GB1578883A - Gamma camera - Google Patents

Description

PATENT SPECIFICATION ( 11) 1 578 883

M ( 21) Application No 31325/79 ( 22) Filed 4 Jan 1977 O O ( 62) Divided out of No 1578881 ( 19) X ( 31) Convention Application No 656304 0 ( 32) Filed 9 Feb 1976 in E ( 33) United States of America (US) ( 44) Complete Specification published 12 Nov 1980 _ ( 51) INT CL 3 GOIT 1/24 G 21 K 1/02 ( 52) Index at acceptance H 4 F D 18 K D 27 M D 30 H D 68 D 83 X L H 5 R 3 ( 72) Inventors PHILIP A SCHLOSSER and JOHN W STEIDLEY ( 54) A GAMMA CAMERA ( 71) We, THE OHIO STATE UNIVERSITY, of 190 N Oval Drive Columbus, Ohio, 43210, United States of America do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:-

This invention relates to a gamma camera 5 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 noninvasive 10 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 15 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 20 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 25 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 the subject of investigation and this scintillation detector crystal When a gamma ray emanating from the region of 30 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 35 Particular interest on the part of investigators has been paid to detectors forms as hybridized diode structures fashioned basically of germanium To provide 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 40 regions of radiation response.

In accordance with this invention therefore we provide, a gamma camera for deriving image defining information of the source distribution of gamma rays providing a photon energy level, E, of interest, said device including an orthogonal strip array semi-conductor for deriving spatial and 45 energy level information corresponding with said distribution, said detector array of strips having a centre-to-centre strip spacing, L, said device further including a collimator operatively associated with said detector and having an inwardly disposed plane defining side spaced from the midplane of said detector a distance, C, an outwardly disposed plane defining side spaced from said inward side to define 5 thickness, A, and spaced from said source a distance, B; wherein said collimator comprises an array of adjacently disposed channels having internal surfaces and disposed intermediate of said inward and outward sides, said array being configured to define a septal thickness, T, intermediate said channels, an effective collimator thickness, AE=A-l 2/p(E)l, where u (E) is the attenuation coefficient of 10 the surface defining material of said channels for said energy level, E, said channels having a channel cross sectional area of effective diameter, D; and said collimator has a collimator resolution R, equal to or greater than about 1 7 (L) and being configured in substantial satisfaction of the expression:

D Rc 15 AE(A+B+C) 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 wherein.

Figure 1 is a schematic representation of a gamma camera arrangement utilizing a collimator in accordance with this invention and showing, in block 20 schematic form, general control functions; Figure 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; Figure 3 is a schematic representation of a solid state strip detector and a 25 schematic collimator functionally associated therewith as such system components relate to a radiation source within a region of clinical interest; Figures 4 (a)-4 (c) are a schematic and graphical representation of the fundamental geometry associated with the interrelationship of a multichannel collimator and a solid state detector; 30 Figure 5 is a pictorial representation of a collimator array invention; Figure 6 is a pictorial view of two internested members of the collimator of Figure 5; Figures 7 (a)-7 (c) respectively and schematically depict representations of a source distribution as related with the geometry of an orthogonal strip detector and 35 image readouts for illustrating aliasing phenomena; Figures 8 (a)-8 (d) portray vertically aligned graphs relating modulation transfer function with respect to resolution as such data relates to aliasing phenomena, Figure 8 (a) showing collimator modulation transfer function (MTFC) with FWHM resolution of 1 33 1, Figure 8 (b) showing a consequent alias frequency 40 spectrum which is processed by the electronics of the camera system, Figure 8 (c) showing electronic MTF for given resolutions, and Figure 8 (d) showing camera system MTF's revealing aliasing introduced by the orthogonal strip solid state detector; Figures 9 (a)-9 (d) provide curves showing the results of aliasing correction as 45 compared with the curves of Figures 8 (a)-8 (d), Figure 9 (a) looking to collimator design as an anti-aliasing filter, Figure 9 (b) showing a consequent aliasing frequency spectrum which is processed by the electronics of the system, Figure 9 (c) showing the consequence of electronics used for anti-aliasing postfiltering, and Figure 9 (d) showing total system MTF revealing the elimination of aliasing 50 phenomena; Figure 10 is an equivalent noise model circuit for solid state detectors; Figure 11 is a circuit model of a detector component and related resistor network, schematically representing a position-sensitive detector arrangement; Figure 12 is a pictorial and schematic representation of an array of detector 55 components showing the interconnections thereof to form a composite detector or region thereof; Figure 13 is a schematic and pictorial representation of another array of detector components, interconnected in accordance with a "row-column" readout geometry; 60 Figure 14 is a schematic and pictorial representation of another array of 1,578,883 3 1,578,883 3 detector components, each of which is formed associated with a surface type impedance arrangement, the components being interconnected in the noted "rowcolumn" fashion; and Figure 15 is a schematic and pictorial representation of another array of detector components interconnected in accordance with the noted "rowcolumn" 5 geometry; In the discourse to follow, a control system 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 10 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 Looking additionally to techniques for improving through-put rate characteristics for the system, the discussion initially is concerned with a control 15 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.

As indicated in the foregoing, during contemplated clinical utilization, a 20 gamma camera arrangement according to the instant invention is used to image gamma radiation within patients Looking to Figure 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 25 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 30 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 35 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 40 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 45 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 50 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 55 generally is retained at a temperaure, of for instance about 770 K, while the FET's, 40, mounted upon plate 42, are retained at about 130 'K to achieve low noise performance.

Mounted outwardly of window portion 30 and in alignment with the detector array 32 is a collimator shown generally at 50 During the operation of the gamma 60 camera, radiation emanating from source 12 is spatially coded initially at collimator 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 65 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 5 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 33 as well as the limitations of the electronic filtering and control of 10 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 which operates upon the charges directed into the resistive chains or networks associated with the orthogonal logic structuring of detector array 32 to 15 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 20 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 for instance, multiple channel analyzer 25 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 level 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 30 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 35 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 Figure 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 40 figure, an exaggerated pictorial representation of such a component of the detector array is revealed at 60 Detector component 60 may be fabricated from ptype high purity germanium by depositing an n-type contacting on one face and a ptype 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 45 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 a region which is depleted of electrons and holes when a reverse bias is applied to the contacts Grooves as at 68 a-68 c are cut into the continuous p-type contact or region at one face of the component to form strips of isolated p-type 50 semiconductor material On the opposite face of the detector component, orthogonally disposed n-type semiconductor strips similarly are formed through the provision of grooves 70 a-70 c 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 55 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 76 a-76 e which, respectively, are tapped at their regions of mutual inter-connection by leads identified, respectively, at 78 a-78 d extending in turn, to the orthogonal strips The 60 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 in coordinate output, for instance, designated as y-axis signal 65 1,578,883 In similar fashion, network 74 is comprised of a string of serially coupled resistors 88 a-88 e, the mutual interconnections of which are coupled with the electrode strips at surface 66, respectively, by leads 90 a-90 e 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 5 axis or axis orthogonally disposed with respect to the output of network 72.

With the assertion of an appropriate bias over detector component 60, 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 10 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 appropriate terminally disposed preamplification stage Such chargesensitive preamplification stage integrates the collected charge to form a voltage pulse proportion to that charge value Assigning charge value designations Q 1 and Q 2, 15 respectively, for the outputs 98 and 96 of network 74, and Q 3 and Q 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 1, Q 2, and their difference i e (Q 1-Q 2), 20 and the y-position is proportional to Q 3, Q 4, their and difference i e (Q 3-Q 4) The energy of the incident gamma ray is proportional to Q 1 +Q 2, and (Q 3 +Q 4), and l(Q 1 +Q 2)-(Q 3 +Q 4)l or in the latter expression, l(Q 3 +Q 4)-(Q 1 +Q 2)l 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 25 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 cleaned from an examination of more or less typical characteristics of that impinging radiation For instance, looking to Figure 3 a 30 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 35 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 travelling nearly perpendicular to the detector, inasmuch as such emanating rays provide true 40 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 45 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 50 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 55 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, 60 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 s 1,578,883 diagnostic implement is directly related to the quality of ultimate image resolution.

As is revealed from the foreoing disclosure, 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 5 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 Figures 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 10 component 60 are shown schematically Figure 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 15 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 Figure 4 (b), is provided under the assumption that the collimator 50 is fixed in position Figure 4 (a), on the other hand, assumes that the collimator 50 moves during an exposure and produces, in 20 consequence, a triangular intensity distribution pattern of photons A location of value -R" designates a full width 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 25 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 Figure 4 (c), considering the similar triangles EFG and LMN, the resolution of collimator 50 generally may be expressed as: 30 D Rc (I) AE(A+B +C) where A=the collimator thickness, AE=the effective collimator thickness due to septal penetration, B=the source to collimator distance, 35 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 crosssectional area of a given collimator channel multiplied by 1 13.

The effective collimator thickness is given approximately by: 40 2 AE=A ( 2) u(E) where pu(E) is the attenuation coefficient of the collimator material at the 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 45 channel, penetrate the septal wall thereof and exit through an adjacent or other channel opening Looking to Figure 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 50 of initial entrance The fraction of photons or rays travelling UV that actually penetrate the septal wall is given by the penetration fraction:

P=exp (-M(E)W) ( 3) It is considered the practice of the art to design the collimator structure such 1.579581 that the penetration fraction, P, is given a value less than about 50 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 5 UVY approximately as:

AT W= ( 4) 2 D+T by assuming A is greater than 2 D+T where T, as noted above, is the septal wall thickness Solving equations ( 3) and ( 4) for the septal wall thickness, T, gives:

-2 D In P T ( 5) 10 A(E) A+ln P 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 arrays emitted by the source Described in terms of the 15 collimator parameters, such efficiency may be given by:

lK l 2 ( 6) 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 a gamma camera imaging system 20 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 efficiencyfor 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 25 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 diminished It has been determined that a multi-channel, 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 30 compared with collimator channels formed has 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 35 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 40 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 ke V, 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, 45 for a selection based upon a mean free path for attenuation, tungsten represents the optimum collimator material Heretofore, however, pragmatic considerations of machine ability or workability have required a dismissal of the selection of tungsten and/or tantalum for collimator fabrication For instance, for multichannel collimators having round channel cross sections, tungsten and tantalum are too 50 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 1,578,883 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 S perspective fashion in Figure 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 Figure 5 is 10 achieved utilizing the earlier described preferred tungsten material or tantalum, such materials 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 retangularly shaped sheet members, as are revealed in the partial assembly of the 15 collimator 114 shown in Figure 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/2 and are 20 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 25 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 30 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 35 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 40 milling techniques, desired tolerances in forming the slots are realizable By utilizing the collimator structure shown in combination with optimal tungsten sheet material, a computable 35 to 40 percent improvement in collimator efficiency may be gained over round hole, hexagonally packed lead collimators of identical dimension, as well as a 50 to 80 percent improvement in septal penetration 45 characteristics and an average 50, improvement in geometric resolution The collimator fabrication technique and structure are seen to offer several advantages over more conventional collimators structures As evidenced from the foregoing such advantages include the availability to the design of the superior shielding capabilities of tungsten; a simplicity of component design and consequent ease of 50 assembly and the use of optimal square hole chamber geometry for maximum geometrical efficiency However, to achieve optimal performance, the assembly technique necessarily introduces small gaps at the intersections of the septal walls of a completed collimator structure These gaps exist by virtue of the tolerances required for the interlocking fit of the septal wall and the effect of gamma ray 55streaming through such gaps should be considered.

In earlier commentary herein, it has been noted that a septal penetration of five percent or less of impinging gamma radiation is preferred for collimator design It follows, therefore that the streaming factor for the particular collimator structure at hand should be assigned the same configurational parameter in the 60 interest of desired unity of system design Through utilization of a geometric analysis of a worst case condition, requisite lowest tolerance required for the interlocking fit of the septal walls and for a desired source to collimator distance can be derived.

Such analysis will reveal that the slot tolerance should preferably be no more than I 1,578,883 9 1,578,883 9 0.001 inch and, more preferably, should be less than that to the extent of practical milling application.

In the discourse given heretofore concerning the functional interrelationships of collimator 50 and detector array 32, no commentary was provided concerning the effect of the discrete electrode strips of the detector upon ultimate image 5 resolution It has been determined that, by virtue of their geometric configuration, orthogonal strip detectors, without appropriate correction, will introduce "alias" frequency components into the output of the system For instance, in a purely linear system, the output of the camera would consist of the same spatial frequency components as the input except with the possibility of reduced contrast Looking to 10 Figures 7 (a)-(c), the aliasing phenomenon is demonstrated in connection with an exemplary and schematic representation of a strip electrode detector 130 In this worst case representation, no collimator is present and the electronic resolution is less than one strip width Looking to Figure 7 (a), a source distribution is shown as may be obtained, for instance, utilizing three discrete collimated point sources 15 spaced at equal distances of 1 5 times the strip spacing The reciprocal, of the periodic spacing of the components depicted may be represented as, v The source distribution shown is one with primary frequency components of v,= 0 and v 2 = 2 v, /3.

Such source input is provided in the instant representation inasmuch as it combines the three qualities which accentuate an aliasing phenomenon, namely, a periodic 20 input, 100 % contrast, and a high signal-to-noise ratio.

Figure 7 (b) reveals a portion of strip electrode detector 130 having the earlier described detector region grooves aligned with respect to the input signals depicted at Figure 7 (a) The one-dimensional spatial image which may be derived, for instance, from a multi-channel analyzer is shown in Figure 7 (c) as curve I 32 By 25 comparison, the corresponding spatial image which would be received within a system incorporating a collimator capable of resolving the input signals, a detector with strip spacing satisfying the anti-aliasing criterion and an antialiasing electronic channel, is revealed at 134 This image shown no aliased components.

Looking more particularly to the aliasing phenomenon represented at curve 30 132, the four lowest spatial frequency components revealed are:

( 1) a component at v= 0, a zero frequency component which represents the average value of the four peaks; ( 2) a component at v= 2 v J 3, which is the frequency equal to the reciprocal of the spacing between one of the two outer peaks and the average position of the two 35 inner peaks; ( 3) a component at v=vs, which is the frequency equal to the reciprocal of the spacing between each of the four peaks; and ( 4) a component at v=v,/3, which is the frequency equal to the reciprocal of the spacing between the two outer peaks 40 The first two components above are the fundamental source components, while the second two components are aliased components of the fundamental source components centered at the first harmonic of the strip sampling frequency, As a prelude to considering a typical representation of the spatial frequency response of a one-dimensional gamma camera as revealed in Figures 8 (a)(d) the 45 modulation transfer functions (MTF) merit comment The MTF is a measure of spatial resolution that can be defined for linear systems and which takes into account the shape of an entire line spread function The rationale for such description of spatial response arises from the fact that any object and its image can be described in terms of the amplitudes and phases of their respective spatial 50 frequency components The MTF is a measure of the efficiency with which modulation or contrast at each frequency is transferred by the imaging system from the object to the image This is analogous to the temporal frequency response of an electronic amplifier or filter Looking now to Figures 8 (a)-8 (d) MTF is plotted against spatial frequency, v, for a series of stages within a gamma camera not 55 accommodating for aliasing phenomena In Figure 8 (a) a collimator modulation transfer function (MTFC) with FWHM resolution of 1 33 1 is revealed, i e, the curve distribution, incorporating some high frequency components, is representative of the signal passed to the semiconductor detector of the camera.

Figure 8 (b) reveals the output frequency spectrum of the detector which is seen by 60 the spatial channel electronics of the camera system An aliased frequency spectrum is revealed, the input signal frequency spectrum being present in the output, centered at zero frequency and additional side bands of the primary input component are present, centered at integer multiples of the strip spacing or sampling frequency, v,= 4 ll Figure 8 (c) represents the MTF of the electronics of 65 1,578,883 10 the system, i e, the transfer function of the spatial channel electronics, while Figure 8 (d) shows the product of the MTF values of the curves of Figures 8 (d) and 8 (c) Accordingly, the curve of Figure 8 (d) shows the spatial frequency response of the entire system, including the introduction of spurious spatial frequency content in the system MTF, represented in the figure as the bump in the frequency range 5 slightly below vs.

Looking by comparison now to Figures 9 (a)-(d) the effect of inserted correction on the part of the collimator design and structure is revealed The collimator 50 design is selected to provide an MTF prefilter to limit the spatial frequency content seen by the detector 32 to frequencies less than v J/2 10 Accordingly, Figure 9 (a) reveals that the collimator MTF is forced to a zero value at spectrum position v,/2 Such design insures that the fundamental input frequency components and the first harmonic frequency components centred at v, do not overlap and this condition obtains in Figure 9 (b), that Figure revealing the alias frequency spectrum which is processed by the electronic pickoff arrangement of 15 the camera from the detector The spatial channel electronics complete the antialiasing filter system by insuring that no spatial frequencies greater than v 3/2 are passed to the imaging system of the camera Such post filtering of the electronics is illustrated in Figure 9 (c) The product of MTF conditions represented by Figures 9 (b) and 9 (c) again are represented in Figure 9 (d) which, particularly when 20 compared with the corresponding Figure 8 (d) reveals the elimination of aliasing phenomena.

Turning now to the prefiltering or corrective function carried out by the collimator in controlling aliasing phenomena it may be observed from the foregoing that the system resolution of an orthogonal strip germanium detector 25 type gamma camera is determined by the collimator resolution, the strip width spacing, and the resolution of the spatial channel readout electronics The collimator is assumed to have a Gaussian point spread function (PSF) and FWHM spatial resolution R, The value of R should be equal to or greater than about 1 7 ( 1), where I is the center-to-center strip spacing in one dimension of the detector A 30 more detailed discussion of aliasing phenomenon value is provided in the following publication:

XXI J W Steidley, et al, "The Spatial Frequency Reponse of Orthogonal Strip Detectors", IEEE Trans Nuc Sci, February, 1976.

Looking now to the specific design parameters of the collimator it may be 35 recalled that collimator resolution, Rc, has been derived geometrically at equation (I) given hereinabove By now substituting the ideal valuation, 1 7 ( 1) determined for anti-aliasing prefiltering on the part of the collimator, the collimator geometry or structure may be defined Accordingly, the collimator is defined under the following expression: 40 D 1.7 ( 1)< ( 7) AE(A+B+C) The collimator further can be defined utilizing equation ( 5) above for septal wall thickness once the values of the parameter of equation ( 7) are determined.

Further, given the value, Rc, for collimator resolution and the geometric parameters determined thereby as described above, the collimator geometric 45 efficiency, O s, as given in equation ( 6) above, can be applied to further maximize the performance of the collimator Additionally, it may be noted that by suppressing frequencies above v/2 input signal contributions to aliasing phenomena are accommodated for.

As has been alluded to earlier herein, discounting entrance geometry, the 50 orthogonal strip position-sensitive detector is resolution limited by noise associated with the detector as well as the charge dividing network Consequently, it is necessary to consider the noise characteristics of the system from the stand-point of minimizing the effects thereof upon resolution as well as treating such phenomena to derive desired imaging effects Generally, it may be concluded that 55 the resistor network is the dominant source of noise within the electronic spatial channel if the system, while the resistor network, coupled with the detector leakage current, represents the dominant noise source in the system's energy channel As will become more apparent as the instant description unfolds, spatial noise dominantly is electrically parallel in nature, whereas energy channel noise may be considered to be electrically series in nature In the discourse to follow, noise treatment and the like are described in conjunction with the singular detector component described heretofore in connection with Figure 2, in the interest of clarity and simplification In later portions of the instant discussion, however, the 5 control system of the camera will be seen to be described in conjunction with detector component array embodiments.

Noise is the random fluctuation of the preamplifier output voltage when there is no stimulus It is generated by imperfections in the preamplifier input device, thermal movement of charge carriers in the resistors and the bulk of the detector 10 and imperfections in the crystal structure of the detector Looking to Figure 10, an equivalent noise model circuit for solid state detector components is revealed.

Note that the model reveals a detector leakage current, i 0, which is assumed to be formed of individual electrons and holes crossing the depletion layer of the detector Such electron hole pairs are thermally generated in the depletion layer 15 Resistive elements which are in parallel with the system input capacitance, CIN, generate thermal noise which is integrated by this capacitance and appears at the preamplifier input as a step function The system input capacitance is the parallel combination of stray capacitance at the preamplifier input and the feedback capacitor of the preamplifier Those resistive components which contribute to this 20 noise term are the high voltage bias resistor, the preamplifier feedback resistor and the detector bulk resistance For a charge dividing resistive strip network, a portion of the dividing resistance, RD, is in parallel with the detector capacitance Since R.

is less than one hundred kilo-ohms, it represents a significant noise source The thermal noise from resistors in series with the detector capacitance appears as a 25 delta function to the preamplifiers For spectroscopy systems, this resistance is minimized and the noise source is neglected The noise developed by the preamplifier input stage is modeled using a resistor, Ra Finally, a noise term which is not shown in Figure 10 is "flicker" noise caused by structural changes and surface effects in the conduction material of the noted preamplifier input stage 30 This noise aspect generally is considered to be insignificant.

Since the noise sources discussed above have a uniform power spectral density, bandwidth limiting filtering or pulse shaping generally is considered appropriate for maximizing the signal-to-noise ratio of the system As suggested earlier, the fundamental noise sources are classifiable as two types, parallel noise 35 representing the charge due to the electron flow which is integrated by the input circuit capacitance, and series noise representing the charge due to the electron flow which is not integrated by input capacitance These noise sources are considered to be mutually related in terms of filtering to the extent that as efforts are made to diminish one, the other increases The high frequency component 40 noise generally is considered a series type while low frequency noise is considered of the parallel variety As has been detailed in the publications given above, the use of a Gaussian and the Gaussian-trapezoidal noise filtering circuits has been found to optimize the energy and spatial resolution values of the camera system.

Turning now to Figure 11, a circuit model of the detector component 60 and 45 the resistor networks of Figure 2 is portrayed The discrete nature of the detector system and the method of read-out is revealed in the figure with the discrete capacitors forming an nxn array Each row and column is defined by the charge measured at the end of the resistor strings The electron-hole pairs which are formed when a gamma ray interacts with the detector are collected on opposite 50 surfaces A charge enters the resistive network and flows to terminal A or B (C or D) in relation to the resistance between its entry point and the virtual earth terminal of each preamplifier (Figure 2) The intersection of the column and row defines the diode position in which the gamma ray energy was deposited Note in the figure, that individual capacitances are represented which are exemplary of the 55 inherent capacitance of the detector itself When considered in conjunction with the resistor networks, as revealed in the figure, it may be noted that a particular time constant or interval is required for any impinging charge to be represented by a charge flow to the output taps of the resistor chains Accordingly, the system must provide an adequate time interval or time constant, TD, for this charge flow to 60 avoid error in information collection In effect, it may be assumed that the detector and each of the resistor strings of the noted impedance networks respond as a diffusive line, and the peaking time of the preamplifier output pulses will vary as a function of the position of interaction, x, of an incident gamma ray The voltage 1 1 1,578,883 1 1 12 1,578,883 12 output of each preamplifier (Figure 2) due to the instantaneous transfer of charge Q O at position xo is:

V(O,x,t) = D 2 1 mx O \ / -m 22 t L x L,D -(Oxa)C 1 Esnexp ( 8) o Xo+ 2 / mitx O \ (-m 2 It 2 t\ V(L,xo,t) = C x L 2 cos (m Tt) sin) exp ( 9) C L m= 1 L D / where C, is the feedback capacitance of a preamplifier in farads, L is a given linear 5 dimension of the detector, Tr D is the time constant of the detector (i e r D= 2RDCD), x O defines the position of interaction and m is a summation variable.

Examination of equation ( 8) and ( 9) show that for a time t> ( 10) i e, an output generation time equivalent to one half of the time constant of the 10 detector, the value of V(O, xo, t) is within 1 %/ of its final value for all x JL< 95 and V(L, xo, t) is within 1 % of its final value for all xj L> 05 Stated otherwise, the error generated from ballistic deficit type characteristics of the system, as it relates to the energy of one preamplifier readout diminishes to a value of 1/ within a period of one half the time constant, TD of the detector 15 By subtracting the output of the one preamplifier of a network, i e at the x=L position from the corresponding amplifier output at the x=O position, i e.

V(O x , t V(L,x ' t) = s CfN L( Li Cf L rn=l L (+cos m exp ( D 2)l 11) + / ( 11).

the following important observations may be observed Equation ( 11) shows that as the spatial location of information impingement alters from O to L, the resulting 20 voltage readout moves from a positive unit value to a negative unit value Stated otherwise the output signal derived from the above signal treatment subtractive approach ranges from +Qo /Cf at xo=O, to -Qo /Cf at xo=L, making the signal twice that of earlier suggested one preamplifier collection technique Further, it may be observed that the odd numbered series terms vanish, thereby reducing the position 25 signal peaking time The value of equation ( 11) is within 1 I of its final value for all values x JL< 45 and x/L> 55 after a time:

TD t> ( 12) Accordingly, it may be observed that through the utilization of a dual preamplifier 13 1,578,883 13 subtractive or "antisymmetric" method of signal analysis, the necessary time constant related signal treatment within the spatial channel is diminished by a factor of 4.

Turning now to the conditions obtaining within the energy channel of the system, the energy channel is derived by summing the output of each preamplifier to 5 obtain the voltage pulse:

V (O, xo, t) + V ( L, xt,) = C O m S in ( L)() Ct m L J omt -M 2 nt 2 t exp () ( 13) Note again, that the peaking time of the pulse is position dependent At x JL= 5, the maximum peaking time occurs and the pulse is within 1 /, of its final value at t ID/2 Accordingly, it may be observed that ballistic deficit or charge 10 collection type considerations within the energy channel will require a charge collection period, for practical purposes, equivalent to one half of the time constant of the detector.

Now considering noise phenomena, as earlier discussed in combination with ballistic deficit considerations, as derived immediately hereinabove, dominant 15 spatial noise, which is parallel noise, may be expressed as follows:

M 1 112 Nq Sl, 4 k T Da plo ( 14) q RD where N,,, is the equivalent noise charge in number of electrons for one preamplifier spatial measurements, RD is the total resistance of the resistive chain, T O is the temperature of the detector and chain, ap is a weighting factor of the filter, 20 q is the magnitude of the charge on an electron, and k is Boltzmanns constant.

In the expressions given above, i e equations 8 through 14, the term RD is intended as the value reprsenting the average of the total resistance of each resistive network For the exaggerated exemplary detector component shown in Figure 2, the term RD represents one-half the sum of the resistance values of 25 networks 72 and 74 Note from equation ( 14) that the noise is proportional to the square root of the temperature as well as the weighting factor and the time constant of the system As disclosed earlier, this time constant is limited by the ballistic deficit conditions of the system Note further that the noise is inversely proportional to total resistance of one chain or resistor network Therefore, it is 30 desirable for system efficiency to minimize the temperature under which it operates as well as the weighting factor and time constant and to elevate the resistance value to the extent practical Equation ( 14) is for one preamplifier readout Reconfiguring the equation to represent a subtractive or antisymmetric arrangement, the following expression obtains: 35 ll/2 Nq SAS 2 O ap O ( 15) From this equation, note that a subtractive arrangement permits the ballistic deficit dictated time constant to reduce by a factor of 4, while the value of noise increases by a factor of 2 for that same time constant However, since a reduced time constant (factor of 4) is involved in a subtractive arrangement, the noise value, 40 otherwise increased by a factor of 2, remains the same and the signal-tonoise ratio is increased by a factor of 2 Recall the earlier discussion above, that the unit signal value runs from a positive unit to a negative unit within a subtractive system The value RD is difficult to increase inasmuch as a concomitant reduction in energy resolution generally is witnessed for such alteration Temperature drop can be 5 achieved practically and the weighting factor, ap, can be altered to a more or less ideal value by appropriate selection of filtering systems It has analytically been determined that a 43 4 percent improvement in spatial resolution is realized if antisymmetric summation, i e subtractive summation, is used as opposed to the utilization, for instance, of one preamplifier for spatial measurement 10 Looking additionally to the "ballistic deficit" phenomenon, for thin detectors, i.e about five mm in thickness, the detector charge collection time is small and does not affect circuitry treating a detected signal For thick detectors, however, i.e having a thickness in the range of about 2 cm, the bulk charge collection time varies from approximately 100 to 200 nanoseconds Since this collection time is 15 approximately the same as the collection time of the charge dividing network, its contribution to ballistic deficit problems must be considered For such systems, theoptimum filtering arrangement consists of a time invariant pre-filter followed by a gated integrator circuit Such filters generally are referred to as gatedintegrators or trapezoidal filters The filter preferred for the purpose is a Gaussian trapezoidal 20 filter which consists of a time invariant Gaussian filter followed by a gated integrator circuit For a detailed discourse concerning the utilization of antisymmetric summation as well as the utilization of trapezoidal filtering within the spatial channel of the system, reference is made to the following unpublished work: 25 XXII Hatch, K F, "Semiconductor Gamma Camera" Ph D Dissertation, Massachusetts Institute of Technology, Cambridge, Massachusetts, February, 1972.

The equivalent noise charge in number of electrons for Gaussian trapezoidal spatial measurements may be represented by the following expression: 30 Ncq'3 GT = q ( 4 k T ap Tt, T79) /z ( 16) where ap is the parallel noise weighting function value for Gaussian trapezoidal systems and T, is the integration time Analysis of the foregoing shows that an excellent improvement in spatial resolution is obtained by using antisymmetric Gaussian trapezoidal filtering This improvement is realized because the effects 35 of -ballistic deficit" are greatly reduced.

The corresponding equivalent noise charge in number of electrons for the energy channel of the system may be expressed by the following formulation:

112 Nq E 51= q^ 2 qi Da Pti, + 4 k TD R ( 17) An important aspect of the above energy channel and spatial channel analyses 40 has been observed In this regard, it may be recalled that opposed relationships stem from a consideration of parallel vs series noise phenomena For instance, it has been described that energy noise is considered serial in nature whereas spatial noise is considered to be parallel in nature The energy noise equation, as shown at ( 17) above, represents a straight summation of two preamplifier outputs and the 45 initial parallel noise factor presented within the brackets thereof is of dismissable magnitude When compared with the spatial noise equation ( 16) above, it may be observed that two separate time constant values, To' Te, respectively, for spatial resolution and energy resolution may be incorporated within the circuitry treating the output of the system detector For instance, the energy resolution filtering of 50 the system requires a relatively extended time constant, whereas corresponding spatial filtering requires a relatively short one for highest signal to noise ratio considerations Inasmuch as the outputs of the filtering media reach the output displays of the camera or imaging system simultaneously, any multiple pulse errors 1,578,883 introduced into the longer time constant energy filter individually will be integrated to achieve a peak value above a predesignated window function of the energy channel (block 54, Figure 1) Accordingly, false information generated from pulse pile-up-phenomena and the like may be rejected without recourse to more involved discrimination circuitry Such a desired system circuit arrangement will be revealed 5 in the description of the control system to follow.

Turning to Figure 12, a composite detector, formed as an array of discrete detector components, is revealed generally at 360 This sub-grouping of four detector components, as identified at 362, 364, 366, and 368, may, for instance, be combined with three additional subgroupings to form a full detector array 10 comprising four subgroupings incorporating a total of 16 detector components Of course, a greater or smaller number of detector components may be combined to form an array of desired dimension In the interest of clarity, only one such quadrant designated sub-array, as at 360, is described in conjunction with a control system Detectors 362-366 are dimensioned having mutually equivalent areas 15 designated for the acceptance of impinging gamma radiation Such equivalency serves to achieve accurate ultimate image readout from the camera system The detector components 362-366 are of the earlier-described orthogonal strip array variety, each strip thereof being defined by grooves Note in this regard, that detector component 362 is formed having strips 370 a-370 d located at its upwardly 20 disposed surface and defined by grooves cut intermediate adjoining ones of these said strips The opposite face of the detector component 362, similarly, is formed having strips 372 a-372 d defined by intermediately disposed grooves arranged orthogonally with respect to the grooves at the upper surface Detector component 364 is identically fashioned, having strips 374 a-374 d at its upwardly disposed 25 surface, each being defined by intermediately disposed grooves; the lower surface of the detector being formed having orthogonally disposed strips 376 a376 d defined by intermediately disposed grooves The corresponding strip arrays of detector component 366 are shown to comprise identically disposed strip groupings as at 378 a-378 d and 380 a-380 d Similarly, detector component 368 is shown to 30 be formed of identically structured mutually orthogonally disposed strip arrays 382 a-382 d and 384 a-384 d.

Components 362-368 are illustrated expanded from one another for purposes of illustration only, it being understood that in an operational embodiment these 351 components are internested together in as a practical a manner as possible To 35 achieve an informational spatial and energy output from the discrete detector components, the strip arrays each are mutually associated along common coordinate directions This association is carried out between components 362 and 364 by leads 386 a-386 d, coupling respective strips 374 a-374 d of component 364 with strips 370 a-370 d of component 362 In similar, parallel coordinate fashion, 40 leads 388 a-388 d are provided for connecting respective strips 382 a-382 d of component 368 with strips 378 a-378 d of component 366.

The outputs of the thus mutually coupled strip arrays of the upwardly disposed faces of the detector components are coupled with an impedance network, represented generally at 390 Network 390 is configured comprising serially 45 interconnected discrete resistors 392 a-392 i Interconnection between respective strips 370 a-370 d and points intermediate resistors 392 e-392 i is provided by leads 394 a-394 d, while corresponding interconnection between strips 378 a-378 d with the intermediate connections of resistors 392 a-392 d is provided by leads 396 a396 d 50 In similar fashion, the arrayed strips 372 a-372 d at the lower surface of component 362 are coupled with respect to strips 380 a-380 d of component 366 by leads 398 a-398 d Similarly, strips 376 a-376 d at the lower face of component 364 are respectively coupled with corresponding strips 384 a-384 d of component 368 by leads 400 a-400 d The thus associated strip arrays of the lower faces of the 55 detector components are connected with a second impedance network, identified generally at 402, in similar fashion as the orthogonally disposed upward surfaces.

Note, for instance, that strips 380 a-380 d of the lower surface of components 366 are connected to intermediate respective discrete resistors 404 a-41)4 e of network 402 by leads 406 a-406 d Similarly, strips 384 a-384 d of the lower surface of 60 component 368 are connected with respective discrete resistors 404 f-404 i of network 402 through leads 408 a-408 d Thus interconnected, the four discrete detector components provide spatial coordinate parameter outputs; i e xdesignated coordinate outputs at lines 410 and 412, which are identified thereat as (x A) and (x B) In like manner, the spatial coordinate parameter outputs of the 65 1,578,883 lower surfaces of the detector components are present at lines 414 and 416 and are y-designated, being labeled in the drawing, respectively, as (y 1 A) and (y B).

Referring now to Figure 13, another composite detector formed as an array of discrete detector components is revealed generally at 680 Illustrated in exploded fashion, the detector 680 is comprised of a plurality of detector components, four 5 of which are shown at 682, 684, 686 and 688 Components 682-688 are dimensioned having mutually equivalent areas as are intended for acceptance of impinging radiation This required equivalency serves to achieve an accurate ultimate image readout from the camera system In the absence of such equivalency, distortion at such readout, exhibiting a discontinuity of image 10 information, would result The detector components illustrated are of the earlier-described orthogonal strip array variety, each strip thereof b'eing defined by grooves Note, in this regard, that detector component 682 is formed having strips 690 a-690 d located at its upward surface and defined by grooves cut intermediate adjoining ones of the said strips The opposite face of detector 15 component 682 similarly is formed having strips 692 a-692 d defined by intermediately disposed grooves arranged orthogonally with respect to the grooves at the upper surface Detector component 684 is identically fashioned, having strips 694 a-694 d at its upwardly disposed surface and lower surface, orthogonally disposed strips 96 a-96 d each strip being defined by intermediately formed 20 grooves Similarly, detector component 686 is formed having strips 698 a698 d at its upward surface defined by intermediately disposed grooves, while its lower surface similarly is formed having strips 700 a-700 d defined by intermediately disposed grooves arranged orthogonally with respect to the grooves of the upward surface Detector component 688 may be observed having strips 702 a-702 d at its 25 upward surface defined by intermediately designated grooves, while its lower surface is formed with strips 704 a-704 d separated by intermediately disposed grooves arranged orthogonally to the grooves of the upward surface of the component.

Detector components 682-688 as well as similar components in later figures 30 are illustrated expanded from one another for purposes of illustration only, it being understood that in an operational embodiment these components are internested together in as practical a manner as possible To achieve an informational spatial and energy output from the discrete detector components, which essentially is equivalent to that output which would be realized from a large detector of 35 equivalent size, the strip arrays are functionally associated under a geometry which,:

as noted above, may be designated "row" and "column" in nature In this regard, note that an impedance network, shown generally at 707, is associated with the strips 694 a-694 d of detector component 684 This network incorporates discrete resistors 706 a-706 e which are tapped at their common junctions by leads 708 a 40 708 d extending, respectively, to strips 694 a-694 d Thus configured, network 707 closely resembles the impedance networks described herein in connection with Figure 2 Note however, that output lines 710 and 712 of network 707 extend to and are coupled in parallel circuit relationship with the corresponding output of a similar impedance network, identified generally at 714 Network 714 incorporates 45 discrete resistors 716 a-716 e which are tapped at their common interconnections by leads 718 a-718 d Leads 718 a-718 d, in turn, respectively extend to strips 690 a-690 d of detector component 682 Accordingly, the upwardly disposed surfaces of detector components 682 and 684 are identically associated with respective impedance networks 714 and 707, while the latter are interconnected in 50 row fashion and in parallel circuit relationship to extend to principal output terminals, as are depicted generally at 720 and 722 It may be noted, that the information collected at these principal terminals represents one imaging spatial coordinate parameter of a select directional sense i e along a designated row.

Looking now to the functional interrelationship of detector components 686 55 and 688, a similar coordinate parameter direction or row-type informational collection network is revealed In this regard, note that the impedance network, shown generally at 724, is configured comprising discrete resistors 726 a726 e, the points of common interconnection of which are coupled with respective leads 728 a-728 d Leads 728 a-728 d, in turn, respectively, are connected with strips 60 698 a-698 d at the upwardly disposed surface of detector component 686 Likewise, an impedance network, shown generally at 730 incorporating discrete resistors 732 a-732 e, is associated with detector component 688 by leads 734 a-734 d extending, respectively, from strips 702 a-702 d to the points of common interconnection of network discrete resistors 732 a-732 e Additionally, the output 65 I 1,578,883 lines as at 736 and 738 of network 730 are connected in parallel circuit relationship with the output of network 724 to provide row readout termini, respectively, at 740 and 742 Here again, a row-type directional spatial coordinate parameter is provided at the upwardly disposed surface of the composite detector 680.

Looking now to the lower surfaces of the detector components, it may be 5 observed that the orthogonally disposed strips of detector component 682 are associated with an impedance network identified generally at 744 Network 744 incorporates discrete resistors 746 a-746 e which are coupled from their mutual interconnections by leads 748 a-748 d, respectively, to strips 692 a-692 d of detector 682 Similarly, the orthogonally disposed strips of detector component 686 10 are associated with an impedance network 750 In this regard, network 750 is formed of discrete resistors 752 a-752 e, which, in turn, are coupled, respectively, with strips 700 a-700 d by leads 754 a-754 d The output of impedance network 744 is connected by leads 756 and 758 to the corresponding output of impedance network 750 to provide column directional coordinate parameter outputs, as at 760 15 and 762, which serve to collect all spatial information of the associated paired surfaces of detectors 682 and 686.

Looking to the lower surface of detector component 684, note that a network, designated generally at 764, incorporating discrete resistors 766 a-766 e is functionally associated with strips 696 a-696 d, respectively, by leads 768 a-768 d 20 In similar fashion, an impedance network, designated generally at 770, is associated with the orthogonally disposed strips 704 a-704 d at the lower surface of detector components 688 Note that the network, incorporating discrete resistors 772 a-772 e, is functionally associated with the array of strips 704 a704 d, respectively, by leads 774 a-774 d Networks 764 and 770 are electrically coupled in 25 parallel circuit fashion by collector leads 776 and 778 extend to principal collection points of termini 780 and 782 Thus interconnected, the lower surfaces of detectors 684 and 688 are coupled in column readout fashion to provide another spatial coordinate parameter of direction parallel with the corresponding lower surface strip array readout arrangement of detector components 682 and 686 30 With the row and column readout intercoupling of the detector components as shown in the figure, it may be observed that the capacitance exhibited by all discrete detector components taken together, remains the same as if only a single detector were operating within a camera Accordingly, the signal treating circuitry and logic of the camera, advantageously, may be designed to accommodate for the 35 charge collection time constant of a single detector Connection with the row and column readouts for given spatial coordinate parameters outputs will be seen to be provided by treating circuits which distribute coordinate channel spatial and energy channel signals into analyzing and distributing circuitry Preamplification stages, as described in connection with Figure 2, are coupled with each row 40 readout point as at 720 and 722 or 740 and 744, as well as with each column readout as at 760 and 762 and 780 and 782 Such preamplification stages generally are located within or near the cryogenic environment of the detector itself The mounting of the contact leads between each of the networks and an associated strip array surface of a detector generally may be carried out by resort to biased contact 45 configurations.

The composite detector arrangement or interrelated detector component mosaic also may be formed utilizing detector structures which incorporate surface disposed resistive layers to achieve spatially proportioned charge readout characteristics Such a detector composite is revealed generally in Figure 14 at 800 50 Referring to that figure, the composite detector, or portion thereof, 800, is shown to comprise four discrete detector components 802-808 The opposed surfaces of the detector components, which are situated generally normally to impinging radiation, are formed having a resistive character This resistance is provided, for instance, by so lightly doping the n-type surface as to achieve a region of resistive 55 character, while, similarly, so lightly doping the opposite surface with a p-type acceptor as to achieve a surface resistive character thereat The readout from these resistive surfaces is collected by conductive strips which, for the case of detector component 802, are shown on the upward surface at 810 and 812 and at the lower surface at 814 and 816 Conductive surfaces 810-816 may be deposited upon the 60 detector component 802, for instance, by conventional evaporation techniques utilizing a highly conductive metal such as a noble metal, i e gold.

Concerning the techniques for developing the noted resistive regional character within the surfaces of detector components 802-808, mention may be made of the following publications: 65 1,578,883 XXIV Owen, R P Awcock, M L, "One and Two Dimensional Position Sensing Semiconductor Detectors", IEEE, Trans Nucl Sci, Vol N S -15, June 1968, Page 290.

XXV Berninger, W H, "Pulse Optical and Electron Beam Excitation of Silicon Position Sensitive Detectors", IEEE, Trans Nucl Sci, Vol V S 21, Page 5 374.

With the impingement of radiation upon detector component 802 and resultant development of an interaction therewithin, charge will be collected on the opposed surfaces, as discussed above, and will split proportionally at the impedance define surfaces and collect at the conductive strips 810-816 For the 10 upwardly disposed surface, these charges then are collected along conduit 818, coupled with conductive strip 812, and conduit 819, coupled with conductive strip 810 The adjacently disposed detector 804 is fashioned in similar manner, the upward surface thereof incorporating a resistive surface layer or region formed in cooperation with conductive strips 820 and 822 The lower surface of detector 15 component 804 is formed incorporating a similar resistive layer or region functionally associated with conductive strips 824 and 826 Note that the latter conductive strips are arranged orthogonally with respect to those at 820 and 822.

Conductive strip 820 is coupled by a lead or conduit 828 to conductive strip 812 of the detector component 802, while conductive strip 822 is coupled by lead or 20 conduit 830 to conductive strip 810 of detector 802 Thus interconnected, it will be apparent that any interaction occurring within detector component 804 will be ,seen" as a charge division between strips 820 and 822, for one coordinate parameter, along leads 828 and 830, as well as output conduits 818 and 819 As is apparent, a desirable simplification of the structure of the composite detector is 25 available with this form of row readout.

Looking to the adjacently disposed row of detector components 806 and 808, it may be noted that detector component 806 is formed incorporating resistive layers or regions in its opposed surfaces aligned for the acceptance of radiation and, additionally, incorporated conductive strips as at 832 and 834 at the extremities of 30 its upward surface as well as orthogonally oriented conductive strips 836 and 838 about the extremities of its lowermost and oppositely disposed surfaces.

Identically structured detector component 808, similarly, is formed having resistive surfaces or regions arranged normally to the direction of radiation impingement The surfaces also incorporate conductive strips, as at 840 and 842 at 35 the upwardly disposed side and, at 844 and 846, orthogonally disposed at the lowermost surface.

Coupled in similar row-type fashion as detectors 802 and 804, the conductive strips of detectors 806 and 808 are directly electrically associated by leads 848 and 850 Note, in this regard that lead 848 extends between conductive strips 840 and 40 832 while lead 850 extends between conductive strips 842 and 834 The output of that particular row at the upward surface of the composite detector is represented by leads 852 and 854.

A columnar interconnection of the detector components is provided between the orthogonally disposed conductive strips 814 and 816 of detector 802, 45 respectively, as by leads 856 and 858, to similarly dispose conductive strips 836 and 838 of detector 806 The columnar readouts for the paired detector components are present at conduits 860 and 862 extending, respectively, from conductive strips 836 and 838.

In similar fashion, the columnar association of detector components 804 and 50 808 is provided by leads 864 and 866 which, respectively, extend between conductive strips 824 and 826 of detector 804 to corresponding conductive strips 844 and 846 of detector component 808 The readouts for the column association of detectors 804 and 808 are provided by conduits 868 and 870 extending, respectively, from conductive strips 844 and 846 of detector component 808 55 As in the embodiment of Figure 13, the output conduits 818, 819 and 852, 854 are of a "row" variety having a designated spatial coordinate parameter and are addressed to initial preamplification stages prior to their association with logic circuitry for deriving imaging information for that particular spatial coordinate.

Similarly, the "columnar" outputs at conduits 860, 862 and 868, 870 are directed to 60 preamplification stages, thence to appropriate circuitry for treating that spatial coordinate parameter It will be understood, of course, that the number of detector components formed within a matrix or array thereof depends upon the field of view desired for a particular camera application as well as the practicalities for retaining 1,578,883 such components under appropriate cryogenic temperature conditions during operation.

The foregoing examination of the composite detector structures, represented in Figures 13 and 14 reveals certain consistent characteristics between the embodiments For instance, as alluded to above, the effective areas presented to 5 radiation impingement of the discrete detector components must be substantially equivalent, in order to avoid distortion in an ultimately developed image.

Additionally, these components should be as closely nested as possible and aligned such that the spatial coordinate which may be designated for each surface evolves what has been termed as a "row-column" orientation In the latter regard, an 10 observation of this geometry shows that the leads interconnecting the impedance networks or the impedance structure i e at the surface region of the detector components, connect them directly, whether in the parallel-series connection of the embodiment of Figure 13 or the interconnection of conductive strips shown in Figure 14 Another aspect typifying the structure, reveals that any two adjacent 15 surfaces of any two adjacent detector components exhibit spatial coordinate parameters of a common directional sense and, more particularly, two adjacent of the coplanar surfaces of any two adjacent detector components are disposed within a linearally oriented grouping arranged to exhibit a common spatial coordinate parameter directional sense Because the composite detector embodiments shown 20 in Figures 13 and 14 operate substantially in the same functional manner, their outputs are identified with the same spatial coordinate directional labels For instance, the x-designated coordinate outputs at lines 722 and 720 of the embodiment of Figure 12, respectively, are identified as (X 1 A) and (X 1 B); while the parallel row y-designated coordinate outputs as at lines 742 and 740, respectively, 25 are identified as (X 2 A) and (X 2 B) Similarly, the orthogonally disposed y-designated coordinate parameter outputs, as represented for instance, at lines 762 and 760, respectively, are identified as (Y 1 A) and (Y 1 B) Next adjacent to that column of the composite detector, are the detectors whose outputs are represented at 780 and 782 and are identified, respectively, as (Y 2 A) and (Y 2 B) This same labelling procedure 30 will be seen to be utilized in the composite detector embodiment of Figure 14.

An important aspect of the "row-column" interconnection of the discrete detector components resides in the realization of an effective reduction in that detector linear dimension over which resolution is evaluated More specifically, an improvement is experienced in the resolution of the camera system which may be 35 expressed by the equation:

AEL Ax= ( 20) E Where, Ax, represents spatial resolution in terms of distance; AE, an absolute energy resolution; L, is length of a detector component as measured parallel to the directional sense of an associated impedance network; and, E, represents the 40 energy of an incident photon interacting with the detector Within the right hand side of equation ( 20) above, the expression, AE, E is readily identified as the fraction (or percentage) of energy resolution and is fixed for a given input energy Accordingly any increase in the value of, L, directly and 45 adversely affects the spatial resolution Where the detector components are not interconnected by the "row-column" technique, the value, L, in the expression above becomes larger For example if the detector pictured in Figure 22 were connected as a single detector the measuring distance would be 2 L, effecting a doubling of the noted spatial resolution value to the detriment of final imaging 50 Another feature characteristic of a detector "row-column" interconnection resides in the presence of a common detector component for each combination of an associated row and column Stated otherwise, a row or column configuration also may be designated as an orthogonally disposed linearly oriented grouping of charge collecting surfaces Any interaction within any given common comporient will 55 provide x and y-designated coordinate output signals from the thus associated linear surface groupings.

1,578,883 A third embodiment for "row-column" interconnection of detector components exhibiting this spatial resolution advantage is revealed in Figure 15.

Referring to that figure, a composite detector formed as an array of discrete detector components is revealed generally at 880 As in the earlierdiscussed embodiments, detector or detector portion 880 is shown in exploded fashion for 5 purposes of clarity and comprises a plurality of detector components four of which are shown at 882, 884, 888, and 886 Components 882-888 are dimensioned having mutually equivalent areas as are intended for acceptance of impinging radiation and are formed as of an orthogonal strip array variety, each strip thereof being defined by grooves formed within the detector surfaces Of course, other, strip 10 defining configurations will occur to those skilled in the art Detector 882 is formed having strips 890 a-890 d defined by grooves cut within its upward charge collecting surface The opposite face of detector component 882 similarly is formed having strips 892 a-892 d defined by intermediately positioned grooves arranged orthogonally with respect to the grooves at the upper surface Detector component 15 884 is identically fashioned, having strips 894 a-894 d formed at its upwardly disposed charge collecting surface; and it its lower surface, orthogonally disposed strips 896 a-896 d, adjacent said strips being defined by intermediately formed grooves Similarly, detector component 886 is formed having strips 898 a898 d atits upward surface, adjacent ones of the strips being defined by intermediately 20 disposed grooves, while its lower surface similarly is formed having strips 900 a900 d defined by intermediately disposed grooves arranged orthogonally with respect to the grooves of the upward surface Detector component 888 may be observed to have strips 902 a-902 d at its upward surface adjacent ones of which are defined by intermediately designated grooves, while its lower surface is formed 25 with adjacently disposed strips 904 a-904 d separated by intermediately disposed grooves arranged orthogonally to the grooves of the upward surface thereof.

In the instant embodiment, strips 894 a-894 d of detector component 884 are directly, electrically associated with corresponding row strips 890 a-890 d of component 882 by electrical leads, respectively identified at 906 a-906 d Note, that 30 no impedance network is interposed intermediate the strip groupings as in the earlier embodiments However, an impedance network, designated generally at 908, is associated with the termini of strips 890 a-890 d opposite the edges thereof coupled with electrical leads 906 a-906 d Network 908 comprises serially .5 associated discrete resistors 910 a-910 e which are tapped at their common 35 junctions by leads 912 a-912 d extending, respectively, to strips 890 a890 d The output, or readout points for the thus defined "row" of the composite detector assembly are represented at 914 and 916 and are provided the same respective spatial or x-designated coordinate parameter output labelling, (x B), (x A) as are present in the corresponding "row" of the embodiments of Figures 20 and 21 40 The corresponding upwardly disposed surfaces of component 886 and 888 are connected in similar fashion For instance, strips 902 a-902 d are electrically coupled with strips 898 a-898 d by respective electrical leads 918 a-918 d The "row" coupling thus provided is associated with an impedance network shown generally at 920 Network 920 is formed comprising serially associated discrete 45 resistors 922 a-922 e which are tapped at their common interconnections by leads 924 a-924 d Leads 924 a-924 d, respectively, extend to strips 898 a-898 d of detector 886 The principal termini of the thus defined "row" are identified at 926 and 928, having outputs respectively labelled (X 2 B, x 2 A).

Looking now to the lower surfaces of the detector components, the 50 orthogonally disposed strips of detector component 882 are electrically coupled as shown with the corresponding strips of detector component 886 by electrical leads 930 a-930 d The thus coupled strip arrays of those detector components are associated in "columnar" fashion with an impedance network identified generally at 932 Network 932 comprises serially associated discrete resistors 934 a934 e, the 55 interconnections between which are connected as shown with strips 900 a900 d of component 886 by leads respectively identified at 936 a-936 d The readout termini for the thus defined "column" association of detectors 886 and 882 are present at 938 and 940 and the corresponding spatial or y-designated coordinate parameter outputs are identified respectively as (y A) and (y B) 60 The lower surfaces of detector components 884 and 888 similarly are associated in "columnar" readout fashion, strips 896 a-896 d of the former being electrically connected through respective leads 940 a-940 d to strips 904 a-904 d of the latter The thus established "columnar" readout is associated with an impedance network identified generally at 942 and comprising serially associated 65 I 1,578,883 21 1,578,883 21 discrete resistors 944 a-944 e Strips 904 a-904 d, respectively, are coupled with the interconnection of the resistors 944 a-944 e of network 942 by leads 946 a-946 d.

As in the earlier embodiments, the principal readouts of the thus defined "columnar" detector component coupling are represented at 948 and 950 and their spatial coordinate parameter outputs are labeled, respectively, (y 2 A and (y 2 B) 5 From the foregoing description of the composite detector arrangement 880 it may be observed that the row-column association of the components thereof enjoys the noted spatial resolution advantages, however, the time constant characteristic thereof will reflect a higher capacity evaluation.

Attention is drawn to our co-pending application 80/77 (Serial No 1,578, 881) 10 and 7931324 (Serial No 1,578,882).

Claims (8)

WHAT WE CLAIM IS:-

1 A gamma camera for deriving image defining information of the source distribution of gamma rays providing a photon energy level, E, of interest, said device including an orthogonal strip array semi-conductor for deriving spatial and 15 energy level information corresponding with said distribution, said detector array of strips having a centre-to-centre strip spacing, L, said device further including a collimator operatively associated with said detector and having an inwardly disposed plane defining side spaced from the midplane of said detector a distance, C, an outwardly disposed plane defining side spaced from said inward side to define 20 thickness, A, and spaced from said source a distance, B; wherein said collimator comprises:

an array of adjacently disposed channels, having internal surfaces and disposed intermediate of said inward and outward sides, said array being configured to define a septal thickness, T, intermediate said channels, an effective 25 collimator thickness, AE=A-l 2/,u(E)l, where M(E) is the attenuation coefficient of the surface defining material of said channels for said energy level, E, said channels having a channel cross sectional area of effective diameter, D; and said collimator has a collimator resolution R, equal to or greater than about 1 7 (L) and being configured in substantial satisfaction of the expression: 30 D RcAE(A+B+C)

2 A gamma camera as claimed in Claim I wherein said internal surfaces of said array of adjacently disposed channels are configured as channel sides defining a square internal channel cross-section.

3 A gamma camera as claimed in Claim 1 or Claim 2 wherein said septal wall 35 thickness, T, is equal to or about:

-2 D In P g(E)A+ I n P and wherein, P, is the penetration fraction of said surface-defining material and has a value about equal to or less than 0 05.

4 A gamma camera as claimed in any one of Claims I to 3, wherein said array 40 of channels comprises a plurality of sheet members of height, h, each having a plurality of mutually equally spaced, parallel slots of length equal to or about h/2, and of width w; said members being mutually internested along said slots to define said array of adjacently disposed channels.

5 A gamma camera as claimed in Claim 4, wherein said sheet members have a 45 thickness equal to said septal wall thickness, T, and said height, h, is substantially equal to said collimator means thickness, A.

6 A gamma camera as claimed in Claim 5 wherein said sheet members are formed of said side defining material, said material exhibiting said attenuation coefficient M(E), for a said energy level, E, of about 140 ke V 50

7 A gamma camera as claimed in Claim 5, wherein said slots have a width corresponding with said sheet member thickness plus a tolerance equal to or less than 0 001 inch.

8 A gamma camera as claimed in any one of the preceding claims, having an optimal collimator geometric efficiency 55 22 1,578,883 22 l o 2822 12 l AE ( D T) 9 A gamma camera as claimed in any one of the preceding claims wherein said surface-defining material is made of tungsten, tantalum or lead.

A gamma camera as claimed in any one of the preceding claims, wherein:

said solid state detector array is formed of germanium 5 TREGEAR, THIEMANN & BLEACH, Chartered Patent Agents, Enterprise House, Isambard Brunel Road, Portsmouth, P 01 2 AN.

and 49/51, Bedford Row, London, WC 1 V 6 RU.

Agents for the Applicants.

Printed for Her Majesty's Stationery Office, by the Courier Press, Leamington Spa, 1980 Published by The Patent Office, 25 Southampton Buildings, London, WC 2 A IAY, from which copies may be obtained.