . , n wherein n denotes an integer representing the number of said unsharp masks, ii) denoting a single coefficient corresponding to single said unsharp mask or a plurality of coefficients corresponding to a plurality of said unsharp masks by .beta.k where k=1, 2, . . . , n, iii) adjusting at least a single coefficient .beta.l, where l denotes an integer within the range of 1 to n, among said coefficients .beta.k, where k=1, 2, . . . , n, to be a function shifting from .beta.l<0 to .beta.l>0 as a dose of said X-rays irradiated to each point on said photographic film increases, and iv) carrying out an operation represented by a formula ##EQU27## where Db1 and Db2 each denote said original image density or an image density obtained by carrying out intermediate processing of a signal representing said original image density, and D' denotes an image density obtained by the operation processing, by use of said coefficient .beta.l. The term "shifting from .beta.<0 to .beta.>0" as used herein for the third X-ray image processing method and apparatus and the term "shifting from .beta.l<0 to .beta.l>0" as used herein for the fourth X-ray image processing method and apparatus embrace the case where, as shown in FIG. 11C by way of example, a region of .beta.=0 or .beta.l=0 is present at an intermediate region. In the course of the operations expressed as Formulas (21) to (24), a signal (Dorg=k.multidot.E where k is a constant) proportional to the optical amount E of the light passing through the original photograph or the light reflected by the original photograph may be used as the signal representing the original image density Dorg, and the operations may be carried out by use of Dus, .beta., Dus.k, .beta.k, Db1 and Db2 corresponding to said signal. Alternatively, from the viewpoint of signal amount compression or the like, the signal representing the original image density Dorg (Dorg=k'.multidot.log E where k' is a constant) proportional to a logarithmic value of the aforesaid optical amount E may be used, and the operations may be carried out by use of Dus, .beta., Dus.k, .beta.k, Db1 and Db2 corresponding to said signal. In general, both a region of a high X-ray dose and a region of a low X-ray dose are present in a single X-ray image in accordance with the distribution of various tissues constituting the object, a difference in thickness of the object, and the like. In the case where the signals representing the image densities obtained by the X-ray image read-out are uniformly subjected to an operation for emphasizing the contrast, the sharpness and the like by use of the method disclosed in, for example, U.S. Pat. No. 4,317,179, grain noise is emphasized and the image becomes rough in the region of a low X-ray dose including more grain noise even though the image quality is improved in the region of a high X-ray dose originally including less grain noise. On the other hand, in the case where grain noise is positively reduced by use of the aforesaid second X-ray image processing method in accordance with the present invention in order to restrict grain noise in the region of a low X-ray dose, the sharpness and the contrast are deteriorated slightly in the region of a high X-ray dose. With both of these methods, it is necessary for image processing to be carried out by ascertaining the balance among the image quality factors of the overall image. In the third X-ray image processing method in accordance with the present invention, by considering that both the region of a high X-ray dose and the region of a low X-ray dose are present in a single X-ray image, the coefficient .beta. is shifted from .beta.<0 to .beta.>0 as the X-ray dose increases in the course of carrying out image processing represented by Formula (21). In this manner, grain noise can be reduced positively in the region of a low X-ray dose including more grain noise in the X-ray image, and the image quality factors such as the sharpness and the contrast can be improved positively in the region of a high X-ray dose originally including less grain noise in the X-ray image. Therefore, the image quality of a reproduced visible image can be improved markedly over the case where image processing is carried out uniformly for the overall image. The X-ray dose in each region of the X-ray image can be detected by investigating the signal representing the original image density obtained by scanning and reading out the original photograph. Also, in order to carry out image processing suitable for each region of the X-ray image as mentioned above, the method as disclosed in U.S. Pat. No. 4,317,179 and the second X-ray image processing method in accordance with the present invention may be combined with each other, and an operation may be carried out as represented by a formula where Dorg denotes the original image density, Dus' and Dus" denote densities of two unsharp masks subjected to appropriate frequency response processing, .beta.' and .beta." (.beta.', .beta.">0) denote coefficients each having an appropriate function form as the function of the original image density (the function of the X-ray dose), and D' denotes the image density obtained by processing. However, with this method, both the operation of the second term .beta.'(Dorg-Dus') and the operation of the third term .beta."(Dorg-Dus") must at least be carried out for each scanning point on the X-ray image. On the other hand, in the case where Formula (21) which is the most basic formula in the third X-ray image processing method in accordance with the present invention is used, only a single term of .beta.(Dorg-Dus) may be calculated, and the operation can be completed in a time approximately half the operation time of Formula (25). Also, in the case where the apparatus is constituted to carry out the operation by hardware, the apparatus configuration is simplified markedly. As indicated by Formula (22), the fourth X-ray image processing method in accordance with the present invention includes other operations as well as the operation represented by Formula (21). Formula (22) can be rewritten into the form of ##EQU28## When the first term Db1 and Db2 of the second term in Formula (26) are expressed as the original image density Dorg, the combination Db1+.beta.l(Db2-Dus.l) of the first term with the second term becomes identical with Formula (21). Specifically, for a single X-ray image, various kinds of image processing such as various kinds of noise reducing processing and window processing for taking up only the necessary spatial frequency components are often carried out as well as the processing in accordance with the present invention. Therefore, in the course of using the fourth X-ray image processing method in accordance with the present invention, the original image density Dorg obtained by reading out the X-ray image need not necessarily be used directly, and an image density obtained by subjecting the signal representing the original image density Dorg to intermediate processing, for example, of the type as mentioned above may be used. Also, in this case, nearly the same effects as Formula (21) can be obtained, and the operation can be combined efficiently with other operation processing. The image density obtained by intermediate processing may also be the image density generated i |