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Home | Alpha Telephone | Domain Names | Web Hosting | Get Traffic | xrEvidence | xrSoccer United States Patent
Apparatus and methods for scatter reduction in radiation imaging The apparatus reduces scatter in radiation imaging and may be used with a detector of ionizing radiation that is partly unscattered and partly Compton scattered. The detector produces an energy signal representing values of energy of the radiation and produces coordinate position information for the radiation. Data storage means for holding numerical values and means for displaying an image based on the numerical values in the data storage means are also used with the inventive apparatus. The scatter reduction apparatus includes circuitry responsive to the energy signal from the detector for producing first and second signals which indicate whether each value of energy represented by the energy signal at a given time is in a first energy range or in a second energy range less than half as wide as the first energy range and having at least some energies in common with the first energy range. Further included is circuitry responsive to the first and second signals and the coordinate position information for generating numerical values for each coordinate position and storing them in the data storage means, the numerical values being a function of the difference of the number of occurrences of radiation in the first energy range at each coordinate position less a second number proportional to the number of occurrences of radiation in the second energy range at that coordinate position. Other apparatus and methods are also described.
Primary Examiner: Smith; Jerry Assistant Examiner: Meyer; Charles B. Attorney, Agent or Firm: CROSS REFERENCE TO RELATED APPLICATION The present application is a continuation-in-part of coassigned application Ser. No. 604,989 "Radiation Imaging Apparatus and Methods" filed Apr. 27, 1984, now U.S. Pat. No. 4,755,680. The U.S. Pat. No. 4,755,680 is hereby incorporated herein by reference. What is claimed is: 1. Apparatus for use in reducing scatter in radiation imaging for use with a detector of ionizing radiation, where the ionizing radiation is partly unscattered and partly Compton scattered, the detector producing an energy signal representing values of energy of the ionizing radiation and producing coordinate position information for the ionizing radiation, and where the apparatus is for use with both data storage means for holding numerical values and with means for displaying an image based on the numerical values in the data storage means, the apparatus comprising: means responsive to the energy signal from the detector for producing first and second signals which indicate whether each value of energy represented by the energy signal at a given time is in a first energy range or in a second energy range less than half as wide as the first energy range and having at least some energies in common with the first energy range; and means responsive to the first and second signals and the coordinate position information for generating numerical values for each coordinate position and storing them in the data storage means, the numerical values being a function of the difference of the number of occurrences of ionizing radiation in the first energy range at each coordinate position less a second number proportional to the number of occurrences of ionizing radiation in the second energy range at that coordinate position. 2. Apparatus as set forth in claim 1 wherein the second energy range is included in the first energy range. 3. Apparatus as set forth in claim 1 wherein the first energy range has a higher energy part and a lower energy part and the second energy range is the lower energy part. 4. Apparatus as set forth in claim 3 wherein the first signal indicates whether each value of energy represented by the energy signal at a given time is in the higher energy part of the first energy range and the second signal indicates whether each value of energy represented by the energy signal at a given time is in the lower energy part of the first energy range. 5. Apparatus as set forth in claim 3 wherein the first signal indicates whether each value of energy represented by the energy signal at a given time is anywhere in the first energy range and the second signal indicates whether each value of energy represented by the energy signal at a given time is in the lower energy part of the first energy range. 6. Apparatus as set forth in claim 3 wherein the first signal indicates whether each value of energy represented by the energy signal at a given time is anywhere in the first energy range and the second signal indicates whether each value of energy represented by the energy signal at a given time is in the higher energy part of the first energy range. 7. Apparatus as set forth in claim 1 wherein the second number is at least twice the number of occurrences of radiation in the second energy range at each coordinate position. 8. Apparatus as set forth in claim 1 wherein the second number is equal to twice the number of occurrences of radiation in the second energy range at each coordinate position. 9. Apparatus as set forth in claim 1 wherein the second number is related by a constant of proportionality to the number of occurrences of radiation in the second energy range at each coordinate position and the apparatus further comprises means for selectively establishing the constant for the generating means. 10. Apparatus as set forth in claim 9 wherein said means for producing the first and second signals includes means for varying the second energy range in width as a function of the constant selectively established. 11. Apparatus as set forth in claim 1 wherein said means for producing the first and second signals includes means for supplying an electrical reference signal representing a predetermined energy in the first energy range, and means for comparing the energy signal with the electrical reference signal to produce the first and second signals respectively depending on whether the energy signal represents an energy which is greater or less than the predetermined energy. 12. Apparatus as set forth in claim 1 wherein the second number is related by a proportionality constant to the number of occurrences of radiation in the second energy range at each coordinate position and said means for producing includes means for supplying an electrical reference signal for comparison with the energy signal, the electrical reference signal representing a predetermined energy in the first energy range to define an upper limit of the second energy range as a function of the proportionality constant. 13. Apparatus as set forth in claim 12 further comprising means connected to said means for producing for adjustably establishing the proportionality constant. 14. Apparatus as set forth in claim 1 wherein the second number is related by a proportionality constant to the number of occurrences of radiation in the second energy range at each coordinate position and said means for producing further includes means for supplying an electrical reference signal for comparison with the energy signal, the electrical reference signal representing a predetermined energy in the first energy range as a function of the proportionality constant, by premeasuring a scatter spectrum in the first energy range and predetermining the energy at which the total scatter in the first energy range below said energy is substantially equal to the scatter in the first energy range divided by the proportionality constant. 15. Apparatus as set forth in claim 1 wherein said means for generating includes means for recursively computing a first set of numbers in response to occurrences of the radiation at each coordinate position satisfying a first predetermined energy condition, recursively computing a second set of numbers in response to occurrences of the radiation at each coordinate position satisfying a second predetermined energy condition, for subsequently computing the numerical values as a function of the numbers recursively computed and storing the numerical values in the data storing means. 16. Apparatus as set forth in claim 1 wherein said means for generating includes means for accumulating respective counts of occurrences of the radiation at each coordinate position satisfying a first predetermined energy condition, recursively computing a set of numbers corresponding to each coordinate position in response to occurrences of the radiation at each coordinate position satisfying a second predetermined energy condition, for subsequently computing the numerical values as a function of the accumulated counts and of the numbers recursively computed and storing the numerical values in the data storing means. 17. Apparatus as set forth in claim 1 wherein said means for generating includes means for accumulating first and second counts of occurrences of the radiation at each coordinate position satisfying first and second predetermined energy conditions respectively, subsequently computing the numerical values as a function of the first and second counts and storing the numerical values in the data storing means. 18. Apparatus as set forth in claim 1 wherein said means for generating includes memory means for storing values corresponding to each coordinate position and means for incrementing or decrementing the value for a particular coordinate position depending on whether an occurrence of the radiation at that coordinate position satisfies a first or a second predetermined energy condition respectively, and for producing the numerical values as a function of the values resulting from the incrementing and decrementing over a period of time. 19. Apparatus as set forth in claim 1 wherein said means for generating the numerical values includes a memory and means connected to said producing means and said memory for accumulating a first set of numbers of occurrences of the radiation at each coordinate position when the energy signal represents a value of energy in the first energy range, and a second set of numbers of occurrences of the radiation at each coordinate position for which occurrences the energy signal represents a value of energy in the second energy range; said accumulating means including means for computing the numerical values as a function of the two numbers so accumulated for each particular coordinate position and storing the numerical values in the data storage means. 20. Apparatus as set forth in claim 1 wherein said means for generating the numerical values includes a memory and means connected to said producing means and said memory for accumulating a first set of numbers of occurrences of the radiation at each coordinate position for which occurrences the energy signal represents a value of energy in the second energy range, and a second set of numbers of occurrences of the radiation at each coordinate position for which occurrences the energy signal represents a value of energy in the first energy range and not the second energy range; said accumulating means including means for computing the numerical values as a function of the two numbers so accumulated for each particular coordinate position and storing the numerical values in the data storage means. 21. Apparatus as set forth in claim 1 wherein said means for generating the numerical values includes a memory and means connected to said producing means and said memory for accumulating a first set of numbers of occurrences of the radiation at each coordinate position when the energy signal represents a value of energy in the first energy range, a second set of numbers of occurrences of the radiation at each coordinate position for which occurrences the energy signal represents a value of energy in the first energy range and not the second energy range, said accumulating means including means for computing the numerical values as a function of the two numbers so accumulated for each particular coordinate position and storing the numerical values in the data storage means. 22. Apparatus as set forth in claim 1 wherein said means for generating includes memory means for storing first and second values corresponding to each coordinate position and means for incrementing or decrementing the first value for a particular coordinate position depending on whether an occurrence of the radiation at that coordinate position satisfies a first or a second predetermined energy condition respectively, and if the first value has reached a preset level then instead incrementing the second value in magnitude and producing the numerical values as a function of the second values resulting over a period of time. 23. Apparatus as set forth in claim 1 wherein said means for generating includes memory means for storing first and second values corresponding to each coordinate position and means for incrementing or decrementing the first value for a particular coordinate position depending on whether the energy signal for an occurrence of radiation at that coordinate position represents an energy that is respectively greater or less than a predetermined energy in the first energy range, and if the first value has reached a preset level then instead incrementing the second value in magnitude and producing the numerical values as a function of the second values resulting over a period of time. 24. Apparatus as set forth in claim 1 wherein said means for generating includes: memory means for storing values at respective addresses corresponding to coordinate positions of the radiation; means for replacing a value already stored at a particular address in the memory means with a latest value which is the sum of the already-stored value plus a first predetermined value having a first sign, when the energy signal represents a value of energy in the second energy range and the radiation occurs at a coordinate position corresponding to that particular address; means for comparing the latest value at the particular address with a preset value when radiation subsequently occurs at the coordinate position corresponding to said address and the energy signal represents a value of energy in a part of the first range outside the second range; means for supplying an unblank pulse and coordinate position information for the radiation when the comparing means indicates that the latest value at said address is not less than the preset value when the first sign is negative or when the comparing means indicates that the value at said address is not greater than the preset value when the first sign is positive, and otherwise replacing the latest value at said address with another value equal to the sum of the latest value and a second predetermined value having a second sign opposite to that of the first predetermined value; and means connected to said supplying means for accumulating counts of the occurrences of the unblank pulse corresponding to each coordinate position and storing a function of the counts as the numerical values. 25. Apparatus as set forth in claim 1 further comprising a memory having addresses and stored contents corresponding to particular coordinate positions of the radiation and wherein said means for generating the numerical values includes means responsive to the coordinate position information and to the first and second signals for decrementing the contents of the memory at a particular address when radiation having an energy value in the second energy range occurs at a coordinate position corresponding to that address, for incrementing the contents of the memory at that particular address when radiation having an energy value in the first range occurs at a coordinate position corresponding to that particular address, and when the memory contents exceed a preset value, supplying an unblank pulse and coordinate position information for the radiation; and further comprising means for accumulating a function of the number of occurrences of the unblank pulse corresponding to each particular coordinate position as the numerical values. 26. Apparatus as set forth in claim 25 wherein each instance of incrementing increases the contents at the particular address by a first amount and each instance of decrementing decreases the contents at the particular address by a second amount which is at least twice the first amount. 27. Apparatus as set forth in claim 1 further comprising a memory having addresses and stored contents corresponding to particular coordinate positions of the radiation and wherein said means for generating the numerical values includes means responsive to the coordinate position information and to the first and second signals for decrementing the contents of the memory at a particular address when radiation having an energy value in the second energy range occurs at a coordinate position corresponding to that address, for incrementing the contents of the memory at that particular address when radiation having an energy value in the first range and not the second range occurs at a coordinate position corresponding to that particular address, and when the memory contents exceed a preset value, supplying an unblank pulse and coordinate position information for the radiation; and further comprising means for accumulating a function of the number of occurrences of the unblank pulse corresponding to each particular coordinate position as the numerical values. 28. Apparatus as set forth in claim 27 wherein each instance of incrementing increases the contents at the particular address by a first amount and each instance of decrementing decreases the contents at the particular address by a second amount which is at least equal to the first amount. 29. Apparatus as set forth in claim 1 further comprising a memory having addresses and stored contents corresponding to particular coordinate positions of the radiation and wherein said means for generating the numerical values includes means responsive to the coordinate position information and to the first and second signals for incrementing the contents of the memory at a particular address when radiation having an energy value in the first energy range but not the second energy range occurs at a coordinate position corresponding to that address, for decrementing the contents of the memory at that particular address when radiation having an energy value in the first range occurs at a coordinate position corresponding to that particular address, and when the memory contents exceed a preset value, supplying an unblank pulse and coordinate position information for the radiation; and further comprising means for accumulating a function of the number of occurrences of the unblank pulse corresponding to each particular coordinate position as the numerical values. 30. Apparatus as set forth in claim 1 wherein said means for generating the numerical values includes a memory and means connected to said producing means and to said memory for accumulating a first set of numbers of occurrences of the radiation at each coordinate position satisfying a predetermined energy condition, and recursively computing a second set of numbers which represents a scatter-related fraction of each first number, said accumulating means including means for computing the numerical values as a function of the two numbers so accumulated for each particular coordinate position and storing the numerical values in the data storing means. 31. Apparatus as set forth in claim 30 wherein said scatter-related fraction is a scatter-free fraction. 32. Apparatus as set forth in claim 30 wherein said scatter-related fraction is a fraction which represents scatter itself. 33. Apparatus as set forth in claim 30 wherein said first set of numbers represents the number of occurrences of radiation in the first energy range for each coordinate position. 34. Apparatus as set forth in claim 30 wherein said first set of numbers represents the number of occurrences of radiation in the second energy range for each coordinate position. 35. Apparatus as set forth in claim 30 wherein said first set of numbers represents the number of occurrences of radiation in the first energy range and not the second energy range for each coordinate position. 36. Apparatus as set forth in claim 1 wherein said means for generating the numerical values includes a memory and means connected to said producing means and said memory for accumulating the first number of occurrences of the radiation at each coordinate position when the energy signal represents a value of energy in the first energy range, and recursively computing another number representing a fraction of the first number which represents that part of the first number of occurrences at each coordinate position which is the number of occurrences of the radiation with a value of energy in the second energy range at each same coordinate position, said accumulating means including means for computing the numerical values as a function of the two numbers so accumulated and computed for each coordinate position and storing the numerical values. 37. Apparatus as set forth in claim 1 wherein said means for generating the numerical values includes a memory and means connected to said producing means and to said memory for accumulating a number of occurrences of the radiation at each particular coordinate position when the energy signal represents a value of energy in the first energy range and not the second energy range, and a second number representing a fraction with respect to the accumulated number for occurrences of the radiation with a value of energy in the second energy range at each same coordinate position, said accumulating means including means for computing the numerical values as a function of the accumulated number and the fraction for each particular coordinate position and storing the numerical values in the data storage means. 38. Apparatus for use in reducing scatter in radiation imaging of ionizing radiation where the ionizing radiation has a photopeak in a first energy range and where the apparatus is for use with data storage means for holding numerical values and with means for displaying an image based on the numerical values in the data storage means, the apparatus comprising: means for detecting ionizing radiation that is partly unscattered and partly Compton scattered to produce an energy signal representing values of energy of the ionizing radiation in the first energy range and to produce coordinate position information for the ionizing radiation; and means responsive to the energy signal and the coordinate position information for generating numerical values for each coordinate position and storing them in the data storage means, the numerical values being a function of the difference of the number of occurrences of ionizing radiation in the first energy range at each coordinate position less a second number proportional to and at least two times the number of occurrences of ionizing radiation at that coordinate position in a second energy range. 39. Apparatus as set forth in claim 38 wherein the second energy range is included in the first energy range. 40. Apparatus as set forth in claim 38 wherein the first energy range has a higher energy part and a lower energy part and the second energy range is the lower energy part. 41. Apparatus as set forth in claim 38 wherein the second energy range is less than half as wide as the first energy range and has at least some energies in common with the first energy range. 42. Apparatus as set forth in claim 38 wherein the second number is related by a constant of proportionality to the number of occurrences of radiation in the second energy range at each coordinate position and the apparatus further comprises means for selectively establishing the constant for the generating means. 43. Apparatus as set forth in claim 42 wherein said means for generating includes means for varying the second energy range in width as a function of the constant selectively established. 44. Apparatus as set forth in claim 38 wherein said means for generating includes means for recursively computing a first set of numbers in response to occurrences of the radiation at each coordinate position satisfying a first predetermined energy condition, recursively computing a second set of numbers in response to occurrences of the radiation at each coordinate position satisfying a second predetermined energy condition, for subsequently computing the numerical values as a function of the numbers recursively computed and storing the numerical values in the data storing means. 45. Apparatus as set forth in claim 38 wherein said means for generating includes means for accumulating respective counts of occurrences of the radiation at each coordinate position satisfying a first predetermined energy condition, recursively computing a set of numbers corresponding to each coordinate position in response to occurrences of the radiation at each coordinate position satisfying a second predetermined energy condition, for subsequently computing the numerical values as a function of the accumulated counts and of the numbers recursively computed and storing the numerical values in the data storing means. 46. Apparatus as set forth in claim 38 wherein said means for generating includes means for accumulating first and second counts of occurrences of the radiation at each coordinate position satisfying first and second predetermined energy conditions respectively, subsequently computing the numerical values as a function of the first and second counts and storing the numerical values in the data storing means. 47. Apparatus as set forth in claim 38 wherein said means for generating includes memory means for storing values corresponding to each coordinate position and means for incrementing or decrementing the value for a particular coordinate position depending on whether an occurrence of the radiation at that coordinate position satisfies a first or a second predetermined energy condition respectively, and for producing the numerical values as a function of the values resulting from the incrementing and decrementing over a period of time. 48. Apparatus as set forth in claim 38 wherein said means for generating includes memory means for storing first and second values corresponding to each coordinate position and means for incrementing or decrementing the first value for a particular coordinate position depending on whether an occurrence of the radiation at that coordinate position satisfies a first or a second predetermined energy condition respectively, and if the first value has reached a preset level then instead incrementing the second value in magnitude, and producing the numerical values as a function of the second values resulting over a period of time. 49. Apparatus as set forth in claim 38 wherein said detecting means has an event processing cycle time and said means for generating includes memory means for storing first and second values corresponding to each coordinate position and means for incrementing or decrementing the first value for a particular coordinate position depending on whether the energy signal for an occurrence of radiation at that coordinate position represents an energy that is respectively greater or less than a predetermined energy in the first energy range, and if the first value has reached a preset level then instead incrementing the second value in magnitude, and producing the numerical values in real time within a cycle time interval less than the event processing cycle time of said detecting means and as a function of the second values resulting over a period of time. 50. Apparatus as set forth in claim 49 wherein said means for generating further includes means for supplying an electrical reference signal representing the predetermined energy in the first energy range, and means connected to the means for incrementing or decrementing for comparing the energy signal with the electrical reference signal. 51. Apparatus as set forth in claim 49 wherein the second number is related by a proportionality constant to the number of occurrences of radiation in the second energy range at each coordinate position and said means for generating further includes means for supplying an electrical reference signal representing the predetermined energy in the first energy range as a function of the proportionality constant. 52. Apparatus as set forth in claim 51 further comprising means connected to said means for generating for adjustably establishing the proportionality constant. 53. Apparatus as set forth in claim 49 wherein the second number is related by a proportionality constant to the number of occurrences of radiation in the second energy range at each coordinate position and said means for generating further includes means for supplying an electrical reference signal representing the predetermined energy in the first energy range as a function of the proportionality constant by premeasuring a scatter spectrum in the first energy range and predetermining the energy at which the total scatter in the first energy range below said energy is substantially equal to the scatter in the first energy range divided by the proportionality constant. 54. Apparatus for use in reducing scatter in ionizing radiation imaging comprising: means for detecting ionizing radiation that is partly unscattered and partly Compton scattered by producing an energy signal representing values of energy of the ionizing radiation and producing coordinate position information for the ionizing radiation; means responsive to the energy signal from the means for detecting ionizing radiation for producing first and second signals which indicate whether each value of energy represented by the energy signal at a given time is in a first energy range or in a second energy range which is less than half as wide as the first energy range and which has at least some energies in common with the first energy range; data storage means for holding numerical values; means responsive to the first and second signals and the coordinate position information for generating numerical values for each coordinate position and storing them in the data storage means, the numerical values being a function of the difference of the number of occurrences of ionizing radiation in the first energy range at each coordinate position less a second number proportional to the number of occurrences of ionizing radiation in the second energy range at that coordinate position; and means for displaying an image based on the numerical values in the data storage means. 55. Apparatus for use in reducing scatter in radiation imaging for use with a detector of ionizing radiation, where the ionizing radiation is partly unscattered and partly Compton scattered, the detector producing an energy signal representing values of energy of the ionizing radiation and producing coordinate position information for the ionizing radiation, and where the apparatus to be used with data storage means for holding numerical values corresponding to each coordinate position, with a circuit for incrementing each corresponding numerical value in response to an unblank signal and the coordinate position information, and with means for then displaying an image based on the numerical values in the data storage means, the apparatus comprising: memory means for storing tabular values corresponding to each coordinate position; means for incrementing or decrementing type tabular value for a particular coordinate position depending on whether the energy signal produced by an occurrence of the ionizing radiation at that coordinate position satisfies a first or a second predetermined energy condition respectively, and if the tabular value at that coordinate position has reached a preset value then instead producing an unblank signal to actuate the incrementing circuit, whereby that circuit increments the numerical value for that coordinate position in producing numerical values over a period of time for display purposes. 56. Apparatus as set forth in claim 55 wherein the memory means is initialized to the preset value for each of the coordinate positions. 57. Apparatus as set forth in claim 55 wherein the first and second predetermined energy conditions relate to a first energy range having a center energy value and a width less than one fourth of the center energy value and the first predetermined energy condition comprises the energy signal representing an energy in the first energy range above a predetermined energy value in that range, and the second predetermined energy condition comprises the energy signal representing an energy in the first energy range less than the predetermined energy value. 58. Apparatus as set forth in claim 57 wherein said means for incrementing or decrementing decrements by an amount which exceeds the amount by which it increments at each coordinate position. 59. Apparatus as set forth in claim 55 wherein said means for incrementing or decrementing includes: counter means having an input connected to said memory means and an output for communicating to said memory means, means for supplying an address to said memory means corresponding to the coordinate position information in response to an occurrence of the radiation to cause said memory means to supply said counter means with a tabular value from the address, means for comparing the tabular value with the preset value, and means for supplying the unblank signal when the comparing means indicates that the tabular value is equal to the preset value if the energy signal represents an energy in a first energy range above a predetermined energy in that range, and otherwise operating said counter means to decrement the tubular value when the energy is less than the predetermined energy or increment the tabular value when the energy is greater than the predetermined energy, and storing the result from said counter means in said memory means as an updated tabular value at the address for that coordinate position. 60. Apparatus as set forth in claim 59 wherein said means for supplying and operating includes microcode store means having addresses and an output for microprogram instructions which supply the unblank signal and operate said counter means, at least one of the microprogram instructions including a jump address to which operations should jump if a predetermined output of said comparing means occurs, and program counter means for incrementing addresses and connected to receive the jump address from the output of said microcode store means and for addressing said microcode store means with the jump address when the predetermined output of said comparing means occurs, and clock circuit means connected to said program counter means for actuating said program counter means and said microcode store means to execute cycles of operation in the incrementing or decrementing means at a rate in excess of one million cycles per second. 61. Apparatus as set forth in claim 55 wherein said means for incrementing or decrementing includes electronic circuit means for supplying the unblank signal and for executing operations on the tabular values in less than five millionths of a second in order to reduce the scatter as rapidly as occurrences of radiation are detected in the detector. 62. Apparatus as set forth in claim 55 wherein the means for decrementing and incrementing has a characteristic ratio of amount of each decrementing to amount of each incrementing and the apparatus further comprises means for supplying an electrical reference level representing a predetermined energy in a first energy range of the radiation by premeasuring a scatter spectrum in the first energy range and predetermining the energy at which the total scatter in the first energy range below said energy is substantially equal to the scatter in the first energy range divided by a number which is a function of the characteristic ratio, and means for signalling the means for decrementing or incrementing when the energy signal is below or above the electrical reference level. 63. Apparatus for use in reducing scatter in radiation imaging for use with a detector of ionizing radiation, where the ionizing radiation is partly unscattered and partly Compton scattered, the detector producing an energy signal representing values of energy of the ionizing radiation in a first energy range around a photopeak for the radiation and producing coordinate position information for the radiation, and where the apparatus is for use with data storage means for accumulating numerical values representing a number of occurrences of an unblank signal corresponding to each coordinate position, and means for displaying an image based on the numerical values from the data storage means, the apparatus comprising: memory means for storing values at respective addresses corresponding to coordinate positions of the ionizing radiation; means for replacing a value already stored at a particular address in the memory means with a latest value which is the sum of the already-stored value plus a first predetermined value having a first sign, when the energy signal represents a value of energy in a second energy range having at least some energies in common with the first range and less than half as wide as the first range and the ionizing radiation occurs at a coordinate position corresponding to that particular address; means for comparing the latest value at the particular address with a preset value when ionizing radiation subsequently occurs at the coordinate position corresponding to said address and the energy signal represents a value of energy in a part of the first range outside the second range; and means for supplying the unblank signal and coordinate position information for the ionizing radiation to the data storage means when the comparing means indicates that the latest value at said address is not less than the preset value when the first sign is negative or when the comparing means indicates that the value at said address is not greater than the preset value when the first sign is positive, and otherwise replacing the latest value at said address with another value equal to the sum of the latest value and a second predetermined value having a second sign opposite to that of the first predetermined value. 64. Apparatus for scatter reduction in radiation imaging for use with a detector of ionizing radiation that is partly unscattered and partly Compton scattered, the detector producing an energy signal representing values of energy of the radiation in a first energy range around a photopeak for the radiation and producing coordinate position information for the radiation, the apparatus comprising: memory means for holding a spectrum of intensity values representing a scatter spectrum; means for supplying an electrical reference level representing a predetermined energy in the first energy range of the radiation by premeasuring the scatter spectrum in the first energy range and predetermining the energy at which the total scatter in the first energy range below said energy is substantially equal to a predetermined fraction of the scatter in the first energy range; and means for producing a signal indicating when the energy signal is below or above the electrical reference level. 65. Apparatus as set forth in claim 64 wherein said means for supplying the electrical reference level includes a digital computer and said means for producing the signal includes means for comparing the energy signal with the electrical reference level. 66. Apparatus for use in reducing scatter in radiation imaging of ionizing radiation where the ionizing radiation has a photopeak in a first energy range and where the apparatus is for use with data storage means for holding numerical values and with means for displaying an image based on the numerical values in the data storage means, the apparatus comprising: means for detecting ionizing radiation that is partly unscattered and partly Compton scattered to produce an energy signal representing values of energy of the ionizing radiation in the first energy range and to produce coordinate position information for the ionizing radiation; and means responsive to the energy signal and the coordinate position information for generating numerical values for each coordinate position and storing them in the data storage means, the numerical values being a function of the difference of the number of occurrences of ionizing radiation in the first energy range at each coordinate position less a second number proportional to the number of occurrences of ionizing radiation at that coordinate position in a second energy range which is less than half as wide as the first energy range and having at least some energies in common with the first energy range. 67. Apparatus for scatter reduction in radiation imaging for use with a detector of ionizing radiation that is partly unscattered and partly Compton scattered, the detector producing an energy signal representing values of energy of the radiation in a first energy range around a photopeak for the radiation and producing coordinate position information for the radiation, the apparatus comprising: memory means for holding a spectrum of intensity values representing a scatter spectrum; and means for premeasuring the scatter spectrum in the first energy range and producing an electrical signal representing a characteristic number from the scatter spectrum as a function of the ratio of the scatter in the first energy range to the total of the scatter in a predetermined lower energy fraction of the first energy range. 68. Apparatus as set forth in claim 67 wherein said premeasuring and predetermining means includes means for producing a reference signal representing a particular energy value in the first energy range that separates the lower energy fraction from the rest of the first energy range. 69. Apparatus as set forth in claim 68 further comprising means for comparing the reference signal with the energy signal to produce an electrical identification signal indicating whether the energy signal represents an energy value that is greater or less than the particular energy value. 70. Apparatus as set forth in claim 69 further comprising means responsive to the electrical identification signal, to the electrical signal representing the characteristic number and to the coordinate position information for generating numerical values for each coordinate position and storing them for display purposes, the numerical values being a function of the difference of the number of occurrences of radiation in the first energy range at each coordinate position less a second number which is a function of the characteristic number and the number of occurrences of radiation at that coordinate position in the predetermined lower energy fraction of the first energy range. 71. Apparatus as set forth in claim 67 further comprising means for selectively establishing the predetermined fraction corresponding to each of a plurality of radionuclides. 72. Apparatus as set forth in claim 67 further comprising means responsive to the electrical signal representing the characteristic number and to the coordinate position information for generating numerical values for each coordinate position and storing them for display purposes, the numerical values being a function of the difference of the number of occurrences of radiation in the first energy range at each coordinate position less a second number which is a function of the characteristic number and the number of occurrences of radiation at that coordinate position in a second energy range. 73. A method for scatter reduction in radiation imaging for use with a detector of ionizing radiation that is partly unscattered and partly Compton scattered, the detector producing an energy signal representing values of energy of the radiation and producing coordinate position information for the radiation, and for use with data storage means for holding numerical values and means for displaying an image based on the numerical values in the data storage means, the method comprising the steps of: producing first and second signals which indicate whether each value of energy represented by the energy signal at a given time is in a first energy range or in a second energy range less than half as wide as the first energy range and having at least some energies in common with the first energy range; and generating numerical values for each coordinate position and storing them in the data storage means, the numerical values being a function of the difference of the number of occurrences of radiation in the first energy range at each coordinate position less a second number proportional to the number of occurrences of radiation in the second energy range at that coordinate position. 74. A method for scatter reduction in radiation imaging of ionizing radiation that has a photopeak in a first energy range and for use with data storage means for holding numerical values and means for displaying an image based on the numerical values in the data storage means, the method comprising the steps of: detecting ionizing radiation that is partly unscattered and partly Compton scattered to produce an energy signal representing values of energy of the radiation in the first energy range and to produce coordinate position information for the radiation; and generating numerical values for each coordinate position and storing them in the data storage means, the numerical values being a function of the difference of the number of occurrences of radiation in the first energy range at each coordinate position less a second number proportional to and at least two times the number of occurrences of radiation at that coordinate position in a second energy range. 75. A method for scatter reduction in radiation imaging for use with a detector of ionizing radiation that is partly unscattered and partly Compton scattered, the detector producing an energy signal representing values of energy of the radiation and producing coordinate position information for the radiation, and for use with data storage means for holding numerical values corresponding to each coordinate position, a circuit for incrementing each corresponding numerical value in response to an unblank signal and the coordinate position information, and means for then displaying an image based on the numerical values in the data storage means, the method comprising the steps of: storing tabular values corresponding to each coordinate position; incrementing or decrementing the tabular value for a particular coordinate position depending on whether the energy signal produced by an occurrence of the radiation at that coordinate position satisfies a first or a second predetermined energy condition respectively, and if the tabular value at that coordinate position has reached a preset value then instead producing an unblank signal to actuate the incrementing circuit, whereby that circuit increments the numerical value for that coordinate position in producing numerical values over a period of time for display purposes. 76. A method for scatter reduction in radiation imaging for use with a detector of ionizing radiation that is partly unscattered and partly Compton scattered, the detector producing an energy signal representing values of energy of the radiation in a first energy range around a photopeak for the radiation and producing coordinate position information for the radiation, and for use with data storage means for accumulating numerical values representing a number of occurrences of an unblank signal corresponding to each coordinate position, and means for displaying an image based on the numerical values from the data storage means, the method comprising the steps of: storing values at respective addresses corresponding to coordinate positions of the radiation; replacing a value already stored at a particular address with a latest value which is the sum of the already-stored value plus a first predetermined value having a first sign, when the energy signal represents a value of energy in a second energy range having at least some energies in common with the first range and less than half as wide as the first range and the radiation occurs at a coordinate position corresponding to that particular address; comparing the latest value at the particular address with a preset value when radiation subsequently occurs at the coordinate position corresponding to said address and the energy signal represents a value of energy in a part of the first range outside the second range; and supplying the unblank signal and coordinate position information for the radiation to the data storage means when the comparing means indicates that the latest value at said address is not less than the preset value when the first sign is negative or when the comparing means indicates that the value at said address is not greater than the preset value when the first sign is positive, and otherwise replacing the latest value at said address with another value equal to the sum of the latest value and a second predetermined value having a second sign opposite to that of the first predetermined value. 77. A method for scatter reduction in radiation imaging for use with a detector of ionizing radiation that is partly unscattered and partly Compton scattered, the detector producing an energy signal representing values of energy of the radiation in a first energy range around a photopeak for the radiation and producing coordinate position information for the radiation, the method comprising the steps of: electronically storing a spectrum of intensity values representing a scatter spectrum; electronically determining the energy at which the total scatter in the scatter spectrum in the first energy range below said energy is substantially equal to a predetermined fraction of the scatter in the first energy range; supplying an electrical reference level representing the energy so determined; and producing a signal indicating when the energy signal is below or above the electrical reference level for different occurrences of the radiation. 78. A method for scatter reduction in radiation imaging for use with memory means for storing numerical values at respective addresses, and means for displaying an image based on the numerical values in the memory means, the method comprising the steps of: detecting ionizing radiation wherein some of the radiation is unscattered and some Compton scattered by producing an energy signal representing values of energy of the radiation and producing coordinate position information for the radiation when the energy signal represents a value of energy in a first energy range; accumulating in the memory means a first number of occurrences of the radiation at each particular coordinate position when the energy signal represents a value of energy in a second energy range less than half as wide as the first energy range and having at least some energies in common with the first energy range and a second number of occurrences of the radiation at each particular coordinate position when the energy signal represents a value of energy in the first energy range and not in the second energy range; storing at respective addresses in the memory means corresponding to each particular coordinate position numerical values which are a function of the first and second numbers so accumulated for each particular coordinate position; and supplying the numerical values to the displaying means for each particular coordinate position. 79. A method for scatter reduction in radiation imaging for use with memory means for storing numerical values at respective addresses, and means for displaying an image based on the numerical values in the memory means, the method comprising the steps of: detecting ionizing radiation wherein some of the radiation is unscattered and some Compton scattered by producing an energy signal representing values of energy of the radiation and producing coordinate position information for the radiation when the energy signal represents a value of energy in a first energy range; accumulating in the memory means a first number of occurrences of the radiation at each particular coordinate position when the energy signal represents a value of energy in a second energy range less than half as wide as the first energy range and having at least some energies in common with the first energy range and a second number of occurrences of the radiation at each particular coordinate position when the energy signal represents a value of energy anywhere in the first energy range; storing at respective addresses in the memory means corresponding to each particular coordinate position numerical values which are a function of the first and second numbers so accumulated for each particular coordinate position; and supplying the numerical values to the displaying means for each particular coordinate position. 80. A method for scatter reduction in radiation imaging for use with memory means for storing numerical values at respective addresses, and means for displaying an image based on the numerical values in the memory means, the method comprising the steps of: detecting ionizing radiation wherein some of the radiation is unscattered and some Compton scattered by producing an energy signal representing values of energy of the radiation and producing coordinate position information for the radiation when the energy signal represents a value of energy in a first energy range; accumulating in the memory means a first number of occurrences of the radiation at each particular coordinate position when the energy signal represents a value of energy anywhere in the first energy range and a second number of occurrences of the radiation at each particular coordinate position when the energy signal represents a value of energy in a higher energy part of the first energy range; storing at respective addresses in the memory means corresponding to each particular coordinate position numerical values which are a function of the first and second numbers so accumulated for each particular coordinate position; and supplying the numerical values to the displaying means for each particular coordinate position. 81. A method for scatter reduction in radiation imaging of ionizing radiation that has a photopeak in a first energy range and for use with data storage means for holding numerical values and means for displaying an image based on the numerical values in the data storage means, the method comprising the steps of: detecting ionizing radiation that is partly unscattered and partly Compton scattered to produce an energy signal representing values of energy of the radiation in the first energy range and to produce coordinate position information for the radiation; and generating numerical values for each coordinate position and storing them in the data storage means, the numerical values being a function of the difference of a number of occurrences of radiation in the first energy range at each coordinate position less a second number proportional to and at least two times a number of occurrences of radiation at that coordinate position in a second energy range. 82. A method for scatter reduction in radiation imaging for use with a detector of ionizing radiation that is partly unscattered and partly Compton scattered, the detector producing an energy signal representing values of energy of the radiation in a first energy range around a photopeak for the radiation and producing coordinate position information for the radiation, the method comprising the steps of premeasuring a scatter spectrum in the first energy range and producing an electrical signal representing a characteristic number from the scatter spectrum as a function of the ratio of the scatter in the first energy range to the total of the scatter in a predetermined lower energy fraction of the first energy range. NOTICE A portion of the disclosure of this patent document contains material to which a claim of copyright protection is made. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but reserves all other rights whatsoever. BACKGROUND OF THE INVENTION The present invention relates to apparatus and methods of reducing scatter such as Compton scatter in radiation imaging systems such as gamma cameras and positron annihilation cameras. More particularly, the present invention relates to various apparatus and methods some of which reduce Compton scatter by subdividing the photopeak window into lower and upper portions and subtracting the lower portion from the upper portion for each of many regions of an image. Radiation imaging apparatus such as scintillation cameras are well known devices used in nuclear medicine. A radiopharmaceutical (e.g. technetium 99m (Tc-99m), thallium 201 (Th-201), gallium 67 (Ga-67) or indium 111 (In-111)) is administered to the patient and temporarily accumulates in areas of the body where it is desired to image lesions. The radioactive isotope emits gamma rays (or positrons which decay into oppositely directed pairs of gamma rays) which then impinge on a scintillation detector such as sodium iodide adjacent to which is located one or more photomultiplier tubes or other means of converting scintillations to electrical pulses. Electronic apparatus is used to process the pulses and to determine coordinate position values of the source of each gamma ray which are used in forming an image. The radioisotope source in other applications is located outside the patient and produces a fan beam or a cone beam that passes through the patient or specimen and impinges on an extended detector as in bone mineral densitometry for osteoporosis. The prior art has recognized that gamma rays produced by radioactive decay may travel directly to the detector or may be scattered by mechanisms such as Compton scattering. When a gamma ray initially emitted in a particular direction is scattered, a gamma ray of somewhat different energy travels in another direction which then reaches the detector as information which interferes with or can be confused with gamma rays reaching the detector directly. Therefore, if the camera is designed to determine that the source of a particular gamma ray lies along a line coinciding with its direction of incidence on the detector, then Compton scatter photons will be misinterpreted as indicating a source distribution of the radionuclide in the patient which is actually different than its actual distribution. In practice the effect of Compton scattering is to provide fuzzy and indistinct images instead of desirable distinct images of the radionuclide distribution in the patient. For example, sodium iodide as a camera detector has a relatively poor energy resolution, and images obtained with conventional techniques employing a pulse height analyzer (PHA) window centered around the photopeak portion of the radiotracer spectrum will contain high (20-50%) fractions of scattered photon events. As a result, devices designed to image the photon emissions from these radioactive materials are presented with a geometrically widespread mixture of scattered photons and non-scattered photons. There is no geometric method to separate scattered non-scattered photons since the usual source of radioactivity is distributed over a significant volume within the patient, and the emission of photons from radioactive decay is isotropic. The inclusion of scatter photons in the measurements obtained with imaging devices not only degrades the image contrast, but also the degradation is amplified by image reconstruction in emission computed tomographic imaging. The physics of Compton scattering is well known and the energy of a scattered photon is given by the equation ##EQU1## where e.sub.s is the energy of the Compton scattered photon e.sub.o is the energy of the original emitted gamma ray, mc.sup.2 is the equivalent energy of a particle such as an electron which scatters the original gamma ray (mc.sup.2 for an electron is 511 KeV), and A is the Compton angle or angle between the direction of incidence of the original photon and the direction of travel of the scattered photon. A scintillation camera typically has one or more pulse height analyzers (PHAs) by means of which the energy of radiation can be readily determined. Gating circuits detect incident photons upon the detector which have radiation energies within a predetermined band or "window" and all of the other radiation not within the window is excluded for imaging purposes. By inspection of the Compton scattering formula (1) it is apparent that a window which is set for a particular energy also determines the Compton angle of any scattered radiation that may enter. For example, a given radionuclide produces gamma rays having a particular energy e.sub.o. Due to statistical factors the actual energy which is sensed at the detector will lie in a more or less peaked distribution centered on an energy called the photopeak energy and the distribution is conventionally called the photopeak. In a typical gamma camera a photopeak window or energy window is centered so as to have lower and upper energy boundaries bracketing the center of the photopeak e.sub.o. Unavoidably some Compton scattering produces radiation that is within the photopeak window which therefore is confused by the radiation imaging apparatus with gamma rays which have come directly from the radionuclide distribution in the patient. The boundaries of the photopeak window define the maximum angle of Compton scattering which can occur and still lie within the photopeak window and thus be confused with the true source distribution. The Compton angle is determined by solving the Compton formula (1) for Compton angle A after substituting the photopeak energy e.sub.0 and the lower boundary of the photopeak window for e.sub.s. In one prior art approach a second entirely separate PHA window or energy range distinctly below the photopeak window is provided at lower energies where the Compton scattering is more numerous to acquire a scatter image. An empirically determined proportion of Compton events is subtracted from the number of detected incident gamma rays in the photopeak window as a rough means of estimating the actual Compton scattering in the photopeak and thus reducing its blurring effect on the image. In emission computed tomography (ECT), the scatter image may be used to construct a scatter ECT image which is subtracted from the regular ECT image at the same geometric position. Unfortunately, the amount of Compton scattered radiation in a lower energy window in general bears little relationship to the actual amount of Compton scatter in the photopeak region itself. Compton scattering occurs due to radiation that originates in regions of the specimen adjacent to the region which is desired to be imaged and the distribution of the radionuclide in any given specimen is precisely the unknown which is to be imaged. As put by one worker "the contribution of adjacent regions over a particular site to the measured activity is almost entirely in the scatter region when collimated detector systems are used. Thus, the peak-to-scatter ratio would be diminished by the presence of appreciable amounts of radioactivity in adjacent structures. This localization must be determined in the course of mapping the peak-to-scatter ratio in and around the region(s) of interest, and the scatter contribution from the adjacent region(s) taken into account . . . [I]t appears that a more comprehensive formalism is needed, since an almost infinite number of phantom configurations and calibrational studies would be needed . . . The suggestion that the scatter window be used as an index of the scatter contribution contained in the photopeak region seems destined to failure because of the phenomena described above. If all the scatter could be related to the activity distribution along the axis of the collimator, the procedure . . . might be useful. Since, however, a variable but large fraction of scatter counts arise from surrounding regions, a single correction factor would not be very useful." R. E. Johnston et al., "Inherent Problems in the Quantitation of Isotope Scan Data" Medical Radioisotope Scintigraphy, Vol. I, IAEA, Vienna, 1969, pp. 617-631, at pp. 630-631. A Compton scattering energy window approach is also discussed in "Four-View Computer Scintiscanning: Image Structuring Through Multi-Window Pulse-Height Analysis" by S. Genna et al. in Medical Radioisotope Scintigraphy 1972, IAEA, Vienna 1973, pps. 133-154 and R. J. Jaszczak "Scatter Compensation Techniques for SPECT", IEEE Trans. Nuclear Science, Vol. 32, February 1985. In another approach to reducing Compton scatter a gamma camera has scintillation events displayed at X,Y coordinates on a cathode ray tube by unblanking the tube with z pulses applied to its control electrode. A Compton scattered radiation deemphasizer determines where the peaks of total energy pulses fall in a part of the energy spectrum or window which is the photopeak window for present purposes. The deemphasizer causes small Z pulses to be produced in the part of the spectrum where Compton scatter is most prevalant and causes increasingly larger Z pulses as the total energy increases up to the midpoint of the photopeak window where Compton scatter is less significant. Constant amplitude Z pulses are produced at and after the midpoint of the photopeak window. In FIG. 3 of this patent a distribution of energy at the photopeak window and the method of deemphasis is shown. A microprocessor implementation for accomplishing the deemphasis is shown in FIG. 6 of U.S. Pat. No. 4,258,428 to E. M. Woronowicz. In U.S. Pat. No. 4,575,810 a "simplistic" and "completely synthetic" example (col. 4 lines 29-47) features three windows. The patent asks the reader to suppose that events in an upper section of the photopeak were known to provide twice as much image information as those in the lower section, and events in a region just below the photopeak could be subtracted to remove scatter. An upper window image would be added twice and the lower window image added once, while subtracting the below range window image. However, the patent states that it is likely that signal-to-noise ratios in the weighted image would not be larger than normal images, due to the simplistic nature of the example, and instead teaches an approach using a carefully constructed weighting function. In U.S. Pat. No. 4,415,982 to M. Nishikawa a background suppression approach to improving the images is produced and no suggestion that scatter reduction is specifically intended is present. In the Nishikawa patent a scintillation camera has a memory with addresses corresponding to the elements of a matrix-like divided image of a scintigram and these addresses correspond to incident position signals of points of radiation determined by a position calculating circuit. A pulse height analyzer receives the radiation events registered by the camera head and issues an unblanking signal to control apparatus if the count rate is significantly higher than background noise. The content at the designated address in memory is compared with a predetermined minimum value of radiation in a comparator. When the content is less than the minimum value the control apparatus adds one to the content and restores the increased content to the designated address. When the content is at least equal to the minimum value, the control apparatus issues the unblanking signal to display apparatus to display the radiation at the designated address. In effect this apparatus accomplishes a kind of subtraction so that only those image positions which receive a number of events in excess of a predetermined number are displayed at all on the display apparatus. The various approaches of the prior art evidently provide some reduction of the Compton scatter blurring of images and increase the contrast and distinctness of the images somewhat. However, the subtraction approach using a separate Compton window has been criticized in the cited literature because such approach is dependent on particular source distributions and is dismissed in the cited U.S. Pat. No. 4,575,810 as having low signal-to-noise ratio. A deemphasizer as described in U.S. Pat. No. 4,258,428 allows some Compton scatter to be detected because it passes all z pulses below the midpoint of the photopeak window except that these are deemphasized. The scintillation camera of Nishikawa in U.S. Pat. No. 4,415,982 does not purport to reduce Compton scattering. Compton scatter which is included in regions of the image which exceed the predetermined minimum of radiation which is blanked out by Nishikawa will not be reduced at all. SUMMARY OF THE INVENTION Among the objects of the present invention are to provide improved apparatus and methods to substantially reduce the blurring and distorting contribution of Compton scatter or other scatter to images produced by radiation imaging apparatus such as is used in nuclear medicine or other fields; to provide improved apparatus and methods to produce on-line real-time active removal of the scatter in nuclear medicine imaging signals and in other uses of radioactive tracers or radionuclides; to provide improved apparatus and methods which can be incorporated either in original equipment or can be retrofitted to radiation imaging apparatus presently in the field; to provide improved apparatus and methods which reduce Compton scatter and increase contrast in parts of the image in which the source intensity varies and already exceeds a predetermined minimum of radiation; to provide improved apparatus and methods which directly eliminate the scatter fraction normally included in images obtained with conventional pulse height analyzer (PHA) windows as normally utilized on Anger type gamma cameras, and other ionizing photon imaging systems; to provide improved apparatus and methods which receive total image signals (scatter plus non-scatter photons within PHA windows) from the imaging system electronics, remove the scatter portion and output substantially scatter-free image signals to typical image display and recording devices; to provide improved apparatus and methods for calibrating scatter reduction apparatus; to provide improved computerized processes for more effective scatter reduction in radiation imaging apparatus and methods; to provide improved scatter reduction apparatus and methods which can be used with radionuclides that have either a single photopeak or multiple photopeaks; to provide improved scatter reduction apparatus and methods which are calibrated conveniently, accurately and speedily; to provide improved scatter reduction apparatus and methods which will automatically adapt themselves to one or more energy windows established in radiation imaging apparatus such as a gamma camera with which the scatter reduction apparatus and methods are to be used; and to provide improved scatter reduction apparatus and methods which are more convenient, versatile, reliable, effective and economical. Generally, one form of the invention is an apparatus for scatter reduction in radiation imaging for use with a detector of ionizing radiation that is partly unscattered and partly Compton scattered. The detector produces an energy signal representing values of energy of the radiation and produces coordinate position information for the radiation. Data storage means for holding numerical values and means for displaying an image based on the numerical values in the data storage means are also used with the inventive apparatus. The scatter reduction apparatus includes circuitry responsive to the energy signal from the detector for producing first and second signals which indicate whether each value of energy represented by the energy signal at a given time is in a first energy range or in a second energy range less than half as wide as the first energy range and having at least some energies in common with the first energy range. Further included is circuitry responsive to the first and second signals and the coordinate position information for generating numerical values for each coordinate position and storing them in the data storage means, the numerical values being a function of the difference of the number of occurrences of radiation in the first energy range at each coordinate position less a second number proportional to the number of occurrences of radiation in the second energy range at that coordinate position. Generally, another form of the invention is an apparatus for scatter reduction in radiation imaging of ionizing radiation that has a photopeak in a first energy range and for use with data storage means for holding numerical values and means for displaying an image based on the numerical values in the data storage means. The apparatus includes means for detecting ionizing radiation that is partly unscattered and partly Compton scattered to produce an energy signal representing values of energy of the radiation in the first energy range and to produce coordinate position information for the radiation. The scatter reduction apparatus further includes circuitry responsive to the energy signal and the coordinate position information for generating numerical values for each coordinate position and storing them in the data storage means, the numerical values being a function of the difference of the number of occurrences of radiation in the first energy range at each coordinate position less a second number proportional to and at least two times the number of occurrences of radiation at that coordinate position in a second energy range. In general, still another form of the invention is an apparatus for scatter reduction in radiation imaging for use with a detector of ionizing radiation that is partly unscattered and partly Compton scattered. The detector produces an energy signal representing values of energy of the radiation and produces coordinate position information for the radiation. The apparatus is further for use with data storage means for holding numerical values corresponding to each coordinate position, a circuit for incrementing each corresponding numerical value in response to an unblank signal and the coordinate position information, and means for then displaying an image based on the numerical values in the data storage means. The inventive scatter reduction apparatus includes memory means for storing tabular values corresponding to each coordinate position combined with means for incrementing or decrementing the tabular value for a particular coordinate position depending on whether the energy signal produced by an occurrence of the radiation at that coordinate position satisfies a first or a second predetermined energy condition respectively, and if the tabular value at that coordinate position has reached a preset value then instead producing an unblank signal to actuate the incrementing circuit, whereby that circuit increments the numerical value for that coordinate position in producing numerical values over a period of time for display purposes. Generally, a further form of the invention is an apparatus for scatter reduction in radiation imaging for use with a detector of ionizing radiation that is partly unscattered and partly Compton scattered, the detector producing an energy signal representing values of energy of the radiation in a first energy range around a photopeak for the radiation and producing coordinate position information for the radiation. The apparatus includes memory means for holding a spectrum of intensity values representing a scatter spectrum combined with means for supplying an electrical reference level representing a predetermined energy in the first energy range of the radiation by premeasuring the scatter spectrum in the first energy range and predetermining the energy at which the total scatter in the first energy range below said energy is substantially equal to a predetermined fraction of the scatter in the first energy range, and means for producing a signal indicating when the energy signal is below or above the electrical reference level. Generally, still another form of the invention is an apparatus for scatter reduction in radiation imaging for use with a detector of ionizing radiation that is partly unscattered and partly Compton scattered, the detector producing an energy signal representing values of energy of the radiation in a first energy range around a photopeak for the radiation and producing coordinate position information for the radiation. The apparatus includes memory means for holding a spectrum of intensity values representing a scatter spectrum, and means for premeasuring the scatter spectrum in the first energy range and producing an electrical signal representing a characteristic number from the scatter spectrum as a function of the ratio of the scatter in the first energy range to the total of the scatter in a predetermined lower energy fraction of the first energy range. Other apparatus and method forms of the invention for achieving the above-stated and other objects of the invention are also disclosed and claimed herein. Other objects and features will be in part apparent and in part pointed out hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph of a typical energy spectrum of a radionuclide which graph shows radiation intensity in counts per unit time on the ordinate versus energy (or camera energy pulse height) on the abscissa; FIG. 2 is a pictorial diagram of a radioactive source element and a radioactive interfering element in the head of a patient with a cone drawn to show where most Compton scatter interference in a given image pixel would originate; FIG. 3 is a detail of FIG. 1 showing a bell-shaped photopeak spectrum and a sloping Compton scatter spectrum, with division of a first energy range S into upper part A and lower part C for purposes of some embodiments of the inventive apparatus and methods; FIG. 4 is a block diagram of an improved radiation imaging system of the invention having a Compton Scatter Filter of the invention operating according to methods of the invention; FIG. 5 is a pictorial diagram of a memory of the Compton Scatter Filter of FIG. 4, the memory holding tabular values according to a method of the invention; FIG. 6 is a chart showing a hypothetical series of operations on one memory cell of the memory of FIG. 5 corresponding to one pixel X.sub.1, Y.sub.1 where events E occur in either the C or A window of FIG. 3 and produce outputs OUT according to a method of the invention; FIG. 7 is a number line further illustrating a method of the invention of FIG. 6; FIG. 8 is a number line further illustrating an alternative method of the invention for computing tabular values in opposite sense to that shown in FIGS. 6 and 7; FIG. 9 is a block diagram of a Compton Scatter Filter of the invention combined with a gamma camera and a film formatter according to the invention; FIGS. 10, 11 and 12 together are a schematic diagram of an embodiment of the inventive Compton Scatter Filter of each of FIGS. 4 and 9, wherein FIG. 10 is a schematic diagram of an inventive calibrating computer circuit operating according to methods of the invention; FIG. 11 is a schematic diagram of further inventive circuitry including the memory of FIG. 5 for reducing scatter according to methods of the invention; FIG. 12 is a schematic diagram of further circuitry of the invention for controlling the circuitry of FIG. 11 according to methods of the invention; FIG. 13 is a flowchart of operations in the calibrating computer circuit of FIG. 10 according to a method of the invention for measuring a bell-shaped photopeak spectrum of a radioactive test source in air and establishing the first energy range S; FIG. 14 is a flowchart of further operations in the calibrating computer circuit of FIG. 10 according to a method of the invention for measuring a spectrum of the radioactive test source with an aluminum scattering plate added to produce a spectrum which is the sum of the bell-shaped spectrum and the scatter spectrum in the first energy range S and then producing a signal representing an energy level L2 in the first energy range to establish the dividing line between the A and C windows; FIG. 15 is a flowchart of further operations in the calibrating computer circuit of FIG. 10 according to a method of the invention for showing in greater detail an inventive method of determining energy level L2 between the A and C windows; FIG. 16 is a block diagram of an inventive microprocessor-based circuit operating according to inventive methods in substitution for the circuits of FIGS. 11 and 12 in a Compton Scatter Filter of the invention; FIGS. 17, 18 and 19 are flowcharts of three alternative inventive methods of operating the apparatus of FIG. 16 for Compton scatter filtering purposes; FIG. 20 is a block diagram of an inventive accumulating computer circuit operating according to methods of the invention for reducing Compton scatter in radiation imaging without the use of a separate Compton Scatter Filter apparatus; FIGS. 21, 22 and 23 are flowcharts of three alternative inventive methods of operating the accumulating computer circuit of FIG. 20 for reducing Compton scatter in a single memory space M1; FIGS. 24, 25 and 26 are flowcharts of three alternative inventive methods of operating the accumulating computer circuit of FIG. 20 for reducing Compton scatter using plural memory spaces M1, M2 and M3; FIGS. 27 and 28 are both halves of a flowchart of a fraction memory process for a further alternative method embodiment to operate the accumulating computer circuit of FIG. 20 for reducing Compton scatter using plural memory spaces M1, M2 and M3; FIG. 29 is a block diagram of a further apparatus embodiment of the invention operating according to methods of the invention and utilizing a Calibration Computer System of FIG. 10 providing computed values of a characteristic ratio R to an accumulating computer operating according to any of flowcharts 21-28; and FIG. 30 is a flowchart of operations of the Calibration Computer System of FIG. 10 for use in the apparatus of FIG. 29 in order to compute the values of R. Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In FIG. 1 a photopeak 11 of radionuclide Tc-99m is centered around a center energy e.sub.o. The observed spectrum is actually the sum of a bell-shaped spectrum 13 of light photon events corresponding to true photopeak photons (gamma rays) and a relatively broad spectrum 15 of Compton events resulting from Compton scattering of photopeak photons in the specimen to be imaged. It is to be understood that gamma rays can be scattered at no more than 180.degree., which according to Compton scattering formula (1) is an energy of: ##EQU2## A typical scintillation material such as sodium iodide (NaI) produces scintillations of light which have an energy that is probabilistically related, according to an approximately bell-shaped curve, to the energy of any incident gamma ray, whether it be a true photopeak photon or a Compton scatter photon. As a result, Compton scatter photons are sometimes detected in the vicinity of the photopeak even when they have an energy outside a first energy range (e.g. with a range width of 25% or less of the center energy e.sub.o). Therefore, a gamma camera having energy selection circuitry set to reject all photon events outside the first energy range does ordinarily pass Compton scatter. Such Compton scatter is both low angle scatter which produces a gamma ray with energy inside the first energy range itself, and higher angle scatter which produces a gamma ray outside the first energy range but which has some probability of being detected in the first energy range anyway, as just mentioned. In FIG. 2, a head of a patient has a complex unknown source distribution of a radionuclide which has recently been administered. A camera such as in FIG. 1 of U.S. Pat. No. 4,755,680 produces an image comprising numerous pixels, only one pixel 27 being shown in FIG. 2 herein. For illustration a line 25 passes through pixel 27 in a direction of collimation of the camera. A source element 31 which it is desired to image emits radiation along the line 25. Unfortunately, an element 33 lying away from the line 25 emits radiation toward parts of the head lying on line 25 and produces Compton scattered photons which travel along line 25 into the camera and contribute interference to the pixel 27. As shown in FIG. 3, a gamma camera utilizes a photopeak energy window S which is of limited width e.sub.w (12-25% of center energy e.sub.o). Most of the interfering sources of Compton scattering are limited to a cone in FIG. 2 which has line 25 as its axis and its apex located where line 25 intersects the surface of the specimen. The cone has an apex angle A which is the angle determined from equation (1) at which Compton scattering produces gamma photons of energy e.sub.1 equal to the low energy boundary of window S. It is to be understood that Compton scattering can occur at any angle, and that the energy of the Compton scattered radiation may be regarded as broadened according to a Gaussian distribution a tail of which lies within the photopeak window. Therefore, some amount of Compton scattering that emanates from the specimen along the line 25 and reaches pixel 27 originates outside the cone. For qualitative purposes, the illustration shows a distinct cone, while it is to be understood that in actuality a fuzziness in the cone boundary allows some contribution to scatter from outside the cone. In FIG. 2, the contribution of source distribution interfering areas or elements is maximum when the interfering areas are located at an angle A1 equal to about half the cone apex angle A. Angle A1 is defined as the angle of line 25 relative to a line segment 35 joining interfering element 33 with a point P where line 25 intersects the surface of the head or other specimen. This maximum contribution at half of angle A is due to two factors: first, that element 33 can scatter anywhere along line 25 through an angle A-A1, and second that the volume that interfering elements can occupy increases approximately with the square of the angle A1. Scatter image degradation does occur due to interfering regions of the radionuclide source distribution in the patient at very low angles of A1 (near line 25). However, this low angle scatter degradation is small notwithstanding that A-A1 is large because of the low volume at the low angles. The scatter image degradation due to interfering regions at high angles A1 (near cone surface 39) is also small, but because A-A1 is small. In contrast, interfering elements like element 33 can occur in a substantial volume and scatter into line 25 through a substantial angle A-A1 when angle A1 is about half of apex angle A. The physical distribution in the specimen of numerous radiation sources such as element 33 theoretically has some effect on the shape of a tail 41 of the scatter spectrum 15 in FIG. 3. However, it is believed that the shape of tail 41 is substantially independent of source distribution for most practical purposes involving complex extended source distributions in specimens encountered in nuclear medicine and other applications. One reason is that the Compton scatter received at each pixel such as 27 is a volume integrated quantity to which elements such as 33 throughout the cone of FIG. 2 contribute. Accordingly, there is a volume averaging effect that tends to also keep the shape of the energy distribution 41 of Compton scatter interference in the photopeak window about the same regardless of pixel position. Some of the inventive embodiments make advantageous use of the essentially source-independent property of the shape of the Compton scatter spectrum 41 (FIG. 3), as now discussed in further detail. One concept used in some of the embodiments uses a ratio R of total scatter "+SCATTER" above an energy L2 divided by total scatter "-SCATTER" below energy L2. Then for each pixel, R is multiplied by the number of events in window S below L2 whether they be scatter or not. The product is subtracted from the total of events in window S above L2. Because of the predictable shape of scatter spectrum 41, the scatter is advantageously cancelled in the process as indicated by "-" and "+" boxes 43 and 45. The remainder 47 represents true photopeak events, numerical values of which are output for each pixel to define a scatter-free image. A theory of operation of some of the embodiments is next discussed. It is to be understood that the theory is presented to motivate some of the concepts and the utility of the embodiments to which it pertains and is not exhaustive or meant to necessarily pertain to all embodiments comprehended in the spirit and scope of the invention. In a typical nuclear medicine imaging system the spectrum of pulse heights falling within a pulse height analyzer (PHA) window when the window encompasses a photopeak is composed of a mixture of scattered and nonscattered photons. The majority of scattered photons are from scattering events occurring in the specimen, and the non-scattered photons are those that have traveled from the site of emission with no interaction in the specimen, or in any collimator of the imaging apparatus. In a gamma camera, this mixed spectrum is present at all locations on the face of the detector crystal which receive any photons originating in the patient. At a typical location with rectangular coordinates X, Y (or angular and axial coordinates in a cylindrical camera of parent application Ser. No. 604,989), a mixture of photons cause electronic pulses to be sent to a PHA, or pulse height analyzer also known as a single channel analyzer 127 as in FIG. 6 of the parent application. The total number of photon events falling in a PHA window symmetrically positioned around the photopeak, is S for some time period T. Then number of events S can be represented in two parts: where a1 is the fraction of events S that occur from scattered photons, and b1 is the fraction of events S which occurs from non-scattered photons. Constants a1 and b1 vary substantially over the pixels of an image of a complex source distribution. Note that: Next consider a second PHA window A receiving a portion of the same spectrum of pulses as the entire photopeak window. Window A is narrower than the entire photopeak window, and positioned asymmetrically toward the high energy side of the photopeak as shown in FIG. 3. The number of events with pulses falling in window A is The first term a2a1S is the scattered photon pulses which fall within window A, and the second term b2b1S is the non-scattered photon pulses which fall in window A. The constant a2 is the fraction of scattered photons in the entire photopeak window that also occurs in window A. The constant a2 can be experimentally measured as discussed later hereinbelow. As discussed above regarding shape of spectrum 41, constant a2 is essentially independent of the total scatter fraction a1. The second constant in equation (5), b2, is the fraction of non-scatter photons in the entire photopeak window that also appear in window A. Due to the predictable nature of bell-shaped photopeak spectrum 13, the value b2 can be measured by imaging a point source in air where no scatter occurs and the bell-shaped spectrum 13 can be isolated. Both a2 and b2 can be determined experimentally for any imaging system, and for a variety of selections of photopeak window S and window A. Since S, the number of counts at the typical location X, Y allowed by the photopeak window is a measured value, as is the number of counts A that correspond to window A (both S and A are measured during the same time interval T), Equations (3), (4), and (5) can be solved for the non-scattered events in the photopeak window by substituting b1=(1-a1) from Equation (4) into Equation (5), solving for alS and substituting for a1S in Equation (3) and solving for b1S with the result: Note that Equation (6) states the non-scattered photon portion of the events with pulses that fall in the photopeak window in terms of the experimentally measured constants a2 and b2, and the number of events S and A corresponding to the photopeak window and its included window A. The previous discussion and the results for the non-scattered events described by Equation (6) are independent of the location on the camera crystal X,Y. Equations (3)-(6) can therefore be generalized to the entire image area and the variables A and S can represent two dimensional fields or matrices of pixels which are the same as digital images. In summary, the above discussion indicates that by using two simultaneous subwindows A and C in the photopeak window S of FIG. 3 it is possible to filter out a separate image of the non-scatter photons that are originally mixed with scattered photons. Advantageously, the narrow C window substantially overlaps or is completely inside or totally encompassed by the entire photopeak window. Several different inventive methods can be recognized for collecting the image data and calculating the non-scatter image as described by Equation (6). These methods are described below. Essentially, data is collected for images in the photopeak window or first energy range S and the lower subwindow C which is a second energy range included in S, where C is given by In words, equation (7) expresses the concept that the number of radiation counts in window C is equal to the number of counts in window S less the number of counts in window A. The method for collecting an A image and a C image corresponds to collecting data for events that have pulse heights that fall anywhere in the upper, or higher energy, window part A, and data that correspond to pulse heights that fall in lower energy window part C. Using Equation (7) to eliminate S in the righthand side of Equation (6), the scatter-free image b1S is obtained as follows: By factoring out the quantity (1-a2) and designating by R a characteristic ratio of scatter in A to scatter in C expressed by quantity a2/(1-a2), the following expression for scatter-free image b1S is obtained: Note that the factors (1-a2)/(b2-a2) and R in Equation (9) are a function only of the imaging system and the windows A and C selected and can be determined in a separate calibration sequence prior to the normal imaging procedure. The factor that determines the scatter removal is (A-RC). Using Equation (7) to eliminate A in the right-hand side of Equation (6), the scatter-free image b1S is alternatively obtained as follows: Note that the factor that determines the scatter removal here is (S-C(1+R)). By eliminating C, equation (10) is equivalently expressed as: In Equation (11) the factor that determines the scatter removal is (A(1+R)-RS). The basic design of a scatter filter hardware embodiment described hereinbelow utilizes the signals generated by a typical gamma camera, or other ionizing radiation imaging apparatus. The particular hardware example advantageously performs in real-time the operation described in Equation (9) above as well as achieving other objects. In other words, the scatter is eliminated in a two-dimensional memory array that accumulates scatter-free information in electronics associated with the camera. FIG. 4, for instance, shows an embodiment suitable for retrofit or with original equipment. In FIG. 4 the scatter is subtracted or filtered from the photopeak photon events as they are counted in real-time processing to obtain scatter-free data for each position or pixel in the image field of the imaging apparatus. A gamma camera 51 acts as a detector of ionizing radiation that is partly unscattered and partly Compton scattered. The detector produces an energy signal representing values of energy of the radiation and X,Y coordinate position information for the radiation. Gamma camera 51 in original equipment is used with a processor 53, a data store 55 for holding numerical values of accumulated radiation counts for each pixel of an image, and display equipment for displaying an image based on the numerical values in data store 55, including a film formatter 61, cathode ray tube (CRT) monitor 63 and ECT (emission computed tomography) computer and display. Processor 53 is suitably any commercially available nuclear medicine computer, for example. By way of improvement in FIG. 4, an inventive Compton Scatter Filter 71 receives a Start/Stop signal from the gamma camera 51, as well as unblank pulses on a line UNBL1, analog signals indicative of X,Y coordinate positions of an occurrence of radiation, and an energy signal on an Energy line. Compton Scatter Filter 71 advantageously produces a lesser number of unblank pulses on an unblank line UNBL2, together with corresponding X,Y coordinate position information for processor 53 corresponding to the image of the specimen with the scatter events removed. One or more circuits 73 identical to circuit 71 are added in parallel with circuit 71 and are tuned to one or more other photopeaks of the radionuclide in use, when it is desired to filter Compton scatter from the other photopeaks. Alternatively, simpler circuits are used to merely bypass circuit 71 when occurrences of energy pulses having a level indicative of other photopeaks occur. Circuit 73 may be dispensed with if the skilled worker elects not to filter scatter from other photopeaks. In FIG. 4, the output signals of Compton Scatter Filter 71 are essentially the same shape and size as those produced by the gamma camera 51 so that compatible operation of processor 53 is achieved. The function of the Compton Scatter Filter 71 is thus to filter out at all positions in the image field of the gamma camera the fraction of events that are scatter radiation, and to pass through to the signal output connections the fraction of data corresponding to non-scattered events. A scenario for processing a single event is as follows. When an ionizing event occurs in the camera crystal that causes an event signal of a height corresponding to an energy anywhere in the entire photopeak window, then camera output signals are generated. These signals are Energy, X position, Y position, Z or unblank, and are input to the Compton Scatter Filter 71 which determines whether the energy signal is in the A or C range for example. Event processing functions are then triggered. Since most cameras produce analog and not digital X and Y position signals, both the X and Y analog signals are fed to sample and hold circuits in Filter 71 followed by analog-to-digital converters (ADCs). The Z unblank pulse is used to enable the sample and hold circuits. For each event only one of two window signals A and C are active (high). The X and Y ADCs determine a digital X and Y pair corresponding to a memory address or location in an image field memory 75 illustrated in FIG. 5. An electronic processor (implemented in hardware, firmware, or as a microprocessor programmed with appropriate software) in Filter 71 reads the X and Y digital values and the A and C logic levels. FIG. 6 shows operations in Filter 71 depicted for a hypothetical series of events E which occur either in the A or C window. Only some of the events E cause an output OUT on line UNBL2, as indicated. The operations are discussed in more detail next. A1l of the memory 75 locations corresponding to pixels are initialized to a preset value V0 such as zero, as indicated by reference line 81 of FIG. 6. If an event occurs with an energy in the A window of FIG. 3 in the particular pixel (X.sub.1, Y.sub.1), then a Filter 71 output UNBL2 is generated because the preset value is already present in memory for this pixel. Next in the FIG. 6 example, a C event 83 subsequently occurs at the same pixel, and the number stored in memory location X, Y is decreased by the value R in Equation (9). (The constant R is determined experimentally during prior calibration of the system.) For simplicity of illustration, it is assumed that amount R is unity in FIG. 6. No output signal is generated by the Compton Scatter filter 71 for C events. The X and Y ADCs, the X, Y and Z sample and hold circuits and the A and C event latches are reset and the system is ready to receive the next event from the gamma camera. Another C event 85 occurs at the same pixel, decreasing the number stored at the corresponding location by another amount R. Next, an A event 87 occurs at the pixel, and the number stored in memory location X, Y is tested to determine if it has reached preset value V0, e.g. zero. If not, the stored number is increased by the value 1 (one) (see Equation (9)) since the value in memory is less than zero. In the example, another event 89 in window A in this pixel occurs until the value in memory for the pixel reached the preset value V0. Then on the next subsequent A event 91, an output unblank signal is generated by the scatter rejection filter apparatus and no addition occurs. The output signal representing scatter-free information is sent on to processor 53 for accumulation of a scatter free image. Again, the X and Y ADCs, the X, Y and Z sample and hold circuits and the A and C event latches are reset and the system is ready to receive the next event from the gamma camera. Further operations in the example of FIG. 6 are also shown and should be apparent. In all, the example shows 7 A events and 3 C events. The Compton Scatter Filter in executing its operations correctly produces 4 UNBL2 unblank outputs representing the scatter free image intensity for that pixel. It is to be understood that the operations of FIG. 6 are carried out for each pixel using the contents of memory locations of memory 75 of FIG. 5 as respective tabular values for keeping track of corresponding operations for every pixel in the image field. Further, as shown in FIG. 7, each pixel tabular value can be graphed on a number line with preset value V0 as starting point. When a C event occurs the tabular value is decreased by any amount proportional to constant R. When an A event occurs and V0 has not been reached, the tabular value should be increased, and the ratio which that amount of increase bears to unity should be the same as the ratio that the amount of decrease bears to R. As shown in FIG. 8, the operations can be carried out in a reverse manner in which C events produce increases, and if the tabular value exceeds preset value V0, then a subsequent A event produces a decrease. FIG. 9 shows that when a display device such as film formatter 61 has a circuit that inherently accumulates or increments for counts of radiation in each pixel, then processor 51 of FIG. 4 can be omitted. As shown, gamma camera 51 is connected to Compton Scatter Filter 71, which in turn feeds the film formatter 61. In some embodiments a gamma camera has at least two pulse height analyzers. If the two PHAs are set for windows S and A as shown in FIG. 3, then for pulses that fall within window A, many currently available cameras generate only a single output from the PHA circuitry, even though the pulse falls into both PHA windows. This is done to avoid double counting of single gamma ray events. The output from the PHA circuitry is usually a TTL (transistor-transistor-logic) or similar pulse used to increment a scaler, and to activate circuitry that produces X and Y position output signals. On the other hand, if a pulse is presented to the PHA circuitry which falls in window C, a different output pulse is generated by the PHA circuitry. PHA outputs which correspond to events that fall in window A of FIG. 3 are designated A events, and events in window C are designated C events. Both A and C events trigger the X and Y position output signals even though they are separate and distinct as they emanate from the PHA circuitry, and can be separately monitored by devices outside of the camera. Processing of output signals from gamma cameras and other ionizing radiation imaging systems as described above can be accomplished by a variety of different alternative embodiments. Real-time processing of each individual event as described above advantageously transfers image data, with scatter events removed, to any of the devices normally receiving camera output signals, and without appreciable delay. In FIG. 10, a Calibration Computer System 101 for use in Filter 71 has a microprocessor 103, associated ROM 105 for holding program software to operate it, a RAM 107 for holding energy spectra, a peak detection unit 108, sample-and-hold circuit S/H 109 and an analog-to-digital converter ADC 111 with output latch, and a digital-to-analog converter DAC 112 with an input latch. After a calibration procedure described later herein, a RUN mode is executed in which a comparator 113 compares the camera energy level, representing energy of an occurrence of the radiation, with an electrical reference signal determined b y calibration to represent the energy L2 that separates the A and C windows in photopeak window S. When the camera pulse energy exceeds the energy L2, the comparator 113 output goes high, activating signal A.sub.IN. An inverter 115 connected between the comparator 113 output and a line for C.sub.IN produces an output low at this time. When the camera pulse energy is less than the energy L2, the comparator 113 output goes low, deactivating signal A.sub.IN and activating signal C.sub.IN through inverter 115. In this way A events and C events are identified. The signal representing camera pulse energy supplied to peak detector unit 108 should be the position corrected (Z axis correction) energy signal that is analyzed by the camera pulse height analyzer. Peak detection unit 108 receives and temporarily holds the voltage at the peak of the camera energy pulse. These functions are important because the peak voltage is the quantity that represents the energy of the radiation detected by the camera 51. Also, gating is performed in the circuit of FIG. 10 and unit 108 advantageously holds the energy pulse long enough to coincide with the slightly later-occurring unblank on a line UNLB1. When an unblank pulse occurs line UNBL1 (corresponding to a photopeak from which scatter is to be removed) at the same time as a logic level output from unit 108, an AND gate 119 activates an interrupt pin INT at microprocessor 103 and enables S/H 109 and ADC 111 to digitize the energy information. The camera unblank is usually a +5 volts by 1 microsecond square pulse (usually TTL) timed near the center of the X,Y position signal interval. In the RUN mode of a switch 121, microprocessor 103 is programmed to supply an output P14 to enable an AND-gate 123 to allow the unblank pulse to pass to line Z.sub.IN. Output P14 also lights a LED 124 labeled "READY". In calibration, gate 123 is disabled. When an unblank pulse occurs on a line UNBL3 corresponding to a second higher energy photopeak which is passed without scatter removed, interrupt pin INT is not affected, thereby preventing the higher photopeak from affecting calibration. In RUN mode the energy pulse from the higher energy photopeak is fictitiously treated to contribute to the image as if it occurred in the A window of the lower photopeak, and since the higher energy photopeak always exceeds energy L2, there is never any C output produced in response to such higher photopeak occurrence. The unblank UNBL3 for the higher photopeak passes through an OR-gate 120 and AND gate 123 in RUN mode to activate output Z.sub.IN. A switch 131 is adjusted to a characteristic number R of 1, 2 or 3. In this way, the probable optimum value of R is preestablished by switch 131 when the FIG. 10 circuit is used with the circuits of FIGS. 11 and 12. A Radionuclide switch 133 is set to its OFF position when R is to be preset by switch 131 in this way. Switch 131 is an example of a means for selectively establishing this constant R. Microprocessor 103 provides a two-bit representation (RMSB, RLSB) of number R to a multiplexer 275 in FIG. 12 corresponding to the value of R selected on switch 131. Microprocessor 103 also computes and provides a digital representation of energy L2 to define the appropriate C window corresponding to the preestablished R, which digital representation is converted to analog form and applied as the reference signal for comparator 113. Thus microprocessor 103 is an example of a means for varying the second energy range in width as a function of the constant selectively established and for supplying an electrical reference signal representing a predetermined energy (e.g., L2) in the first energy range and thereby defining the upper limit of the second energy range as a function of the proportionality constant. The reference level can also be manually adjusted by a potentiometer 141 selectable for comparator 113 by an AUTO-MANUAL switch 143. As discussed hereinafter, R can conversely be computed for a preestablished window set by the Radionuclide switch 133, and output through a latch 135 to the circuitry of FIG. 29 in that embodiment. A pair of chip select AND-gates 125 and 127 respectively respond to signals on address lines A1l and A12 to enable DAC 112 or latch 135 on occurrence of a Write signal on a line R/W/under control of microprocessor 103. When Radionuclide switch 133 is not OFF, the value on R switch 131 is ignored by microprocessor 103. FIG. 11 shows a scatter filter interface 201 which has an analog-to-digital X,Y conversion stage 203, an A event/C event register 205, a memory address generating circuit 207, a memory array 75, an up/down counter 209, a digital comparator 211, and a digital-to-analog X,Y conversion stage 213. A microprogrammable controller 215 for interface 201 of FIG. 11 is shown in FIG. 12. Analog-to-digital conversion stage 203 has two HAS 1204 analog to digital converters ADC 221 and 223 respectively feeding two 12 bit registers 225 and 227 and associated discrete logic. ADCs 221 and 223 convert X and Y event position signals, which are nominally square analog pulses 4 to 10 microseconds wide, which may be bipolar in some cameras. The HAS 1204 converts a 2.5 to -2.5 volt signal to 12 bits of digital data in 2 microseconds. The status signal ST. of each converter 221 and 223 is used to strobe the respective converter output into the registers 225 and 227. The status signals are also combined in a NOR gate 229 to inform the controller 215 of FIG. 12 when the converters 221 and 223 have valid data. The A or C signals A.sub.IN and C.sub.IN from the circuit of FIG. 10 are strobed into an A/C event register 205 until needed. Register 205 is used because the A.sub.IN and C.sub.IN signals are only valid during the pulse Z.sub.IN and the controller 215 executes its operations in a time interval longer than that of 1 microsecond pulse Z.sub.IN. Address generation circuit 207 has a 16 bit counter 241 and a 3-to-8-line demultiplexer 243 for memory section selection. A demultiplexer 245 as well as demultiplexer 243 each drive 2 sets 247 and 249 of 8 light emitting diodes (LEDs), to indicate proper function of the interface 201. This combination creates enough address space for a 256 by 256 image in memory 75. Since only 16 bits are needed to create this address space only the top 8 bits of each ADC 221 and 223 are used. If more resolution is needed, then the address counter 241 length is increased and more bits of the ADCs 221 and 223 are used. Three control lines ACLEAR, ALOAD, and INC from controller 215 respectively clear, load, and increment counter 241. One output signal OVER from counter 241 indicates to the PROM controller 215 when and if the address counter 241 has overflowed. RAM memory 75 is a memory array of eight 6264 (64K by 8 bit) memory chips. Two signals R/W and OE from controller 215 control the reading/writing and the output of the memory 75. Up/down counter 209 is an 8 bit device with Q outputs fed to an 8-bit A input of comparator 211 and to the input of an 8-bit Schmitt input inverting tri-state buffer 251. Counter 209, comparator 211 and buffer 251 hold the output of the memory array 75, increment or decrement the counter 209, compare the counter contents to those of a memory location and route the counter output back to the memory array 75. The just-mentioned functions are controlled by five signals CCLEAR, CLOAD, UP, DWN AND COE from the PROM controller 215. Two signals EQUAL and NEGATIVE returned to the PROM controller 215 inform the controller of an equal comparator output or a negative value in the counter 209 respectively. Digital-to-analog conversion stage 213 has two HDH1205 digital to analog converters wh |