WO2016112135A1

WO2016112135A1 - Compact trapezoidal pet detector with light sharing

Info

Publication number: WO2016112135A1
Application number: PCT/US2016/012386
Authority: WO
Inventors: Robert S. Miyaoka; William C. J. HUNTER
Original assignee: University of Washington
Current assignee: University of Washington
Priority date: 2015-01-07
Filing date: 2016-01-06
Publication date: 2016-07-14
Anticipated expiration: 2017-07-07

Abstract

A positron emission tomography (PET) scanner (100) includes trapezoidal sensor modules (101) arranged adjacently to form an annular detector ring (110). The sensor modules include a plurality of scintillation crystals (103) with interleaved light reflective/blocking elements (106). At least the end-face reflective element covers only a portion of the face of the sensor module, such that light is shared between adjacent sensor modules. A photodetector (104) and optional light guide (105) are fixed to an outer face of the plurality of scintillation crystals, wherein the photodetectors are disposed at an angle with respect to adjacent sensor modules. Light sharing between neighboring modules allows the modules to be small, enabling a very compact device. Signals from the photodetectors are processed with a computer system (90) configured to identify the three-dimensional location of scintillation events occurring in the detector ring.

Description

COMPACT TRAPEZOIDAL PET DETECTOR WITH LIGHT SHARING

CROSS-REFERENCE TO RELATED APPLICATION This application claims the benefit of Provisional Application No. 62/100,839, filed January 7, 2015, the entire disclosure of said application is hereby incorporated by reference herein.

STATEMENT OF GOVERNMENT LICENSE RIGHTS

This invention was made with Government support under R21-EB013716 awarded by National Institutes of Health. The Government has certain rights in the invention.

BACKGROUND

Positron Emission Tomography (PET) is a functional imaging technology developed during the latter half of the twentieth century. The first functional PET imager has been variously attributed to Michael E. Phelps at Washington University for a PET system known as PETT I, and to Sy Rankowitz and James Robertson at the Brookhaven National Laboratory in 1962. PET technology has continued to advance and has gained widespread adoption in nuclear medicine and in preclinical research. PET functional imaging is frequently combined with computed tomography or magnetic resonance imaging to obtain concurrent functional and structural image data.

Small animal imaging has emerged as an important research field. Currently, many research efforts have been devoted to multimodality imaging systems. For example, systems that incorporate both positron emission tomography (PET) and magnetic resonance imaging (MRI) provide information for combined functional and anatomic imaging. Due to the relatively small bore size in small animal PET detectors, it is desirable to provide three-dimensional (i.e., including depth of interaction (DOI) position) capability. Desirable characteristics in a small animal imaging PET system include (1) high image resolution; (2) high absolute sensitivity; (3) the ability to work in a strong magnetic field; and (4) a compact size.

The present inventors have created a novel high resolution, monolithic crystal small animal PET detector that provides a number of advantages over discrete crystal detector designs. Relatively large scintillation crystals are preferred for a monolithic crystal detector because of the challenges associated with positioning events near the edges of a crystal, which makes two-dimensional monolithic crystal geometry less than ideal for a compact PET detector. In order to maintain the advantages of monolithic crystal design, such as DOI decoding capability, while reducing the edge effects, the PET detector may use elongate trapezoidal slat crystals (TSC) to produce a compact annular detector. A PET detector suitable for use in a PET/MRI imaging system is disclosed in Xiaoli Li, et al., "Design of a trapezoidal slat crystal (TSC) PET detector for small animal PET/MR imaging," Nuclear Science Symposium Conference Record (NSS/MIC), 2010 IEEE (hereinafter, Li), which is hereby incorporated by reference in its entirety.

A front view of a prior art PET detector 50 is shown schematically in FIGURE 1.

The PET detector 50 includes a plurality of detector modules 52 assembled in an annular arrangement. During imaging, the subject is positioned generally along the center axis of the annular assembly.

A detector module 52 is shown in a detail view in FIGURE 1. The detector module 52 includes a scintillator body 51 that is shaped as an isosceles trapezoid. An optional optical window or light guide 57 is fixed to an outer end of the scintillator body 51, and two rows of coplanar photodetectors 60, for example, silicon photomultipliers (SiPM), are positioned to detect light released from scintillation events in the scintillator body 51. For example, a 2 X 12 array of photodetectors 60 may be provided on each scintillator body 51.

The scintillator body 51 comprises a plurality of elongate slat crystals 54 (8 shown) that are trapezoidal in cross section, and tapers in the radially inward direction. Light-blocking/reflecting elements 56 between each of the slat crystals 54 prevent (full length) or limit (less than full length) light sharing between adjacent slat crystals 54. Full length light-blocking/reflecting elements 56 cover both radial sides of the scintillator body 51. Light-blocking/reflecting elements 56 that extend only partially along the radial length of the adjacent slat crystals 54 permit some light from a scintillation event in one slat crystal 54 to be shared with an adjacent slat crystal 54. In particular, an outer portion of some of the interior slat crystal 54 faces are joined with transparent glue 108 (indicated by dashed lines), that allow light sharing internally within the module 52. This light sharing feature provides information that can be used to estimate the crystal of interaction of the scintillation event occurring in the detector module 52. However, this PET detector provides for light sharing only internally within a module 50, and requires two or more coplanar rows of photodetectors 60. The detector modules 50 are therefore relatively wide, to accommodate the two co-planar photodetectors 60, which limits the practical compactness of the scanner. The detector modules 50 do not incorporate light sharing between adjacent modules 52. It would be beneficial to have narrower detector elements to improve the resolution of the PET imager 50.

SUMMARY

This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

A positron emission tomography (PET) scanner includes a plurality of sensor modules arranged adjacently to form an annular detector ring. The sensor modules include (i) plural scintillation crystals with oppositely disposed converging faces, and plural light blocking elements interleaved with the scintillation crystals to form a scintillator assembly having a trapezoidal cross section; (ii) an end-face light blocking element fixed to an end face of the scintillator assembly, and blocking a radially inner portion of the end face, leaving a radially outer portion unblocked; and (iii) a row of photodetectors fixed to a radially inner or outer face of the scintillator assembly. The sensor modules are configured to share light across the radially outer portion of the end- face with an adjacent sensor module. Light may also be shared between adjacent crystals within a crystal module. In an embodiment the PET scanner includes a computing system operatively connected to the plurality of sensor modules to receive signals from the rows of photodetectors during operation of the PET scanner.

In an embodiment the sensor modules share light with adjacent sensor modules such that the shared light is detected by respective rows of photodetectors that are not coplanar.

In an embodiment the scintillation crystals are slat crystals having a wedge shape with parallel outer and inner faces connected by converging faces.

In an embodiment the scintillation crystals have an inner face with a width of 1.0 mm or less, and in some embodiments the inner face width is 0.5 mm or less. In some embodiments the scintillation crystals are at least 40 mm in length. In an embodiment the sensor modules each comprise six or fewer scintillation crystals. In an embodiment the light block elements are reflective.

In an embodiment at least some of the light blocking elements extend radially from a radially inner end of adjacent scintillation crystals only part way to a radially outer end, such that the radially outer portion is not blocked and light can be shared between the adjacent scintillation crystals

In an embodiment the plurality of scintillation crystals are fixed to each other with a transparent glue.

In an embodiment the sensor modules include a light guide disposed between the scintillator assembly and the row of photodetectors.

A sensor module for a PET scanner includes (i) a plurality of scintillation crystals with oppositely disposed converging faces and a plurality of light blocking elements interleaved with the plurality of scintillation crystals to form a scintillator assembly having a trapezoidal cross section; (ii) an end-face light blocking element fixed to an end face of the scintillator assembly, configured to block only a radially inner portion of the end face of the scintillator assembly such that a radially outer portion of the end face is not blocked; and (iii) a row of photodetectors fixed to an inner or outer face of the scintillator assembly operable to detect scintillation photons generated in the scintillator assembly.

In an embodiment the plurality of scintillation crystals are elongate slat crystals having a wedge shape with parallel outer and inner faces connected by converging faces.

In an embodiment the plurality of scintillation crystals have an inner face with a width of 0.5 mm or less, and a length of at least 40 mm.

In an embodiment the sensor module has six or fewer scintillation crystals. In an embodiment the light blocking elements are reflective.

In an embodiment at least some of the plurality of light blocking elements disposed between adjacent pairs of scintillation crystals in each sensor module extend from a radially inner end of adjacent scintillation crystals only part way to a radially outer end of the adjacent scintillation crystals, such that a radially outer portion of the adjacent crystals is not blocked by the light blocking element such that light can be shared between the adjacent scintillation crystals through the radially outer portion of the adjacent crystals. In an embodiment the sensor module includes a light guide disposed between the scintillator assembly and the row of photodetectors.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a schematic front view of a prior art PET detector, with a detail view of a detector module;

FIGURE 2 is a partially exploded view of a PET detector in accordance with the present invention;

FIGURE 3 is a front view of the scanner ring for the PET detector shown in FIGURE 2;

FIGURE 4 is an exploded view of one detector module of the PET detector shown in FIGURE 2; and

FIGURE 5 is a front view of two detector modules of the PET detector shown in FIGURE 2.

DETAILED DESCRIPTION FIGURE 2 illustrates an ultra-compact positron emission tomography scanner 100 in accordance with the present invention.

Positron emission tomography (PET) is a scintillation crystal medical imaging modality that takes advantage of radioactive decay to measure metabolic activities inside a living organism. A PET imaging system comprises three main components, a radioactive tracer that is administered to the subject to be scanned, a scanner that is operable to detect the location of the radioactive tracer (indirectly, as discussed below), and a tomographic image processing system.

The radioactive tracer includes a radioactive isotope and a metabolically active molecule. The tracer is injected into the body to be scanned, wherein the metabolically active molecule allows the tracer to be metabolized in active cells. After allowing time for the tracer to concentrate in certain tissues, the body (or a portion of the body) is positioned in the center channel of the annular PET scanner 100. The radioactive decay event for tracers used in PET studies is positron emission. An emitted positron travels a short distance in the body tissue until it interacts with an electron. The positron and electron interact in an annihilation event that produces two 511 KeV anti-parallel photons. The PET scanner 100 detects pairs of photons from annihilation events from detector modules on opposite sides of the annihilation event, essentially simultaneously.

A 511 KeV photon has a high energy and will pass through many materials, including body tissue. While the high energy allows the photon to travel through and exit the body, the high-energy photons are difficult to detect directly. Therefore, PET detectors are configured to detect the high-energy photons indirectly. Photon detection is the task of scintillator crystals 103 in the PET scanner 100. To detect a high-energy photon, sometimes referred to herein as a gamma photon, the scintillator crystal 103 absorbs or scatters (e.g., via Compton scattering) the gamma photon and emits a large number of low-energy photons (scintillation photons), which may be visible light photons. The incident gamma photons typically produce lantthousands of scintillation photons in a very short flash or scintillation event. The number of scintillation photons produced in the slat crystal 103 is proportional to the energy deposited by the gamma photon. As used herein, "slat crystal" is expressly defined to mean a scintillation crystal having a wedge shape with parallel inner and outer faces connected by converging faces.

The scintillation photons are readily detected with photodetectors 104 that may be placed on the radially outer or inner surface of the scintillator crystals, or on both the inner and outer surfaces. Exemplary photodetectors 104 include photomultiplier tubes (PMT), avalanche photodiodes (APDs), Si-PIN photodiodes, silicon drift photodiodes, and silicon photomultipliers (SiPM). The signals from the photodetectors 104 are analyzed to identify the relevant scintillation crystal 103 and determine the location of the scintillation event within the scintillation crystal 103 (preferably, in three spatial dimensions). The time of the scintillation event and the total energy of the event are also obtained.

Exemplary scintillation crystals include Nal(TI) (thallium-doped sodium iodide),

BGO (bismuth germinate), LSO (lutetium oxyorthosilicate), GSO (gadolinium orthosilicate), LYSO (cerium-doped lutetium yttrium orthosilicate), LuAP (lutetium aluminum perovskite), LGSO (LU_Q 4Gdi gSiC^: 22.0 mol% Ce), LaBr₃ (lanthanum bromide), lutetium fine silicate, and the like. Additional suitable scintillation crystals are disclosed in U.S. Pat. 7,132,060 to Zagumennyi et al., which is hereby incorporated by reference. For example, when BGO interacts with high-energy radiation, such as gamma- rays or x-rays, it emits a green fluorescent light with a peak wavelength of 480 nm. BGO is used for a wide range of applications in high-energy physics, nuclear physics, space physics, nuclear medicine, geological prospecting, and other industries. LYSO crystal has the advantages of high light output and density, quick decay time, excellent energy resolution, and moderate cost. These properties make LYSO a good candidate for a range of detection applications in nuclear physics and nuclear medicine, which require improved timing and energy resolution. Not all scintillation materials are technically crystals. As used herein, the term "scintillation crystal" and "slat crystal" is defined to include a component formed from any suitable scintillation material.

The PET scanner 100 shown in FIGURE 2 comprises two scanner rings 110 and related front-end electronics 90. Refer also to FIGURE 3, which shows a front view of the scanner ring 110. The scanner rings 110 comprise a plurality of relatively narrow sensor modules 101 assembled to form an annulus. More or fewer scanner rings 110 are contemplated, and may be selected to accommodate a desired axial operative length.

In this embodiment the scanner ring 110 has 24 sensor modules 101. More or fewer modules may be used, and are contemplated by the present invention. For example, a scanner ring 110 in accordance with the present invention may include 100 detector modules 101, or more. In particular embodiments teach detector ring 110 includes between 50 and 70 detector modules 101.

The sensor modules 101 are generally trapezoidal in cross section and are formed with a plurality of elongate slat-shaped scintillation crystals (slat crystals) 103. Light-blocking/reflecting element 106, for example, an opaque or reflective panel or coating, are interposed between opposing faces of the elongate scintillation crystals 103. Optionally, a light guide 105 is attached to a radially outer end of the scintillation crystals 103 in the sensor module 101. An axially oriented row of photodetectors 104 are attached to the light guide 105, if present, or to the radially outer end of the scintillation crystals 103 in the sensor module 101. For example, ten or more photodetectors 104 may be provided along the length of the scintillation crystals 103. The photodetectors 104 are configured to detect low-energy scintillation photons in the scintillator crystals 103.

The photodetectors 104 are operably connected to the front-end electronics 90. The signals or information received from the photodetectors 104 is processed by front- end electronics 90 to determine the parameters or characteristics of the detected pulse (e.g., location, energy, timing). The front-end electronics 90 may include, for example, one or more low-pass filters 96, analog-to-digital converters 97, and field programmable gate arrays 98. The sensors comprise scintillators 103 and photodetectors 104. In an exemplary embodiment the data is sent from the front-end electronics to a host computer 95 that performs tomographic image reconstruction.

An exploded view of the sensor module 101 is shown in FIGURE 4 (only one of the photodetectors 104 from the row of photodetectors is visible). In this embodiment the sensor module 101 comprises five trapezoidal slat crystals 103A-103E. The outboard slat crystals 103 A, 103E are similar in shape and mirror images in cross section. Interior slat crystals 103B, 103D are similar in shape and mirror images in cross section. The center slat crystal 103C has an isosceles cross-sectional shape. In other embodiments more or fewer slat crystals, and different crystal shapes may be used to form generally trapezoidal crystal modules.

The outboard slat crystal 103 A has an associated light-blocking or reflecting element 106A that is configured to extend along the outer face 113 A, from the inner end 114, and is sized to cover only a portion of the outer face 113A. For example, in various embodiments the light-blocking/reflecting element 106 A may cover between 50% and 80% of the area of the outer face 113 A. As will be clear from FIGURE 5, the corresponding light-blocking/reflecting element 106 A from an adjacent sensor module 101 will similarly cover a portion of the outer face 113B of the outboard slat crystal 103E.

Two intermediate light-blocking/reflecting elements 106B are also provided. One intermediate light-blocking/reflecting element 106B engages opposing faces of slat crystals 103 A and 103B, and is configured to cover only a portion of the opposing faces. The other intermediate light-blocking/reflecting element 106B engages opposing faces of slat crystals 103D and 103E, and is configured to cover only a portion of the opposing faces. For example, in various embodiments the intermediate light-blocking/reflecting elements 106B may cover between 60% and 90% of the area of the opposing faces. Therefore, depending on the location of the scintillation event, scintillation photons generated in slat crystals 103B or 103D may be shared with a neighboring outboard slat crystal 103 A, 103E and with a neighboring sensor module 101, by passing through adjacent slat crystals. Two central light-blocking/reflecting elements 106C are also shown. The central light-blocking/reflecting elements 106C are located on either side of the center slat crystal 103C and abut neighboring slat crystals 103B and 103D respectively. In this embodiment the central light-blocking/reflecting elements 106C extend along the entire face of the associated slat crystals, and therefore prevent light sharing with or across the central slat crystal 103C.

Although not shown in FIGURE 4, in this embodiment the slat crystals 103 and light-blocking/reflecting elements 106 are joined with transparent glue 108. In particular, the transparent glue 108 is provided between the light-sharing portions of the opposing faces of the slat crystals 103 that are not separated by a light-blocking/reflecting element.

In this embodiment the light guide 105 is fixed to an upper end of the slat crystals 103A-103E, and the row of photodetectors 104 are fixed to the light guide 105.

FIGURE 5 shows two adjacent sensor modules 101 from the scanner ring 110 shown in FIGURE 2. The row of photodetectors 104 for the sensor module 101 on the left side is oriented at an angle A with respect to the row photodetectors 104 for the sensor module 101 on the right side.

A scintillation event S is illustrated schematically in slat crystal 103E of the sensor module 101 on the left side. The scintillation photons generated are shared with slat crystal 103D of the sensor module 101 on the left side, and with slat crystals 103 A and 103B of the sensor module 101 on the right side.

The disclosed sensor module 101 has many advantages over related prior art PET detectors. An important aspect of the sensor module 101 is that it is very compact, and therefore allows for the construction of very compact PET imaging systems. All of the slat crystals 103 are trapezoidal in cross section, which has advantages in manufacturability and cost. The trapezoidal slat crystals can be fabricated as relatively thin slats, allowing for greater image resolution.

More or fewer slats may be used in each sensor module. For example, in one embodiment each sensor module comprises six slats that are each 8 mm tall and 40 mm long. The thickness of the slat crystals varies from 0.44 mm to 0.64 mm. In another embodiment the sensor modules comprise four crystal slats that are each 10 mm tall and 40 mm long. The thickness of the crystal slats varies from 0.71 mm to 1.11 mm. Therefore, in some embodiments the sensor module 101 comprises a plurality of slat crystals having a minimum thickness of less than 1.0 mm, and in some embodiments the sensor module 101 comprises a plurality of slat crystal having a minimum thickness of less than 0.5 mm.

Other characteristics of the sensor modules in accordance with the present invention may also be customized for a particular application by selecting lengths of the light blocking/reflecting elements to control light sharing within the crystal arrays.

For example, in an exemplary embodiment, to calibrate the sensor module 101 a coincidence line source is stepped along the full length of the sensor module in 1 mm increments, and the light response function versus position is characterized. The line source is perpendicular to the axial direction of the crystal array, and is created using a point source in coincidence with a thin linear array of discrete crystals. About 60,000 events are collected at each calibration position. The data are binned using simple Anger logic. The data are then separated into the crystal slat of interaction.

For the depth of interaction (DOI) calibration, we used the fact that the light spread on the linear array of the photodetectors (MPPC elements, Hamamatsu Photonics, Japan) is correlated with the depth of interaction of the event in the slat crystal. The closer the event occurs to the photodetector array the narrower the light distribution will be. The light distribution of each event is fit to the Lorentzian function. The width of the Lorentzian function is then used as a surrogate for the depth of interaction.

After the detector is calibrated, two testing data sets are collected. The first is with the line source perpendicular to the crystal array, identical to the calibration data set. The second is with the line source entering the crystal array at a 45 degree angle. The detectors were then evaluated for the following performance characteristics: slat decoding - peak to valley ration; energy resolution; intrinsic spatial resolution along the slat dimension; and the DOI positioning resolutions.

A new trapezoidal slat crystal PET detector is described above, using a shared photodetector design that can be implemented with thinner slats to improve spatial resolution and fewer slats, for better efficiency. Reducing the number of slat crystals in the sensor module allows taller slat crystals to be decoded without any loss in intrinsic spatial resolution along the axial and DOI directions.

While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.

Claims

CLAIMS The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A positron emission tomography (PET) scanner comprising:

a plurality of sensor modules having first and second radial faces, and arranged adjacently to form an annular detector ring, each sensor module comprising:

(i) a plurality of scintillation crystals with oppositely disposed converging faces and a plurality of light blocking elements, wherein the plurality of light blocking elements and the plurality of scintillation crystals are interleaved and are assembled to form a scintillator assembly having a trapezoidal cross section with one of the plurality of light blocking elements disposed between each adjacent pair of scintillation crystals;

(ii) an end-face light blocking element fixed to an end face of the scintillator assembly, wherein the end-face light blocking element blocks only a radially inner portion of the end face of the scintillator assembly such that a radially outer portion of the end face is not blocked; and

(iii) a row of photodetectors fixed to an inner or an outer face of the scintillator assembly, and operable to detect scintillation photons generated in the scintillator assembly;

wherein the sensor modules of the plurality of sensor modules are configured to share light across the radially outer portion of the end face that is not blocked by the end- face light blocking element with an adjacent sensor module.

2. The PET scanner of Claim 1, wherein each sensor module is configured to share light with an adjacent sensor module across the unblocked radially outer portion of the end face of the scintillator assembly, such that the shared light is detected by respective rows of photodetectors that are not coplanar.

3. The PET scanner of Claim 2, wherein the plurality of scintillation crystals are slat crystals having a wedge shape with parallel outer and inner faces connected by converging faces.

4. The PET scanner of Claim 3, wherein the plurality of scintillation crystals have an inner face width of 1.0 mm or less.

5. The PET scanner of Claim 3, wherein the plurality of scintillation crystals have an inner face width of 0.5 mm or less.

6. The PET scanner of Claim 3, wherein the plurality of scintillation crystals have an inner face having a width of 1.0 mm or less and a length of at least 40 mm.

7. The PET scanner of Claim 1, wherein the plurality of sensor modules each comprise six or fewer scintillation crystals.

8. The PET scanner of Claim 1, wherein the plurality of light blocking elements are reflective.

9. The PET scanner of Claim 1, wherein at least some of the plurality of light blocking elements disposed between adjacent pairs of scintillation crystals in each scintillator assembly extend from a radially inner end of adjacent scintillation crystals only part way to a radially outer end of the adjacent scintillation crystals, such that a radially outer portion of the adjacent crystals is not blocked by the light blocking element and such that light can be shared between the adjacent scintillation crystals within the scintillator assembly through the radially outer portion of the adjacent scintillation crystals.

10. The PET scanner of Claim 1, wherein the plurality of scintillation crystals in each sensor module are fixed to each other with a transparent glue.

11. The PET scanner of Claim 1, wherein each sensor module further comprises a light guide disposed between the scintillator assembly and the row of photodetectors.

12. The PET scanner of Claim 1, further comprising a computing system operatively connected to the plurality of sensor modules to receive signals from the rows of photodetectors during operation of the PET scanner.

13. A sensor module for a positron emission tomography (PET) scanner, the sensor module comprising:

(i) a plurality of scintillation crystals with oppositely disposed converging faces and a plurality of light blocking elements interleaved with the plurality of scintillation crystals to form a scintillator assembly having a trapezoidal cross section with one of the plurality of light blocking elements disposed between each adjacent pair of scintillation crystals;

(iii) a row of photodetectors fixed to an inner or an outer face of the scintillator assembly, and operable to detect scintillation photons generated in the scintillator assembly.

14. The sensor module of Claim 13, wherein the plurality of scintillation crystals are elongate slat crystals having a wedge shape with parallel outer and inner faces connected by converging faces.

15. The sensor module of Claim 14, wherein the plurality of scintillation crystals have an inner face width of 1.0 mm or less.

16. The sensor module of Claim 14, wherein the plurality of scintillation crystals have an inner face width of 0.5 mm or less.

17. The sensor module of Claim 14, wherein the plurality of scintillation crystals have an inner face having a width of 1.0 mm or less and a length of at least 40 mm.

18. The sensor module of Claim 13, wherein the plurality of sensor modules each comprise six or fewer scintillation crystals.

19. The sensor module of Claim 13, wherein the plurality of light blocking elements are reflective.

20. The sensor module of Claim 13, wherein at least some of the plurality of light blocking elements disposed between adjacent pairs of scintillation crystals in each scintillator assembly extends from a radially inner end of adjacent scintillation crystals only part way to a radially outer end of the adjacent scintillation crystals, such that a radially outer portion of the adjacent crystals is not blocked by the light blocking element such that light can be shared between the adjacent scintillation crystals within the scintillator assembly through the radially outer portion of the adjacent crystals.

21. The sensor module of Claim 13, further comprising a light guide disposed between the scintillator assembly and the row of photodetectors.