Gamma camera system

A gamma camera system having control components operating in conjunction with a solid state detector. The detector is formed of a plurality of discrete components which are associated in geometrical or coordinate arrangement defining a detector matrix to derive coordinate signal outputs. These outputs are selectively filtered and summed to form coordinate channel signals and corresponding energy channel signals. A control feature of the invention regulates the noted summing and filtering performance to derive data acceptance signals which are addressed to further treating components. The latter components include coordinate and energy channel multiplexers as well as energy-responsive selective networks. A sequential control is provided for regulating the signal processing functions, of the system to derive an overall imaging cycle.

 

We claim:

1. In a system for imaging the distribution within a region of interest of isotopic materials emitting radiation of given photon energy, said system including composite solid state detector means having a plurality of discrete components which are operationally associated to provide spatial coordinate parameter outputs representative of the spatial disposition of corresponding interactions of said radiation with said detector means, the improvement comprising:

first output treating means connected to receive said spatial coordinate parameter outputs of said detector means components, actuable to selectively filter and sum said outputs to derive corresponding coordinate channel signals and an energy channel signal having values corresponding respectively with said spatial disposition and given photon energy exhibited at a said interaction, said first output treating means further including control means for effecting said actuation to filter and sum and for deriving a data acceptance signal in correspondence with said coordinate and energy channel signals, said control means being responsive to a received reset signal to reset said first output means to a clear condition;

means including spatial coordinate multiplexer means and energy channel multiplexer means respectively coupled to be addressed by said energy channel signals, each said multiplexer means being responsive to a coded actuating signal to effect a transference of said channel signals addressed thereto;

process control means including memory means for receiving said data acceptance signals and selectively retaining them in received serialized fashion, and actuable to derive said coded actuating signal in correspondence with said serialized data acceptance signals;

sequential control means for selectively actuating said process control means and regulating an operational cycle of said system; and

second treating means responsive to said transferred channel signals for deriving readout information representative thereof.

2. The improved system of claim 1 including; storage means, having receive and hold modes, for receiving, when transferred, each said coordinate and energy channel signal when in said receive mode, and actuable to assume said hold mode retaining each said channel signal over a given interval, said storage means having outputs for asserting each said retained channel signal; and

wherein said sequential control means is configured for selectively actuating said storage means to derive and retain said hold mode for said given interval.

3. The improved system of claim 2 in which said sequential control means is configured for deriving and submitting said reset signal to said control means in correspondence with said storage means actuation.

4. In a system for imaging the distribution within a region of interest of isotopic materials emitting radiation exhibiting two or more levels of photon energy, said system including composite solid state detector means having a plurality of discrete components which are operationally associated to provide spatial coordinate parameter outputs representative of the spatial disposition of corresponding interactions of said radiation with said detector means, the improvement comprising:

first output treating means connected to receive said spatial coordinate parameter outputs of said detector means components, actuable to selectively filter and sum said outputs to derive corresponding coordinate channel signals and an energy channel signal having values corresponding respectively with said spatial disposition and the level of said photon energy exhibited at a said interaction, and including control means for effecting said actuation to filter and sum and for deriving a data acceptance signal in correspondence with said coordinate and energy channel signals, said control means being responsive to a reset said first output means to a clear condition;

means including spatial coordinate multiplexer means and energy channel multiplexer means respectively coupled to be addressed by said coordinate channel and said energy channel signals, each said multiplexer means being responsive to a coded actuating signal to effect a transference of said channel signals addressed thereto;

process control means including means for receiving said data acceptance signals to effect a de-randomization thereof and actuable to provide a said coded actuating signal in correspondence with a said data acceptance signal;

second treating means including divider network means responsive to a said energy channel multiplexer means transferred energy channel signal and to said spatial coordinate multiplexer means transferred signals for normalizing said spatial coordinate channel signals with respect to the said photon energy related value of their corresponding said energy channel signal, said second treating means being configured for deriving readout information corresponding with said normalized spatial coordinate channel signals; and

sequential control means for selectively actuating said process control means and regulating an operational cycle of said system.

5. The improved system of claim 4 wherein said divider network is configured to carry out said normalization by effecting a division of the value of each said spatial coordinate channel signal by the value of said energy channel signal.

6. The improved system of claim 5 wherein said second treating means include means for evaluating the peak value of each said energy channel signal with respect to predetermined upper and lower limit values selected in correspondence with each said level of photon energy exhibited by said isotopic materials.

7. In a system for imaging the distribution within a region of interest of isotopic materials emitting radiation of given photon energy or energies, said system including composite solid state detector means having a plurality of discrete components which are operationally associated within predetermined portions of said detector means to provide spatial coordinate parameter, x-, y- designated outputs representative of the spatial disposition of corresponding interactions of said radiation with said detector means, the improvement comprising:

first output treating means connected to receive said spatial coordinate parameter, x-, y- designated outputs of said components of each said predetermined portion of said detector means, actuable to selectively filter and sum said outputs when operating from a clear condition to derive corresponding x- and y- designated coordinate channel signals and an energy channel signal having values corresponding respectively with said spatial disposition and given energy exhibited at a said interaction, said first output treating means including first evaluating means responsive to the value of said given energy equaling or exceeding a predetermined value to derive a select output, and further including control means responsive to the presence of said evaluating means select output for effecting said actuation to filter and sum and for deriving a data acceptance signal in time correspondence with said coordinate channel and energy channel signals, said control means being responsive to a reset signal when submitted thereto to reset said first output treating means to said clear condition;

means including x-position multiplexer means, y-position multiplexer means and energy multiplexer means respectively coupled to be addressed by said x- and y- designated coordinate and energy channel signals, each said multiplexer means being responsive to a coded actuating signal to transfer said channel signals addressed thereto;

storage means, having receive and hold modes, for receiving each transferred said channel signal then in said receive mode, and actuable to assume said hold mode retaining each said channel signal over a given interval, said storage means having outputs for asserting each said retained channel signal;

process control means including asynchronous memory means for accepting and retaining said data acceptance signals in received serialized fashion and actuable to provide said coded actuating signal;

sequential control means for selectively actuating said process control means to effect selective transfer of said channel signals to said storage means, and for actuating said storage means to derive and retain said hold mode for said given interval; and

second treating means responsive to said channel signals asserted at said storage means outputs for deriving readout information representative thereof.

8. The improved system of claim 7 in which said process control means asynchronous memory means is configured and arranged for de-randomizing the receipt of said data acceptance signals to effect said serialization in a time domain independent of the rate of said receipt.

9. The improved system of claim 7 in which said sequential control means is configured and arranged for generating and submitting a said reset signal subsequent to said actuation of said storage means to assume said hold mode.

10. The improved system of claim 7 wherein:

said second treatment means includes second evaluating means for evaluating the peak value of each said energy channel signal over a given interval of time, and having a select output when said energy channel signal peak value lies within predetermined limits; and

said sequential control means is configured and arranged for generating said reset signal in the absense of said second evaluating means select output.

11. The improved system of claim 10 in which said sequential control means is configured and arranged for generating said reset signal in the absence of said second evaluating means select output and at the termination of said second evaluating means given interval of time.

12. The improved system of claim 7 in which said first output treating means includes peak detector means configured and arranged for receiving and retaining the peak value of a said summed, filtered x-, y- designated output deriving said energy channel signal at least until said storage means actuation.

13. The improved system of claim 7 in which said second treatment means includes divider network means responsive to the said x- and y- designated coordinate channel signals for normalizing said signals with respect to the photon energy of their derivative said interaction.

14. The improved system of claim 7 in which said second treatment means includes: first divider network means coupled to receive said x- designated coordinate channel signal asserted at a said storage means output; and second divider network means coupled to receive said y-designated coordinate channel signal asserted at a storage means output; said first and second divider networks being configured and arranged in operative association with the said energy channel signal output asserted at a said storage means output to effect a division of the values of said coordinate channel signals by the value of the corresponding said energy channel to provide x- and y- designated spatial signals normalized with respect to the said energy channel signal value; said second treatment means deriving said readout information from said normalized x- and y- designated spatial signals.

15. The improved system of claim 7 in which:

said process control means is configured and arranged for de-randomizing the receipt of said data acceptance signal to effect said serialization in a time domain independent of the rate of said receipt; and

wherein said sequential control means is configured and arranged for generating and submitting a said reset signal subsequent to said actuation of said storage means to assume said hold mode.

16. The improved system of claim 7 in which:

said second treatment means includes second evaluating means for evaluating the peak value of each said energy channel signal over a select interval of time, and having a select output when said energy channel signal peak value lies within predetermined limits; and

said sequential control means is configured and arranged for said actuation of said storage means to derive a said hold mode given interval substantially in correspondence with said second evaluating means select interval, and the generating said reset signal in the absence of said second evaluating means select output.

17. The improved system of claim 16 in which said first output treating control means is configured and arranged to reset said first output treating means to said clear condition in the absence of said first evaluating means select output.

18. The improved system of claim 17 in which said sequential control means is configured and arranged for generating and submitting a said reset signal subsequent to said actuation of said storage means to assume said hold mode.

19. The improved system of claim 18 in which said first output treating means includes peak detector means configured and arranged for receiving and retaining the peak value of a said summed, filtered x-, y- designated output deriving said energy channel signal at least until said storage means actuation.

20. The improved system of claim 19 in which said second treatment means includes divider network means responsive to the said x- and y- designated coordinate channel signals for normalizing said signals with respect to the photon energy of their derivative and interaction.

21. The improved system of claim 19 in which said second treatment means includes: first divider network means coupled to receive said x- designated coordinate channel signal asserted at a said storage means output; and second divider network means coupled to receive said y- designated coordinate channel signal asserted at a said storage means output; said first and second divider networks being configured and arranged in operative association with the said energy channel signal output asserted at a said storage means output to effect a division of the values of said coordinate channel signals by the value of the corresponding said energy channel signal to provide x- and y- designated spatial signals normalized with respect to the said energy channel signal value; said second treatment means deriving said readout information from said normalized x- and y- designated spatial signals.

22. The improved system of claim 21 in which said process control means asynchronous memory means is configured and arranged for de-randomizing the receipt of said data acceptance signals to effect said serialization in a time domain independent of the rate of said receipt.

23. In a system for imaging the distribution within a region of interest of isotopic material emitting radiation of given photon energy, said system including composite solid state detector means having a plurality of mutually adjacently disposed discrete detector components each having one of two oppositely disposed charge collecting surfaces positioned substantially within a common plane for exposure to said radiation, said components being arranged to establish linearly oriented groupings of respective said opposed surfaces, each said grouping of surfaces being electrically intercoupled and associated with impedance means for providing x- and y- designated coordinate outputs from respective mutually orthogonally aligned and oppositely disposed ones of said groupings associated with a common said detector component at which an interaction with said radiation corresponding with said coordinate output occurs, the improvement comprising:

x- coordinate output treating means responsive to said x- designated coordinate outputs, actuable to selectively filter and sum said outputs from a clear condition to derive corresponding x- designated coordinate channel signals and an energy signal having values corresponding respectively with a coordinate aspect of the spatial location of said interaction and the value of said photon energy thereof, and including control means for effecting said actuation to filter and sum and for deriving an x- data signal corresponding with said channel and energy signals, said control means being responsive to a reset signal when submitted thereto to reset said treating means to said clear condition;

y- coordinate output treating means responsive to said y- designated coordinate outputs, actuable to selectively filter and sum said outputs from a clear condition to derive corresponding y- designated coordinate channel signals and an energy signal having values corresponding respectively with a coordinate aspect of the spatial location of said interaction and the value of said photon energy thereof, and including control means for effecting said actuation to filter and sum and for deriving a y- data signal corresponding with said channel and energy signals, said control means being responsive to a reset signal when submitted thereto to reset said treating means to said clear condition;

means including x- position multiplexer means, y- position multiplexer means and energy multiplexer means, respectively coupled to be addressed by said x- and y- designated coordinate channel signals and at least one said energy signal, each said multiplexer means being responsive to a coded actuating signal to transfer said signals addressed thereto;

process control means including coincidence network means responsive the said x- data and y- data signals corresponding with a given said interaction for deriving a pair code signal, and asynchronous memory means responsive to said pair code signal for accepting and retaining said x- and y- data signals in received serialized fashion and actuable to provide and assert said coded actuating signal at said multiplexer means;

means responsive to said x- and y- designated coordinate channel signals when transferred from said x- position multiplexer means, said y- position multiplex means, and energy multiplexer means for deriving readout information representative thereof; and

sequential control means for selectively actuating said process control means to effect said transfer of said signals.

24. The improved system of claim 23 including:

storage means, having receive and hold modes, for receiving, when transferred, each said channel and energy signal when in said receive mode, and actuable to assume said hold mode retaining each said channel signal over a given interval, said storage means having outputs for asserting each said retained channel signal; and

wherein said sequential control means is configured and arranged for selectively actuating said storage means to derive and retain said hold mode for said given interval.

25. The improved system of claim 23 in which each said x- coordinate output treating means and said y- coordinate output treating means include first evaluating means responsive to the value of said energy signal equaling or exceeding a predetermined value to derive a select output; and said control means is responsive to the presence of said select output for effecting said actuation to filter and sum.

26. The improved system of claim 23 in which:

each said x- coordinate output treating means and said y- coordinate output treating means include a summing network for deriving said energy signal, said summing network including a stage deriving a derivative signal representing the time derivative of said energy values of said coordinate output signals; and

the said control means of each said output treating means is responsive to said derivative signal for deriving a designated said x- data or y- data signal.

27. The improved system of claim 26 in which:

each said x- coordinate output treating means and said y- coordinate output treating means includes comparator means responsive to the value of said summing network stage derivative signal equaling or exceeding a predetermined reference value for deriving a given output; and

each said control means of a said output treating means is responsive to said given output to effect said actuation and derive a designated said x- data or y- data signal.

28. The improved system of claim 23 in which said process control means asynchronous memory means is configured and arranged for de-randomizing the receipt of said x- and y- data signals to effect said serialization in a time domain independent of the rate of said receipt.

29. The improved system of claim 23 in which said sequential control means is configured and arranged for deriving and submitting said reset signal to said control means subsequent to said actuation of said process control means.

30. The improved system of claim 29 including:

storage means, having receive and hold modes, for receiving, when transferred, each said channel and energy signal when in said receive mode, and actuable to assume said hold mode retaining each said channel signal over a given interval, said storage means having outputs for asserting each said retained channel signal; and

wherein said sequential control means is configured and arranged for selectively actuating said storage means to derive and retain said hold mode for said given interval.

31. The improved system of claim 30 in which each said x- coordinate output treating means and said y- coordinate output treating means include first evaluating means responsive to the value of said energy signal equaling or exceeding a predetermined value to derive a select output; and said control means is responsive to the presence of said select output for effecting said actuation to filter and sum.

32. The improved system of claim 31 in which:

each said x- coordinate output treating means and said y- coordinate output treating means include a summing network for deriving said energy signal, said summing network including a stage deriving a derivative signal representing the time derivative of said energy values of said coordinate output signals; and

the said control means of each side output treating means is responsive to said derivative signal for deriving a designated said x- data or y- data signal.

33. The improved system of claim 31 in which:

each said x- coordinate output treating means and said y- coordinate output treating means include a summing network for deriving said energy signal, said summing network including a stage deriving a derivative signal representing the time derivative of said energy value of said coordinate output signals; and

the said control means of each said output treating means is responsive to said derivative signal for deriving a designated said x- data or y- data signal.

34. The improved system of claim 33 in which:

each said x- coordinate output treating means and said y- coordinate output treating means includes comparator means responsive to the value of said summing network stage derivative signal equaling or exceeding a predetermined reference value for deriving a given output; and

each said control means of a said output treating means is responsive to said given output to effect said actuation and derive a designated said x- data or y- data signal.

35. The improved system of claim 23 including: storage means, having receive and hold modes, for receiving, when transferred, each said channel and energy signal when in said receive mode, and actuable to assume said hold mode retaining each said channel signal over a given interval, said storage means having outputs for asserting each said retained channel signal;

wherein said sequential control means is configured and arranged for selectively actuating said storage means to derive and retain said hold mode for said given interval; and

wherein each said x- coordinate output treating means and said y- coordinate output treating means include first evaluating means responsive to the value of said energy signal equaling or exceeding a predetermined value to derive a select output; and said control means is responsive to the presence of said select output for effecting said actuation to filter and sum.

36. The improved system of claim 35 in which:

each said x- coordinate output treating means and said y- coordinate output treating means include a summing network for deriving said energy signal, said summing network including a stage deriving a derivative signal representing the time derivative of said energy values of said coordinate output signals; and

the said control means of each said output treating means is responsive to said derivative signal for deriving a designated and x- data or y- data signal.

37. The improved system of claim 36 in which said sequential control means is configured and arranged for generating and submitting a said reset signal subsequent to said actuation of said storage means to assume said hold mode.

38. The improved system of claim 36 wherein:

said means for deriving said readout information includes second evaluating means for evaluating the peak value of each said energy signal over a given interval of time, and having a select output then said energy signal peak lies within predetermined limits; and

said sequential control means is configured and arranged for generating and submitting a reset signal in the absence of said second evaluating means select output.

39. The improved system of claim 38 in which said sequential control means is configured and arranged for generating said reset signal in the absence of said second evaluating means select output and at the termination of said second evaluating means given interval time.

40. The improved system of claim 36 in which said means for deriving readout information includes divider network means responsive to the said x- designated coordinate channel signals and y- designated coordinate channel signals for normalizing said signals with respect to the photon energy of their derivative said interaction.

41. The improved system of claim 36 in which said means for deriving readout information includes; first divider network means coupled to receive a said x- designated coordinate channel signal asserted at a said storage means output; and second divider network means coupled to receive a said y- designated coordinate channel signal asserted at a said storage means output; said first and second divider networks being configured and arranged in operative association with the said energy signal output asserted at a said storage means output to effect a division of the values of said coordinate channel signals by the value of their corresponding said energy signal to provide x- and y- designated spatial signals normalized with respect to the said energy signal value; said second treatment means deriving said readout information from said normalized x- and y- designated spatial signals.

BACKGROUND

The field of nuclear medicine has long been concerned with techniques of diagnosis wherein radiopharmaceuticals are introduced into a patient and the resultant distribution or concentration thereof, as evidenced by gamma ray intensities, is observed or tracked by an appropriate system of detection. An important advantage of the diagnostic procedure is that it permits non-invasive investigation of a variety of conditions of medical interest. Approaches to this investigative technique have evolved from early pioneer procedures wherein a hand-held radiation counter was utilized to map body contained areas of radioactivity to more current systems for simultaneously imaging substantially an entire, in vivo, gamma ray source distribution. In initially introduced practical systems, scanning methods were provided for generating images, such techniques generally utilizing a scintillation-type gamma ray detector equipped with a focusing collimator which moved continuously in selected coordinate directions, as in a series of parallel sweeps, to scan regions of interest. A drawback to the scanning technique resides in the necessarily longer exposure times required for the derivation of an image. For instance, such time elements involved in image development generally are overly lengthy to carry out dynamic studies of organ function.

By comparison to the rectilinear scanner described above, the later developed "gamma camera" is a stationary arrangement wherein an entire region of interest is imaged at once. As initially introduced the stationary camera systems generally utilized a larger diameter sodium Iodide, Na I (TI) crystal as a detector in combination with a matrix of photomultiplier tubes. A multiple channel collimator is interposed intermediate the source containing subject of investigation and this scintillation detector crystal. When a gamma ray emanating from the region of investigative interest interacts with the crystal, a scintillation is produced at the point of gamma ray absorption and appropriate ones of the photomultiplier tubes of the matrix respond to the thus generated light to develop output signals. The original position of gamma ray emanation is determined by position responsive networks associated with the outputs of the matrix. For additional information concerning such cameras, see:

I. Anger, H.O., "A New Instrument For Mapping Gamma Ray Emitters," Biology and Medicine Quarterly Report UCRL-3653, 1957.

A continually sought goal in the performance of gamma cameras is that of achieving a high resolution quality in any resultant image. Further, it is desirable to achieve this resolution in combination with concomitant utilization of a highly versatile radionuclide or radiolabel, 99m-Technetium, having a gamma ray or photon energy in the region of 140 keV. A broadened clinical utility for the cameras also may be realized through the use and image identification of radiopharmaceuticals exhibiting more than one photon energy level. With such an arraignment, two or a plurality of diagnostic aspects simultaneously may be availed the operator. For example, in carrying out myocardial imaging, the above-identified 99m-Technetium might be utilized in conjunction with 111-Indium, the latter contributing photon energy in the regions of 173 and 247 KeV. Similarly, 81-Rubidium, exhibiting photon energy in the range of 350 KeV might be utilized in conjunction with 81-Krypton, the latter having gamma ray energy at about 120 KeV. The noted dual energy characteristic of 111-Indium also might be utilized to achieve two aspects of diagnostic data.

The resolution capabilities of gamma cameras incorporating scintillation detector crystals, inter alia, is limited both by the light coupling intermediate the detector and phototube matrix or array as well as by scatter phenomena of the gamma radiation witnessed emanating from within the in vivo region of investigation. Concerning the latter scattering phenomena, a degradation of resolution occurs from scattered photons which are recorded in the image of interest. Such photons may derive from Compton scattering into trajectories wherein they are caused to pass through the camera collimator and interact photoelectrically with the crystal detector at positions other than their point of in vivo derivation. Should such photon energy loss to the Compton interaction be less than the energy resolution of the system, it will effect an off-axis recordation in the image of the system as a photopeak photon representing false spatial information or noise. As such scattered photons record photopeak events, the noise increases and consequent resolution quality of the camera diminishes. For the noted desirable 140 KeV photons, the scintillation detector type camera energy resolution is approximately 15 KeV. With this resolution, photons which scatter through an angle from 0.degree. to about 70.degree. will be seen by the system as such photopeak events.

A continuing interest in improving the resolution qualities of gamma cameras has lead to somewhat extensive investigation into imaging systems incorporating relatively large area semiconductor detectors. Such interest has been generated principally in view of theoretical indications of an order of magnitude improvement in statistically limited resolution to provide significant improvements in image quality. In this regard, for example, reference may be made to the following publications: II. R. N. Beck, L. T. Zimmer, D. B. Charleston, P. B. Hoffer, and N. Lembares, "The Theoretical Advantages of Eliminating Scatter in Imaging Systems," Semiconductor Detectors in Nuclear Medicine, (P. B. Hoffer, R. N. Beck, and A. Gottschalk, editors), Society of Nuclear Medicine, New York, 1971, pp. 92-113.

III. R. N. Beck, M. W. Schuh, T. D. Cohen, and N. Lembares, "Effects of Scattered Radiation on Scintillation Detector Response," Medical Radioisotope Scintigraphy, IAEA, Vienna, 1969, Vol. 1, pp. 595-616.

IV. A. B. Brill, J. A. Patton, and R. J. Baglan, "An Experimental Comparison of Scintillation and Semiconductor Detectors for Isotope Imaging and Counting", IEEE Trans. Nuc. Sci., Vol. NS-19, No. 3, pp. 179-190, 1972.

V. M. M. Dresser, G. F. Knoll, "Results of Scattering in Radioisotope Imaging" IEEE Trans. Nuc. Sci., Vol. NS-20, No. 1, pp. 266-270, 1973.

Particular interest on the part of investigators has been paid to detectors provided as hybridized diode structures formed basically of germanium. To derive discrete regions for spatial resolution of impinging radiation, the opposed parallel surfaces of the detector diodes may be grooved or similarly configured to define transversely disposed rows and columns, thereby providing identifiable discrete regions of radiation response. Concerning such approaches to treating the detectors, mention may be made of the following publications:

VI. J. Detko, "Semiconductor Dioxide Matrix for Isotope Localization", Phys. Med. Biol., Vol. 14, No. 2, pp. 245-253, 1969.

VII. J. F. Detko, "A Prototype, Ultra Pure Germanium Orthogonal Strip Gamma Camera," Proceedings of the IAEA Symposium on Radioisotope Scintigraphy, IAEA/SM-164/135. Monte Carlo, October 1972.

VIII. R. P. Parker, E. M. Gunnerson, J. L. Wankling, and R. Ellis, "A Semiconductor Gamma Camera with Quantitative Output," Medical Radioisotope Scintigraphy.

IX. V. R. McCready, R. P. Parker, E. M. Gunnerson, R. Ellis, E. Moss, W. G. Gore, and J. Bell, "Clinical Tests on a Prototype Semiconductor Gamma-Camera," British Journal of Radiology, Vol. 44, 58-62, 1971.

X. Parker, R. P., E. M. Gunnerson, J. S. Wankling, R. Ellis, "A Semiconductor Gamma Camera with Quantitative Output," Medical Radioisotope Scintigraphy, Vol. 1, Vienna, IAEA, 1969, p. 71.

XI. Detko, J. F., "A Prototype, Ultra-Pure Germanium, orthogonal-Strip Gamma Camera," Medical Radioisotope Scintigraphy, Vol. 1, Vienna, IAEA, 1973, p. 241.

XII. Schlosser, P. A., D. W. Miller, M. S. Gerber, R. F. Redmond, J. W. Harpster, W. J. Collis, W. W. Hunter, Jr., "A Practical Gamma Ray Camera System Using High Purity Germanium," presented at the 1973 IEEE Nuclear Science Symposium, San Francisco, November 1973; also published in IEEE Trans. Nucl. Sci., Vol. NS-21, No. 1 February 1974, p. 658.

XIII. Owen, R. B., M. L. Awcock, "One and Two Dimensional Position Sensing Semiconductor Detectors," IEEE Trans. Nucl. Sci., Vol. NS-15, June 1968, p. 290.

In the more recent past, investigators have shown particular interest in forming orthogonal strip matrix detectors from p-i-n semiconductors fashioned from an ultra pure germanium material. In this regard, reference is made to U.S. Pat. No. 3,761,711 as well as to the following publications:

XIV. J. F. Detko, "A Prototype, Ultra Pure Germanium, Orthogonal Strip Gamma Camera," Proceedings of the IAEA Symposium on Radioisotope Scintigraphy, IAEA/SM-164/135, Monte Carlo, October, 1972.

XV. Schlosser, P. A., D. W. Miller, M. S. Gerber, R. F. Redmond, J. W. Harpster, W. J. Collis, W. W. Hunter, Jr., "A Practical Gamma Ray Camera System Using High Purity Germanium," presented at the 1973 IEEE Nuclear Science Symposium, San Francisco, November 1973; also published in IEEE Trans. Nucl. Sci., Vol. NS-21, No. 1, February 1974, p. 658.

High purity germanium detectors promise numerous advantages both in gamma camera resolution as well as practicality. For instance, by utilizing high purity germanium as a detector, lithium drifting arrangements and the like for reducing impurity concentrations are avoided and the detector need only be cooled to requisite low temperatures during its clinical operation. Readout from the orthogonal strip germanium detectors is described as being carried out utilizing a number of techniques, for instance, each strip of the detector may be connected to a preamplifier-amplifier channel and thence directed to an appropriate logic function and visual readout. In another arrangement, a delay line readout system is suggested with the intent of reducing the number of preamplifiers-amplifier channels, and a technique of particular interest utilizes a charge splitting method. With this method or technique, position sensitivity is obtained by connecting each contact strip of the detector to a charge dividing resistor network. Each end of each network is connected to a virtual earth, charge sensitive preamplifier. When a gamma ray interacts with the detector, the charge released enters the string of resistors and divides in relation to the amount of resistance between its entry point in the string and the preamplifiers. Utilizing fewer preamplifiers, the cost and complexity of such systems is advantageously reduced. A more detailed description of this readout arrangement is provided in:

XVI. Gerber, M. S., Miller, D. W., Gillespie, B., and Chemistruck, R. S., "Instrumentation For a High Purity Germanium Position Sensing Gamma Ray Detector," IEEE Trans. on Nucl. Sci., Vol. NS-22 No. 1, February, 1975, p. 416

To achieve requisite performance and camera image resolution, it is necessary that substantially all sources of noise or false information within the system be accounted for. In the absence of adequate noise resolution, the performance of the imaging systems may be compromised to the point of impracticality. Until the more recent past, charge-splitting germanium detector arrangements have not been considered to be useful in gamma camera applications in consequence of thermal noise anticipated in the above-noted resistor divider networks, see publication VII, supra. However, as will be evidenced in the description to follow, such considerations now are moot.

Another aspect in the optimization of resolution of the images of gamma cameras resides in the necessarily inverse relationship between resolution and sensitivity. A variety of investigations have been conducted concerning this aspect of camera design, it being opined that photon noise limitations, i.e. statistical fluctuations in the image, set a lower limit to spatial resolution. Further, it has been pointed out that the decrease in sensitivity witnessed in conventional high resolution collimators may cancel out any improvements sought to be gained in image resolution. A more detailed discourse concerning these aspects of design are provided, for instance, in the following publications:

XVII. E. L. Keller and J. W. Coltman, "Modulation Transfer and Scintillation Limitations in Gamma Ray Imaging," J. Nucl. Med. 9, 10, 537-545 (1968)

XVIII. B. Westerman, R. R. Sharma, and J. F. Fowler, "Relative Importance of Resolution and Sensitivity in Tumor Detection," J. Nucl. Med. 9, 12 638-640 (1968)

Generally, the treatment of the signals derived at the entrance detection portion of gamma cameras involves a form of spatial or coordinate identification of photons reaching the detector and additionally, a form of analysis of the energy of radiation reaching the detector. Spatial analysis may be carried out by difference summing circuits, while energy determination is carried out by additive summing circuits. Further, pulse height analyzers may be utilized as one discriminating component of a system for determining the presence of true or false imaging information. In any of the systems both treating noise phenomena and seeking a high integrity of spatial information, a control is required which carries out appropriate noise filtering while segregating true from false information. In addition to the foregoing, it is necessary that the "through-put rate" of the system be maximized in order that it may accommodate a highest number of bits or pulses representing spatial and energy data.

Another operational phenomenon tending to derogate from the spatial resolution quality performance of the cameras is referred to as "aliasing". This phenomenon represents a natural outgrowth of the geometry of the earlier-noted orthogonal strip germanium detector. A more detailed discussion of this aspect of the gamma cameras is provided at:

XIX. J. W. Steidley, et al., "The Spatial Frequency Response of Orthogonal Strip Detectors," IEEE Trans. Nucl. Sci., February, 1976.

To remain practical, it is necessary that the imaging geometry of stationary type gamma cameras provide for as large a field of view as practical. More particularly, such considerations require a camera field of view large enough to encompass the entire or a significant extent of the profiles of various organs of interest. Because of limitations encountered in the manufacture of detector crystals, for instance, high purity germanium crystals, the size of solid state detector components necessarily is limited. As a consequence, composite detector configurations are required which conjoin a plurality of smaller detector components to provide an imaging field of view or radiation acceptance geometry of effectively larger size. However, such union of a multitude of detector components must be carried out without the concurrent generation of noise phenomena and without a significant loss of image information validity and acuity. For instance, in the latter regard, spatial information must have a consistency of meaning across the entire extent of an ultimately displayed image of an organ, otherwise, clinical evaluation of such images may be encumbered. Preferred arrangements for inter-coupling the discrete detector components within an overall array thereof is described in a copending application for United States Patent by M. S. Gerber and D. W. Miller, entitled "Gamma Camera System With Composite Solid State Detector" filed Apr. 27, 1976, Ser. No. 680,754 and assigned in common herewith.

The control systems utilized with gamma cameras having multi-component detectors further are called upon to collect image data therefrom at an optimum rate while evaluating the validity thereof and assigning it an appropriate address function. Such address assignment may vary in nature depending upon the selected mode of circuit interrelationship of the discrete detector components with the array. An additional function of the control system is to identify the spatial position of the detector-photon interaction for select but different energy levels. This requires a technique for normalizing the spatial labels of such signals while properly evaluating the energy level states thereof as representing valid image information. The rapidity with which this data is treated, as by assigning spatial regional factors to it, as well as evaluating it for validity becomes a particularly important aspect of the control systems where they are contemplated for use in clinical dynamic function studies. With such studies, dynamic alterations in an image component occuring within any segment of the image area should be followed closely in correspondence with the actual movement of the image source. Accordingly, efficient image signal treatment by the camera system is required.

SUMMARY

The present invention is addressed to an improved system for imaging the distribution within a region of interest of isotopic materials emitting radiation. Characterized as a gamma camera, the system operates in conjunction with a solid state detector, for instance of the ultra-pure germanium variety, which is formed having a plurality of discrete components. These detector components are arranged in mutual adjacency to form a composite detector and, accordingly, are operationally associated with impedance deriving arrangements to provide spatial coordinate parameter outputs representing the spatial disposition of corresponding interactions of the radiation impinging upon the detector.

The association of the detector components may take on a variety of configurations. For example, the components are formed and arranged in the composite detector such that each has one of two oppositely disposed charge collecting surfaces positioned within a common plane for exposure to radiation. These components then are arranged to establish linearly oriented groupings of the respective surfaces, each of the groupings of surfaces being electrically intercoupled and associated with the noted impedance arrangement for providing coordinate outputs which may be designated as x- and y- designated coordinate parameter outputs. These outputs are derived from respective mutually orthogonally aligned and oppositely disposed ones of the groupings associated with a common detector component at which an interaction with radiation corresponding with the output occurs. Such an operational grouping of the components is generally referred to as being "row-column" in nature. The outputs of any predetermined grouping of the detector components are, in accordance with the invention, selectively filtered and summed to derive corresponding coordinate channel signals as well as an energy channel signal which have values related to the noted spatial disposition and given photon energy exhibited at an interaction with a given detector component. A control arrangement associated with the grouping regulates the noted summation and filtering and derives a data acceptance signal as well as carries out resetting functions to permit a next processing procedure to be carried out.

The system further includes spatial coordinate multiplexers and energy channel multiplexers which are arranged so as to be addressed by the noted coordinate channel and energy channel signals. Each of the multiplexers is connected for response to a coded actuating signal to provide proper transference of the channel signals to further processing treatment. In this regard, a process control arrangement including a memory circuit, receives the data acceptance signals and is arranged to selectively retain them in a serialized fashion. The memory circuit is actuable to derive the noted coded actuating signal in correspondence with the serialized data acceptance signals to effect the noted transference of the channel and energy signals. A sequential control means is provided for selectively controlling or actuating the process control and for regulating an overall operational cycle of the system. Further treating arrangements within the system respond to the transferred channel signals for deriving readout information representative thereof which may be used for clinical analysis purposes and the like.

As another object and feature, the invention further contemplates the provision of a storage arrangement within the control system which may be in the form of a series of sample and hold components serving, when in a receive mode, to receive the coordinate and channel signals derived from the noted multiplexers. Upon actuation to a hold mode, the signals are retained over a given interval while being asserted within additional signal treatment functions of the system. The noted sequential control function of the system is further utilized during this interval to effect the carrying out of the noted reset function associated with the control components immediately processing the outputs of the detector component groupings. In consequence, an improved throughput rate for the system is achieved to enhance the imaging capability of the camera.

Another object and feature of the invention is to provide a control system of the type described above wherein isotopic material sources of radiation, i.e. radiopharmaceuticals or the like, exhibiting more than one photon energy level may be provided for purposes of broadened clinical practicality. For such an arrangement, the imaging system incorporates components treating the noted spatial coordinate channel as well as energy channel signals transferred from the multiplexer function of the system and carries out a normalization operation over the spatial channel signals such that they are characterized as representing only accurate spatial information for imaging purposes. This operation is provided utilizing divider networks which are configured and arranged to, in effect, divide the spatial channel signals by their corresponding energy signal. The thus normalized signals then are transmitted to appropriate readout components of the system.

Another aspect and object of the invention provides evaluating features within the imaging system. For instance, an evaluating arrangement in the form of multi-channel analyser is incorporated to evaluate and respond to the peak values of a each energy channel signal submitted thereto as transferred from the noted multiplexing functions and/or the sample and hold components. The analysis performed is one wherein each energy signal peak value is evaluated with respect to predetermined upper and lower level window criteria which are pre-established in accordance with the known photon energy levels of the isotopic material distribution being imaged. In the event of a failure of a given energy channel signal to meet this window criteria, the control feature of the invention carries out a noted reset function to effect a short cycle performance of the system, thereby permitting a more rapid processing of a new quantum of image information. In one embodiment, two evaluating stages are utilized, one associated with that circuitry immediately treating the outputs of a predetermined number of the detector components while the second evaluation is carried out following a first evaluation and subsequent to the transference of the signal into later treatment stages.

A further object of the invention is to provide an improved system for imaging the distribution of isotopic materials which utilizes a composite solid state detector arrangement of the "row-column" variety described hereinabove. In such embodiment, the spatial coordinate signals, which may be designated x- and y- coordinate signals which are derived from select groupings of detector components, for instance four, are initially summed and filtered as described above and in the course of such summation, a time derivative of the energy signal is provided from each x- coordinate output and y- coordinate output processing arrangement to generate corresponding data signals. These data signals are then transmitted to a coincidence network which, in turn, generates a pair code signal which, in turn, is submitted to the earlier described memory arrangement. Accordingly, the spatial coordinate aspects of the x- and y- channel signals are established within processing system to properly locate the resultant spatial information signals within a readout component.

Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.

The invention, accordingly, comprises the system and apparatus possessing the construction, combination of elements, and arrangement of parts which are an exemplified by the following detailed disclosure.

For a fuller understanding of the nature and the object of the invention, reference should be had to the following detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a gamma camera arrangement as may utilize the improvement of the invention, showing, in block schematic form, general control functions;

FIG. 2 is a pictorial representation of a solid state orthogonal strip high purity germanium detector component incorporating a charge splitting resistor network in combination with preamplification electronics;

FIG. 3 is a schematic representation of a solid state strip detector and a schematic collimator functionally associated therewith as such system components relate to a radiation source within a region of clinical interest;

FIGS. 4(a)-4(c) are a schematic and graphical representation of the fundamental geometry associated with the interrelationship of a multi-channel collimator and a solid state detector;

FIG. 5 is a pictorial representation of a collimator array which may be utilized with the system of invention;

FIG. 6 is a pictorial view of two internested members of the collimator of FIG. 5;

FIGS. 7(a)-7(c) respectively and schematically depict representations of a source distribution as related with the geometry of an orthogonal strip detector and image readouts for illustrating aliasing phenomena;

FIGS. 8(a)-8(d) portray vertically aligned graphs relating modulation transfer function with respect to resolution as such data relates to aliasing phenomena, FIG. 8(a) showing collimator modulation transfer function (MTF.sub.c) with FWHM resolution of 1.33l, FIG. 8(b) showing a consequent alias frequency spectrum which is processed by the electronics of the camera system, FIG. 8(c) showing electronic MTF for given resolutions, and FIG. 8(d) showing camera system MTF's revealing aliasing introduced by the orthogonal strip solid state detector;

FIGS. 9(a)-9(d) provide curves showing the results of aliasing correction as compared with the curves of FIGS. 8(a)-8(d), FIG. 9(a) looking to collimator design as an anti-aliasing filter, FIG. 9(b) showing a consequent aliasing frequency spectrum which is processed by the electronics of the system, FIG. 9(c) showing the consequence of electronics used for anti-aliasing post-filtering, and FIG. 9(d) showing total system MTF revealing the elimination of aliasing phenomena;

FIG. 10 is an equivalent noise model circuit for solid state detectors as utilized in accordance with the instant invention;

FIG. 11 is a circuit model of a detector component and related resistor network, schematically representing a position-sensitive detector arrangement;

FIG. 12 is a block schematic diagram of a gamma camera control system configured as it is related to a single detector component output;

FIG. 13 is a schematic block diagram of a gated integrator configuration which may be utilized with the instant invention;

FIG. 14 is a schematic circuit representation of the configuration described in connection with FIG. 13;

FIG. 15 is a schematic representation of the logic components of a control arrangement which may be utilized with the system of the invention;

FIG. 16 is a circuit timing diagram corresponding with the schematic representation shown in FIG. 15;

FIG. 17 is a pictorial and schematic representation of an array of detector components showing the interconnections thereof to form a composite detector or region thereof as may be utilized with the system of the invention;

FIG. 18 is a block schematic representation of a control system utilized to receive and treat the outputs of the detector array configuration of FIG. 17;

FIG. 19 is a block schematic diagram of an embodiment of the control system of the invention as it is utilized for treating the signals developed by the control arrangement of FIG. 18;

FIG. 20 is a schematic and pictorial representation of another array of detector components, interconnected in accordance with a "row-column" readout geometry;

FIG. 21 is a schematic and pictorial representation of another array of detector components, each of which is formed associated with a surface type impedance arraignment, the components being interconnected in the noted "row-column" fashion;

FIG. 22 is a schematic and pictorial representation of another array of detector components interconnected in accordance with the noted "row-column" geometry;

FIG. 23 is a block schematic diagram of a control system utilized in treating one spatial channel output of the noted "row-column" detector component interconnection geometry;

FIG. 24 is a schematic block diagram of a control circuit operating in conjunction and cooperation with the control system of FIG. 23; and

FIG. 25 is a block diagram of a control arrangement for utilization with the noted "row-column" interconnection of detector components, the figure representing an alternate control arraignment within the diagram of FIG. 19.

DETAILED DESCRIPTION

In the discourse to follow, the control system of the invention initially is described in conjunction with the arrangements utilized for physically accepting gamma radiation from a clinically determined region of interest. In particular, initial acceptance techniques for collimating such radiation as well as parameters required for such collimation are set forth. Following that discussion, the discourse sets forth techniques for achieving optimised system performance with respect to noise characteristics which otherwise would be encountered with the solid state detector arrangement of the invention. Looking additionally to techniques for improving through-put rate characteristics for the system, the discussion initially is concerned with a control over a detector arrangement incorporating only a one detector component. Following this basic description, however, preferred techniques are set forth for associating a plurality of solid state detector components within a predetermined array or mosaic configuration. Such configurations and operational criteria therefore being established, the discussion then looks to a control system which may operate with radiopharmaceutical sources of more than one detectible energy level and which serves to treat resulting signals as well as label and address them to achieve practical overall imaging fields of view while maintaining efficient signal treatment.

As indicated in the foregoing, during contemplated clinical utilization, a gamma camera arrangement according to the instant invention is used to image gamma radiation within patients. Looking to FIG. 1, an exaggerated schematic representation of such a clinical environment is revealed generally at 10. The environment schematically depicts the cranial region 12 of a patient to whom has been administered a radio-labeled pharmaceutical, which pharmaceutical will have tended to concentrate within a region of investigative interest. Accordingly, radiation is depicted as emanating from region 12 as the patient is positioned on some supporting platform 14. Over the region 12 is positioned the head or housing 16 of a gamma camera. Extending outwardly from the sides of housing 16 are mounting flanges, as at 18 and 20, which, in turn, may be connected in pivotal fashion with an appropriate supporting assembly (not shown). Housing 16 also supports a vacuum chamber 22 defined by upper and lower vacuum chamber plates shown, respectively, at 24 and 26 conjoined with an angularly shaped side defining flange member 28. Lower vacuum chamber plate 26, preferably, is formed of aluminum and is configured having a thin entrance window portion 30, directly above which is provided an array of discrete solid state detector components, as shown generally at 32. Array 32, in turn, is operationally associated with the "cold finger" component 34 of an environmental control system, which preferably includes a cryogenic region refrigerating unit of a closed-cycle variety, shown generally at 36. An ion pump, as at 38, assures the integrity of the vacuum in chamber 22, such pump, in conjunction with the refrigerating unit 36, being mounted for association with chamber 22 through upper vacuum plate 24, the latter which may be formed, for instance, of stainless steel. Vacuum pump-down of the chamber 22 is accomplished by first using a sorption-type roughing pump, then using the ion pump shown to reduce and maintain the chamber pressure at 10.sup.-6 Torr or less.

Electronics incorporated within chamber 22 include preliminary stages of amplification, for instance field effect transistors (FET's) as at 40 which are mounted upon a plate 42 coupled, in turn, between cold-finger 34 and side channel 28. Thus connected, the plate 40 evidences a temperature gradient during the operation of the unit which provides a selected ideal temperature environment of operation for the amplification stages. The outputs of these stages are directed through subsequent stage electronics, shown within a housing 44, which, in turn, provides electrical communication to externally disposed control electronics through conduit 46 and line 48. To provide for appropriate operation, chamber 22 generally is retained at a temperature of, for instance, about 77.degree. K, while the FET's, 40, mounted upon plate 42, are retained at about 130.degree. K to achieve low noise performance.

Mounted outwardly of window portion 30 and in allignment with the detector array 32 is a collimator, shown generally at 50. During the operation of the gamma camera, radiation emanating from source 12 is spatially coded initially atcollimator 50 by attenuating or rejecting off-axis radiation representing false image information. That radiation passing collimator 50 impinges upon detector array 32 and a significant portion thereof is converted to discrete charges or image signals. Detector array 32 is so configured as to distribute these signals to resistor chains as well as the noted preamplification stages 40 retained within chamber 22 to provide initial signals representative of image spatial information along conventional coordinate axes as well as representing values for radiation energy levels. This data then is introduced, as represented schematically by line 48, to filtering and logic circuitry which operates thereupon to derive an image of optimized resolution and veracity. In the latter regard, for instance, it is desired that only true image information be elicited from the organ being imaged. Ideally, such information should approach the theoretical imaging accuracy of the camera system as derived, for instance, from the geometry of the detector structure 32 and collimator arrangement 50 as well as the limitations of the electronic filtering and control of the system.

Image spatial and energy level signals from line 48 initially, are introduced into Anti-Symmetric Summation and Energy Level Derivation functions represented at block 52. As is described in more detail later herein, the summation carried out at block 52 operates upon the charges directed into the resistive chains or networks associated with the orthogonal logic structuring of detector array 32 to derive discrete signals or charge values corresponding with image element location. Additionally, circuitry of the function of block 52 derives a corresponding signal representing the energy levels of the spatial information. The output of block 52 is directed to Filtering Amplification and Energy Discrimination functions as are represented at block 54. Controlled from a Logic Control function shown at block 56, function 54 operates upon the signal input thereto to accommodate the system to parallel and series defined noise components through the use of Gaussian amplification or shaping, including trapezoidal pulse shaping of data representing the spatial location of image bits or signals. Similarly, the energy levels of incoming signals are evaluated, for instance, utilizing, multiple channel analyzer components controlled by logic circuitry at 56 to establish energy level windows for data received within the system. In this regard, signals falling above and below predetermined energy levels are considered false and are blocked. From Amplification and Discrimination stage 54 and Logic Control 56, the analyzed signals are directed into an Information Display and Readout Function, as is represented at block 58. Components within function block 58 will include display screens of various configurations, image recording devices, for instance, photographic apparatus of the instant developing variety, radiation readout devices and the like, which are controlled at the option of the system operator.

As outlined above, the instant description now looks in more detail to the configuration of the collimator structure 50. To facilitate such description, however, the structure of a single component within the detector array 32 is described in conjunction with FIG. 2. Later discussion and figures will reveal the interrelationships of such impedance networks and their equivalents as they are operatively associated with a multi-component detector array. Looking to that figure, an exaggerated pictorial representation of such a component of the detector array is revealed at 60. Detector component 60 may be fabricated from p-type high purity germanium by depositing an n-type contact on one face and a p-type contact on the opposite face of a rectangular planar crystal. Accordingly, a high purity germanium region of the crystal, as at 62, serves as an intrinsic region between p-type semiconductor region contacts 64 and n-type semiconductor region contacts as at 66. The intrinsic region 62 of the p-i-n detector components forms a region which is depleted of electrons and holes when a reverse bias is applied to the contacts. Grooves as at 68a-68c are cut into the continuous p-type contact or region at one face of the component to form strips of isolated p-type semiconductor material. On the opposite face of the detector component, orthogonally disposed n-type semiconductor strips similarly are formed through the provision of grooves 70a-70c. Configured having this geometry, the detector component 60 generally is referred to as an orthogonal strip detector or an orthogonal strip array semiconductor detector component. The electrode strips about each of the opposed surfaces of component 60, respectively, are connected to external charge splitting resistor networks revealed generally at 72 and 74. Resistor network 72 is formed of serially coupled resistors 76a -76e which, respectively, are tapped at their regions of mutual interconnection by leads identified, respectively, at 78a-78d extending, in turn, to the orthogonal strips. The opposed ends of network 72 terminate in preamplification stages 80 and 82, the respective outputs of which, at 84 and 86, provide spatial output data for insertion within the above-described summation and energy level derivation function 52 to provide one detector component orthogonal or coordinate output, for instance, designated as a y-axis signal.

In similar fashion, network 74 is comprised of a string of serially coupled resistors 88a-88e, the mutual interconnections of which are coupled with the electrode strips at surface 66, respectively, by leads 90a-90e. Additionally, preamplification stages as at 92 and 94 provide outputs, respectively, at lines 96 and 98 carrying spatial data or signals representative of image information along an x axis or axis orthogonally disposed with respect to the output of network 72.

With the assertion of an appropriate bias over detector component 60, as described in U.S. Pat. No. 3,761,711, any imaging photon absorbed therewithin engenders ionization which, in turn, creates electron-hole pairs. The charge thusly produced is collected on the orthogonally disposed electrode strips by the bias voltage and such charge flows to the corresponding node of the impedance networks 72 and 74. Further, this charge divides in proportion to the admittance of each path to the virtual ground input of the apropriate terminally disposed preamplification stage. Such charge-sensitive preamplification stage integrates the collected charge to form a voltage pulse proportional to that charge value. Assigning charge value designations Q.sub.1 and Q.sub.2, respectively, for the outputs 98 and 96 of network 74, and Q.sub.3 and Q.sub.4, respectively, for the output lines 84 and 86 of network 72, the above-noted Summation and Energy Level Derivation functions for spatial and energy data may be designated. In this regard, the x-position of each diode defined by the orthogonal strip geometry is found to be proportional to Q.sub.1, Q.sub.2, and their difference i.e. (Q.sub.1 -Q.sub.2), and the y-position is proportional to Q.sub.3, Q.sub.4, and their difference i.e. (Q.sub.3 -Q.sub.4). The energy of the incident gamma ray is proportional to Q.sub.1 +Q.sub.2, and (Q.sub.3 +Q.sub.4), and [(Q.sub.1 +Q.sub.2) - (Q.sub.3 +Q.sub.4)] or in the latter expression, [(Q.sub.3 +Q.sub.4) - (Q.sub.1 +Q.sub.2)]. As noted above, the operational environment of the detector array 32 and associated amplification stages is one within the cryogenic region of temperature for purposes of avoiding Johnson noise characteristics and the like.

As a prelude to a more detailed consideration of the spatial resolution of gamma radiation impinging upon the entrance components of the gamma camera, some value may be gleaned from an examination of more or less typical characteristics of that impinging radiation. For instance, looking to FIG. 3 a portion of a patient's body under investigation is portrayed schematically at 100. Within this region 100 is shown a radioactively tagged region of interest 102, from which region the decay of radiotracer releases photons which penetrate and emit from the patient's body. These photons are then spatially selected by a portion of collimator 50 and individually detected at component 60 for ultimate participation in the evolution of an image display. The exemplary paths of seven such photons are diagrammed in the figure, as at a-g, for purposes of illustrating this initial function which the camera system is called upon to carry out. In this regard, the function of collimator 50 is to accept those photons which are traveling nearly perpendicular to the detector, inasmuch as such emanating rays provide true spatial image information. These photons are revealed at ray traces, a, and, b, showing direct entry through the collimator 50 and appropriate interaction coupled with energy exchange within detector component 60. Photon path, c, is a misdirected one inasmuch as it does not travel perpendicularly to the detector. Consequently, for appropriate image resolution such path represents false information which should be attenuated, as schematically portrayed. Scattering phenomena within collimator 50 itself or the penetration of the walls thereof allows "non-collimated" photons, i.e. ray traces, d, and e, to reach the detector. Photon path trace, f, represents Compton scattering in the patient's body. Such scattering reduces the photon energy but may so redirect the path direction such that the acceptance geometry of the camera, including collimator 50, permits the photon to be accepted as image information. Inasmuch as the detector component 60 and its related electronics measure both the spatial location and energy of each photon admitted by the collimator, the imaging system still may reject such false information. For example, in the event of a Compton scattering of a photon either in the patient or collimator, the energy thereof may have been reduced sufficiently to be rejected by an energy discrimination window of the system. Photon path, g, represents a condition wherein component 60 exhibits inefficient absorption characteristics such that the incident photon path, while representing true information, does not interact with the detector. As is apparent from foregoing, each of the thousands of full energy photons which are absorbed at the detector ultimately are displayed at their corresponding spatial location on an imaging device such as a cathode ray tube to form an image of the source distribution within region 102 of the patient. Of course, the clinical value of the gamma camera as a diagnostic implement is directly related to the quality of ultimate image resolution.

As is revealed from the foregoing discourse, the imaging resolution of the camera system is highly dependent upon the quality of collimation exhibited at the entrance of the camera by collimator 50. Generally, collimator 50 is of a multichannel, parallel-hole variety, its performance being dictated by its fundamental geometric dimensions, the material with which it is formed, and the technique of its fabrication. Referring to FIGS. 4(a)-4(c), a designation of the geometric aspects of collimator 50, as such aspects relate to photon path travel, and spatial intensity distribution over the corresponding spatial axis of detector component 60 are shown schematically. FIG. 4(b) shows the photon intensity distribution at the mid-plane 60' of the detector due to a line source of radiation at distance B from the collimator 50 outwardly disposed plane defining side. Note that the source position is designated "L." Source point, L, is located, for purposes of the instant analysis, within a plane 104 lying parallel to the outwardly disposed plane defining side of collimator 50 as well as its inwardly disposed plane defining side and the plane defined by the midpoint 60' of detector 60. The intensity distribution pattern of photons, revealed in FIG. 4(b), is provided under the assumption that the collimator 50 is fixed in position. FIG. 4(a), on the other hand, assumes that the collimator 50 moves during an exposure and produces, in consequence, a triangular intensity distribution pattern of photons. A location of value "R" designates a full width at half maximum (FWHM) spatial resolution. Such spatial or position resolution capability of the camera system may be defined utilizing several approaches. However, for the latter designation, FWHM, is derived from a consideration that if a very small spot of radiation exits at the object plane, the image generally will be a blurred spot with radially decreasing intensity. The position resolution then is defined as twice the radial distance at which the intensity is half of the center intensity.

Looking in particular to FIG. 4(c), considering the similar triangles EFG and LMN, the resolution of collimator 50 generally may be expressed as:

R.sub.c = (D/A.sub.E) (A + B + C) (1)

where

A = the collimator thickness,

A.sub.E = the effective collimator thickness due to septal penetration,

B = the source to collimator distance,

C = the collimator to detector midplane distance and

D = the effective diameter of each channel within the multi-channel collimator

Effective diameter, D, is considered to be the square root of the cross-sectional area of a given collimator channel multiplied by 1.13

The effective collimator thickness is given approximately by: ##EQU1## where .mu.(E) is the attenuation coefficient of the collimator material at a photon energy, E.

For a given collimator material, sufficiently thick septal walls are required to reduce the number of photons or gamma rays that enter within a given collimator channel, penetrate the septal wall thereof and exit through an adjacent or other channel opening. Looking to FIG. 4(c), one such gamma ray or photon path is traced as UV. Note, that for this condition, the photon or ray passes through a collimator vane or channel side of thickness, T, along a minimum septal distance, W, thereby allowing the ray or photon to exit from a channel adjacent the channel of initial entrance. The fraction of photons or rays traveling UV that actually penetrate the septal wall is given by the penetration fraction:

P = exp (-.mu.(E) W). (3)

it is considered the practice of the art to design the collimator structure such that the penetration fraction, P, is given a value less than about 5%. In this regard, mention may be made of the following publication:

XX. H. O. Anger, "Radioisotope Cameras," Instrumentation in Nuclear Medicine, G. J. Hine, ed. Vol. 1, Academic Press, New York, 485-552 (1967).

The minimum septal distance, W, is found from the similar triangles IJK and UVY approximatley as: ##EQU2## by assuming A is greater than 2D + T where T, as noted above, is the septal wall thickness. Solving equations (3) and (4) for the septal wall thickness, T, gives: ##EQU3## The value, T, as set forth in equation (5) serves to define that minimal septal thickness for collimator 50 which is required for a given penetration fraction, P.

The geometric efficiency of the collimator is defined as the ratio of the number of gamma rays or photons which pass through the collimator to the number of photons or gamma rays emitted by the source. Described in terms of the collimator parameters, such efficiency may be given by: ##EQU4## where K = 0.238 for hexagonally packed circular holes and 0.282 for square holes or chambers in a square array.

As described above, the clinical value of the gamma camera imaging system stems importantly from the systems' capability for achieving quality image resolution. Given the optimum image resolution which is practically available, it then is desirable to provide a design which achieves a highest efficiency for that resolution. For a collimator design, it is desirable to provide a low septal penetration fraction as well as a practical fabrication cost. Further, an inspection of equations (1) and (6), given above for collimator resolution and geometric efficiency, respectively, reveals that as resolution is enhanced, the efficiency of the collimator is dimenished. It has been determined that a multichannel, parallel-hole collimator, the channels of which are configured having square cross sections represents a preferred geometric design feature. In this regard, where the latter are compared with collimator channels formed having round holes, hexagonally packed arrays or hexagonally packed bundles of tubes all of given identical dimensions, resolution remains equivalent, but the efficiency of the preferred square cross sectional channel array will be a factor of 1.4 times greater than the round hole design, while the efficiency of the hexagonally packed bundle of tubes will be intermediate the efficiency value of the above two designs. Consequently, as noted above, on the basis of maximum efficiency at a desired resolution, the square hole cross sectional chamber design is preferred.

Concerning the materials which may be selected for constructing the collimator, those evidencing a high density, high atomic number characteristic are appropriate for consideration. In particular, mention may be made of tungsten, tantalum and lead for the purpose at hand. The primary criterion for the material is that of providing a short mean free path at the photon energy level of interest. For the desirable energy level of 140 keV, the mean free path for photon attenuation is 0.012 inch in tungsten, 0.015 inch in tantalum and 0.016 inch in lead. Accordingly, for a selection based upon a mean free path for attenuation, tungsten represents the optimum collimator material. Heretofore, however, pragmatic considerations of machineability or workability have required a dismissal of the selection of tungsten and/or tantalum for collimator fabrication. For instance, for multi-channel collimators having round channel cross sections, tungsten and tantalum are too difficult and, consequently, too expensive for drilling procedures and, in general, hexagonally packed arrays providing such cross sections are restricted to fabrication in lead. Similarly, other designs formed out of the desired materials do not lend themselves to conventional machining and forming techniques, the cost for such fabrication being prohibitive even for the sophisticated camera equipment within which the collimator units are intended for utilization.

In the instant preferred arrangement, a square hole collimator design, fabricable utilizing the optimum material tungsten, is provided. Revealed in perspective fashion in FIG. 5, the collimator is shown to comprise an array of mutually parallel adjacently disposed channels having sides defining a square cross section. These channels extend to define inwardly and outwardly disposed sides which are mutually parallel and the channels are formed axially normally to each of these side planes. The highly desirable square structure shown in FIG. 5 is achieved utilizing the earlier described preferred tungsten material or tantalum, such mateials normally being difficult or impractical to subject to more conventional manufacturing procedures. However, practical assembly of the collimator array 50 is achieved through the use of a plurality of discrete rectangularly shaped sheet members, as are revealed in the partial assembly of the collimator 114 shown in FIG. 6. Referring to that figure, note that member 110 is formed as a flat rectangular sheet of height, h, corresponding with desired collimator thickness, A. Formed inwardly from one edge of member 110 are a plurality of slots spaced in regularly recurring parallel fashion and identified generally at 112. Slots 112 are formed having a height equivalent to h/2and are mutually spaced to define a pitch or center-to-center spacing D + T. The slots are formed having a width of T +e, where e will be seen to be a tolerance. When the plurality of sheet members, for instance, as shown at 110 and 114 are vertically reversed in mutual orientation and the corresponding slots, respectively, as at 112 and 116 are mutually internested as shown, the collimator may be built-up to desired dimensions without recourse to elaborate forming procedures. Note that the width of slots 112 and 116 closely approximates the width of each of the sheet members within the array with a controlled allowance for tolerances. In determining the value for the above described pitch of the regularly recurring slots within the sheet members, assuming resolution criteria are met, a spacing may be selected to match the center-to-center electrode strip spacing of a detector component 60 or a multiple thereof so that the septal walls for the collimator 50 can be aligned with less active grooves formed within the detector. Practical fabrication techniques are available for forming the slots as exemplified at 112 and 116. In particular, chemical milling or chemical machining techniques are available for this purpose. With such techniques, a wax type mask is deposited over the sheets to be milled, those material portions designated for removal being unmasked. The sheets then are subjected to selected etchants whereupon the slots are formed. Following appropriate cleaning, the sheet members then are ready for the relatively simple assembly build-up of a completed collimator. Through the use of such chemical milling techniques, desired tolerances in forming the slots are realizable. By utilizing the collimator structure shown in com