WO2018119070A1

WO2018119070A1 - Slanted surface crystal geometry for scintillation detector

Info

Publication number: WO2018119070A1
Application number: PCT/US2017/067586
Authority: WO
Inventors: Arion-Xenofon Fokion HADJIONANNOU; David PROUT; Yanisley VALENCIAGA
Original assignee: The Regents Of The University Of California
Priority date: 2016-12-22
Filing date: 2017-12-20
Publication date: 2018-06-28

Abstract

A scintillator crystal may be formed with a geometry that allows the capturing of DOI information. For example, a slanted surface may be one geometry formed in the scintillator crystal to obtain DOI information. Depth of interaction (DOI) information describes where in the scintillation crystal the gamma photon interacts with the crystal. Obtaining the DOI information and using the DOI interaction information in reconstructing the image obtained by Positron Emission Topography (PET) may improve the quality of the image, such as by improving resolution or by increasing sensitivity.

Description

DESCRIPTION

SLANTED SURFACE CRYSTAL GEOMETRY FOR SCINTILLATION DETECTOR

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS [0001] This application claims the benefit of priority of U.S. Provisional

Patent Application No. 62/437,816 to Arion Hadjioannou et al. filed on December 22, 2016, and entitled "Slanted Surface Crystal Geometry for Scintillation Detector," which is incorporated by reference.

FIELD OF THE DISCLOSURE [0002] The instant disclosure relates to scintillator. More specifically, portions of this disclosure relate to scintillator crystals for Positron Emission Tomography (PET) and other imaging techniques.

BACKGROUND

[0003] Positron Emission Tomography (PET) is a medical imaging technique based on the detection of annihilation photons through their interactions with scintillator crystals. In PET, a subject is injected with a tracer molecule that interacts with the biology of the body. The tracer molecule is designed to decay and emit positrons. A PET system detects the presence of those positrons when they annihilate with electrons in the subject. Upon annihilation, each electron/positron pair creates two gamma photons that travel in opposite directions along a straight line. These annihilation photons are typically detected and imaged with a scintillator detector, conventionally composed by a combination of scintillators and photomultipliers (PMT). This detector performs the detection by converting the high-energy annihilation photon (gamma radiation) to electronic position and time encoding signals. PET scans may be used to perform examinations of various processes within the body by selecting an appropriate tracer. One popular technique is to use PET imaging to detect the presence of cancerous tumors within the body.

[0004] In order to obtain high sensitivity and high spatial resolution, scintillation crystal pixels are conventionally designed to be long and narrow in shape. Increasing sensitivity with such a shape involves increasing the crystal volume to be large enough to stop the annihilation gamma photons. Consequently, the crystals are made longer, with increasingly larger aspect ratios. The narrower the scintillator crystal is, the better the spatial resolution. However, designing long and narrow crystal pixels introduces parallax error and reduces the scintillation light output. Parallax error is a difference in apparent position of an object from different viewpoints. Parallax error in PET imaging can result in lower resolution images and reduces the capability of identifying anomalies in the resulting image.

[0005] The parallax error results from assigning the interaction position of the annihilation gamma photon to a single position in a scintillation crystal, although this interaction can occur anywhere in the volume of the crystal or even in multiple crystals. When reconstruction software assumes a location inside the crystal at an average depth of interaction, there is still a residual position uncertainty that becomes larger for more oblique lines of response and for longer scintillation crystals. That is, the longer the scintillation crystal, the larger possible deviation from the average depth of interaction, and thus the larger error in reconstruction.

[0006] In circular PET scanners, the parallax error is not uniformly distributed throughout the field of view. In these scanners, photons interacting at an oblique angle with crystals at the periphery of the scanner field of view will be mispositioned towards the center of the scanner field of view. This reduces the accuracy of the image obtained from the PET scan.

SUMMARY

[0007] Depth of interaction (DOI) information describes where in the scintillation crystal the gamma photon interacts with the crystal. Obtaining the DOI information and using the DOI interaction information in reconstructing the image obtained by Positron Emission Topography (PET) may improve the quality of the image, such as by improving resolution and/or by increasing sensitivity. For example, the DOI information may be used to reduce parallax error that occurs in the conventional PET systems described above. [0008] The scintillator crystal may be formed with a geometry that allows the capturing of DOI information. For example, a slanted surface may be one geometry formed in the scintillator crystal to obtain DOI information. In some embodiments, the scintillator crystal may be a polished BGO crystal with a slanted surface. Dense and contiguous crystal arrays may be formed from scintillator crystals with at least one slanted surface. The crystal arrays may also include a combination of crystals such that some have at least one slanted surface and some do not have a slanted surface. In some embodiments, slanted surface crystals may be placed in close proximity, such as by attaching a slanted surface of one crystal to a slanted surface of another crystal. [0009] According to one embodiment, an PET detector may include a first scintillation crystal comprising a first edge configured to contact a first sensor and a second edge opposite the first edge, wherein the second edge comprises a slanted surface having at least one adjacent angle greater than ninety degrees. The PET detector may include a plurality of scintillation crystals arranged in an array. The PET detector may be part of a PET scanner configured to reconstruct an image of objects interacting with positrons emitted from a positron source of the PET scanner. In certain embodiments, the PET detector may include a mix of scintillation crystal configurations, such that some have a slanted surface and others do not. In certain embodiments, any slanted surface scintillation crystals may have the same of different angles of slanted surface. [0010] A PET detector may also include dual-read out detectors, which may be mixed with other scintillation crystal geometries in an array. A dual-read out detector in the array may include a first scintillating crystal comprising a first edge and a second edge opposite the first edge, wherein the second edge comprises a slanted surface; a second scintillating crystal comprising a third edge and a fourth edge opposite the third edge, wherein the fourth edge comprises a slanted surface, wherein the fourth edge of the second scintillating crystal abuts a second edge of the first scintillating crystal; a first detector attached to the first edge of the first scintillating crystal and configured to collect a first amount of scintillation light from the first scintillating crystal; and a second detector attached to the third edge of the second scintillating crystal and configured to collect a second amount of scintillation light from the second scintillating crystal.

[0011] The foregoing has outlined rather broadly certain features and technical advantages of embodiments of the present invention in order that the detailed description that follows may be better understood. Additional features and advantages will be described hereinafter that form the subject of the claims of the invention. It should be appreciated by those having ordinary skill in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same or similar purposes. It should also be realized by those having ordinary skill in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. Additional features will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended to limit the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] For a more complete understanding of the disclosed system and methods, reference is now made to the following descriptions taken in conjunction with the accompanying drawings.

[0013] FIG. 1A is a side view of a crystal geometry for obtaining depth of interaction (DOI) information according to some embodiments.

[0014] FIG. IB is a side view of a crystal geometry with a different angle for obtaining depth of interaction (DOI) information according to some embodiments. [0015] FIG. 1C is a side view of a crystal geometry with attached sensor for obtaining depth of interaction (DOI) information according to some embodiments.

[0016] FIG. 2 is a graph illustrating a simulated fraction of light output as a function of location of emission in a crystal with various crystal geometries according to some embodiments. [0017] FIG. 3 is a graph illustrating a measured fraction of light output as a function of location of emission in a crystal with various crystal geometries according to some embodiments. [0018] FIG. 4 is a graph illustrating energy spectra corresponding to light output simulations when using a 165° slanted geometry according to some embodiments of the disclosure.

[0019] FIG. 5 is a graph illustrating energy spectra corresponding to measured light output when using a 165° slanted geometry according to some embodiments of the disclosure.

[0020] FIG. 6 is a graph illustrating a curve fit used to quantify a correlation of depth of interaction (DOI) position in a slanted region of a crystal and light output according to some embodiments of the disclosure. [0021] FIG. 7 is a flow chart illustrating a method of determining depth of interaction (DOI) information from a crystal according to one of the geometries described in some embodiments of the disclosure.

[0022] FIG. 8 is a side view of a dual readout detector configuration that provides depth of interaction (DOI) information according to some embodiments of the disclosure.

DETAILED DESCRIPTION

[0023] A slanted surface may be incorporated into crystal geometry to assist in obtaining depth of interaction (DOI) information. Some example crystal geometries are shown in FIG. 1A, FIG. IB, and FIG. 1C. FIG. 1A is a side view of a crystal geometry for obtaining depth of interaction (DOI) information according to some embodiments. A crystal 102 may include a slanted surface 104, in which the degree of slant of the slanted surface 104 is defined by an angle 104 A between the slanted surface 104 and one of the parallel surfaces of the crystal 102. The crystal 102 may be made of suitable material for a scintillator detector, such as bismuth germinate (BGO), gadolinium oxyorthosilicate (GSO), or lutetium oxyorthosilicate (LSO). The crystal 102 may be treated, such as by polishing to obtain smooth surfaces or by etching to obtain rough surfaces. In some embodiments, the crystal 102 may be a polished BGO crystal. When the term "crystal" is used throughout this description, the "crystal" may be made of any of these example materials or others suitable for a scintillator detector. [0024] A crystal geometry with a smaller angle is shown in FIG. IB. FIG. IB is a side view of a crystal geometry with a different angle for obtaining depth of interaction (DOI) information according to some embodiments. A crystal 112 may include a slanted surface 114, in which the degree of slant of the slanted surface 114 is defined by an angle 114A between the slanted surface 114 and one of the parallel surfaces of the crystal 112. Although two particular angles for the slanted surface 114 are illustrated in FIG. 1A and FIG. IB, any angle other than ninety degrees may produce a slanted surface. Different angles may provide different capabilities for obtaining depth of interaction (DOI) information as described below. For example, the angles may be greater than ninety degrees, such as with angles of 120 degrees, 145 degrees, or 165 degrees.

[0025] A crystal with a slanted surface, such as the geometries illustrated in FIG. 1A and FIG. IB, may be attached to a sensor for obtaining information from interactions within the crystal. FIG. 1C is a side view of a crystal geometry with attached sensor for obtaining depth of interaction (DOI) information according to some embodiments. A crystal 122 may include a slanted surface 124, in which the degree of slant of the slanted surface 124 is defined by an angle 124 A between the slanted surface 124 and one of the parallel surfaces of the crystal 122. The crystal 122 may have a reflector 126 attached to portions of the surface of the crystal 122 by an adhesive 126A. For example, the reflector 126 may be attached to portions of or all of the slanted surface 124 and a parallel surface of the crystal 122 adjacent to the slanted surface 124. Additionally, a detector 132, such as a solid state detector (e.g., SiPM pixel) or another suitable detector, may be attached to the crystal 122 at a surface opposite and not in contact with the slanted surface 124. The detector 132 may be attached, for example, through glass 134 and optical grease 136. The crystal and detector arrangement of FIG. 1C may be used to measure a light source 140 when gamma rays enter the crystal at one of the parallel surfaces of the crystal 122, such as the parallel surface not coated by reflector 126. A controller 150 may be coupled to the detector 132 and configured for receiving and/or processing information. For example, the controller 150 may include an analog-to-digital converter (ADC) for converting an analog output from the detector to a digital signal for further processing. The controller 150 may also include logic circuitry for performing arithmetic operations on the detector data, such as in the converted digital signal. The arithmetic operations may include calculations to determine depth of interaction (DOI) information for annihilation photons in the crystal 122. Furthermore, in some embodiments, the controller 150 may include communications equipment for transmitting the detector data and or processed data to a recipient through wireless or wired communications channels.

[0026] Slanted surface detectors, such as those illustrated in FIG. 1A, FIG. IB, and FIG. 1C were simulated for various angles of the slanted surface through the radiation and optical simulation application, Geant4 Application for Tomographic Emission (GATE) . The simulation was run for locations in 2 mm steps along the length of the crystals. The results of the simulation are shown in FIG. 2. FIG. 2 is a graph illustrating a simulated fraction of light output as a function of location of emission in a crystal with various crystal geometries according to some embodiments. [0027] The fraction of light output as a function of location of the emission in the crystal is shown in FIG. 2 for crystals with different geometries. A line 202 illustrates a baseline result for a crystal geometry with no slanted surface. Lines 204, 206, and 208 illustrate results for crystal geometries with slanted surfaces from angles of 120, 145, and 160 degrees, respectively. The results show that the conventional 900° geometry has a light output that is not affected by the location of the interaction inside the crystal. However, changes in the light output for slanted surface geometries are detected. The light output change may be enhanced when the interaction occurs in the slanted surface region of the crystal. That is, the light output change is larger when the gamma ray enters the crystal and interacts with the crystal to produce a photon in a region such as region 108 of FIG. 1A or region 118 of FIG. IB. As shown in FIG. 2, crystals with 1650° and 1450° slanted surface geometries of lines 206 and 208 show up to about a 60% increase in light collection at an end of the crystal opposite the slanted surface as compared to the baseline rectangular geometry of line 202. For the 1650° case, a decrease in light output was observed when interaction occurs in the rectangular region of the crystal. Crystal with a 1200° slanted surface geometry of line 204 showed up to about a 20% increase in light output compared to a rectangular crystal over most of its length as compared to the baseline rectangular geometry of line 202.

[0028] Crystals in accordance with some embodiments of this disclosure were manufactured to validate the simulated results. Measurements of the crystals are shown in FIG. 3. FIG. 3 is a graph illustrating a measured fraction of light output as a function of location of emission in a crystal with various crystal geometries according to some embodiments. A line 308 illustrates a baseline result for a crystal geometry with no slanted surface. Lines 304, 306, and 302 illustrate results for crystal geometries with slanted surfaces from angles of 120, 145, and 160 degrees, respectively. The results of measurements shown in FIG. 3 are consistent with the simulated results of FIG. 2, although some imperfections in the sample manufacturing process may somewhat reduce the detection rate obtained in simulations.

[0029] The measurements and simulations of light output from crystal geometries with slanted surfaces illustrated in FIG. 2 and FIG. 3 illustrate that the light output depends on the location of interaction of the annihilation gamma within the crystals. That is, there is an increase in light output for interaction locations further in the slanted region of the crystals. This effect can be used to determine depth of interaction (DOI) information. FIG. 4 and FIG. 5 below illustrate energy spectra obtained from a crystal geometry with a 165° slanted surface and describe how those energy spectra can be used to determine depth of interaction (DOI) information.

[0030] FIG. 4 is a graph illustrating energy spectra corresponding to light output simulations when using a 165° slanted geometry according to some embodiments of the disclosure. In this measurement, the crystal is approximately 20 mm long, although embodiments of a crystal geometry may be other dimensions. A line 402 illustrates spectra acquired at locations between 0 mm and 10 mm along the crystal. These locations overlap because in the simulated crystal locations between 0 mm and 10 mm are in a rectangular region of the crystal. Lines 404, 406, 408, 410, and 412 illustrate spectra acquired at locations for 12 mm, 14 mm, 16 mm, 18 mm, and 20 mm. Each of the lines 404-412 illustrate different photo-peaks that allow for the different interaction depths to be distinguished. Thus, depth of interaction (DOI) information may be obtained by examining the photo-peaks. For example, a controller, such as a custom circuit or microprocessor, may be coupled to a detector attached to the crystal, record these photo-peaks, and process the photo-peaks to determine depth of interaction (DOI) information.

[0031] In some embodiments, the determined depth of interaction (DOI) information may be continuous or real-valued. That is, the depth of interaction may be determined as any value between 10 mm and 20 mm, such as 12.23 mm. In other embodiments, the determined depth of interaction (DOI) information may be assigned to bins for processing. For example, two to ten DOI bins may be implemented and photo-peaks assigned to those bins. The DOI resolution and spread of the photo-peaks may be affected by a total length of the crystal along which full energy deposition takes place and/or a width of the collimated beam of a photo source impinging the crystal. The depth of interaction (DOI) information may be created by a photoelectric effect within the crystal. A probability for a photoelectric effect in the first interaction in BGO at 511 keV, is approximately -44%. Thus, a large fraction of gammas will be completely stopped after two, or three interactions, thereby increasing the range of depth where the energy was deposited.

[0032] Measurements on a crystal with a 165° slanted surface were performed and the results are shown in FIG. 5. FIG. 5 is a graph illustrating energy spectra corresponding to measured light output when using a 165° slanted geometry according to some embodiments of the disclosure. The measurements of FIG. 5 produce similar results as the to the simulations of FIG. 4. Lines 502, 504, and 506 illustrate spectra acquired at locations for 10 mm, 14 mm, and 18 mm. The spectra of line 502 at 10 mm may be used to represent spectra acquired anywhere in the rectangular region of the crystal from 0 mm to 10 mm. The measured energy spectra illustrate that data may be grouped into three DOI bins. However, more or less DOI bins may be implemented by changing other factors, such as collimation of light entering the crystal, resolution of a detector attached to the crystal, and/or dimensions of the crystal.

[0033] An example calculation of depth of interaction (DOI) information using measured photo-peaks is described with reference to FIG. 6. FIG. 6 is a graph illustrating a curve fit used to quantify a correlation of depth of interaction (DOI) position in a slanted region of a crystal and light output according to some embodiments of the disclosure. From the energy spectra in the slanted surface region of the crystal (e.g., locations 10 mm to 18 mm of FIG. 4 or FIG. 5), the energy full-width, half-maximum (FWHM) is calculated as the spread of the number of collected scintillation photons and that data graphed in FIG. 6. A curve fit 602 is used to quantify the correlation of DOI position in the slanted surface region of the crystal and light output. From the curve fit 602, the following equation may be obtained:

DOl mm) = a x (# microcells per event— b), where a = 0.0468; b = 283.24

[0034] Using this equation and a value of triggered microcells at the FWHM energy limits, the corresponding positions of interaction are calculated. By subtracting the location of interaction, the DOI FWHM in units of millimeters is obtained and shown in Table 1 below:

TABLE 1.

[0035] Depth of interaction (DOI) information such as obtained according to the measurements and calculations described above may be computed by a controller, such as controller 150 of FIG. 1C, according to a programmed method. One example of such a method is described with reference to FIG. 7. FIG. 7 is a flow chart illustrating a method of determining depth of interaction (DOI) information from a crystal according to one of the geometries described in some embodiments of the disclosure. A method 700 begins at block 702 with receiving an energy spectra of annihilation photons created in a scintillation crystal with slanted surface geometry. Then, at block 704, depth of interaction (DOI) information is determined from the received energy spectra.

[0036] A dual read-out detector one detector structure that implements the slanted surface geometry for a scintillation crystal described above. A dual read-out detector may improve the quality of DOI information. One embodiment of a dual read-out detector is shown in FIG. 8. FIG. 8 is a side view of a dual readout detector configuration that provides depth of interaction (DOI) information according to some embodiments of the disclosure. [0037] In detector 800, two crystals 810 and 820 with slanted surface geometry are positioned opposite each other. For example, the crystals 810 and 820 may be positioned such that slanted surfaces 812 and 822 of the crystals 810 and 820, respectively, align to form one approximately rectangular- shaped detector. Annihilation photos generated within the crystals 810 and 820 may be readout separately by SiPM sensors 814 and 824, respectively. The sensors 814 and 824 may be attached to glass 816 and 826, and optical grease 818 and 828 of the crystals 810 and 820, respectively. DOI information may be assigned by the magnitude of the signal received by each of the sensors 814 and 824. The dual read-out detector 800 may offer higher spatial resolution [0038] The schematic flow chart diagram of FIG. 7 is generally set forth as a logical flow chart diagram. As such, the depicted order and labeled steps are indicative of aspects of the disclosed method. Other steps and methods may be conceived that are equivalent in function, logic, or effect to one or more steps, or portions thereof, of the illustrated method. Additionally, the format and symbols employed are provided to explain the logical steps of the method and are understood not to limit the scope of the method. Although various arrow types and line types may be employed in the flow chart diagram, they are understood not to limit the scope of the corresponding method. Indeed, some arrows or other connectors may be used to indicate only the logical flow of the method. For instance, an arrow may indicate a waiting or monitoring period of unspecified duration between enumerated steps of the depicted method. Additionally, the order in which a particular method occurs may or may not strictly adhere to the order of the corresponding steps shown.

[0039] The operations described above as performed by a controller may be performed by any circuit configured to perform the described operations. Such a circuit may be an integrated circuit (IC) constructed on a semiconductor substrate and include logic circuitry, such as transistors configured as logic gates, and memory circuitry, such as transistors and capacitors configured as dynamic random access memory (DRAM), electronically programmable read-only memory (EPROM), or other memory devices. The logic circuitry may be configured through hard-wire connections or through programming by instructions contained in firmware. Further, the logic circuity may be configured as a general purpose processor capable of executing instructions contained in software. If implemented in firmware and/or software, functions described above may be stored as one or more instructions or code on a computer-readable medium. Examples include non-transitory computer-readable media encoded with a data structure and computer-readable media encoded with a computer program. Computer-readable media includes physical computer storage media. A storage medium may be any available medium that can be accessed by a computer. By way of example, and not limitation, such computer-readable media can comprise random access memory (RAM), read-only memory (ROM), electrically-erasable programmable read-only memory (EEPROM), compact disc read-only memory (CD-ROM) or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer. Disk and disc includes compact discs (CD), laser discs, optical discs, digital versatile discs (DVD), floppy disks and Blu-ray discs. Generally, disks reproduce data magnetically, and discs reproduce data optically. Combinations of the above should also be included within the scope of computer-readable media. [0040] In addition to storage on computer readable medium, instructions and/or data may be provided as signals on transmission media included in a communication apparatus. For example, a communication apparatus may include a transceiver having signals indicative of instructions and data. The instructions and data are configured to cause one or more processors to implement the functions outlined in the claims. [0041] Although the present disclosure and certain representative advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the present disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

Claims

What is claimed is:

An apparatus, comprising: a first scintillation crystal comprising a first edge configured to contact a first sensor and a second edge opposite the first edge, wherein the second edge comprises a slanted surface having at least one adjacent angle greater than ninety degrees.

The apparatus of claim 1, wherein the first scintillation crystal is a polished BGO crystal.

The apparatus of claim 1, wherein the first scintillation crystal is a polished LSO crystal.

The apparatus of claim 1, wherein the at least one adjacent angle is between approximately 120 degrees and approximately 170 degrees.

The apparatus of claim 4, wherein the at least one adjacent angle is approximately 165 degrees.

The apparatus of claim 1, further comprising: a first sensor attached to the first edge of the first scintillation crystal; a second scintillation crystal comprising a third edge comprising a slanted surface configured to contact the second edge, and the second scintillation crystal further comprises a fourth edge opposite the third edge; and a second sensor attached to the fourth edge of the second scintillation crystal.

The apparatus of claim 1, further comprising a solid state light detector attached to the first edge. The apparatus of claim 1, further comprising a controller coupled to the solid state light detector, wherein the controller is configured to determine an energy spectra of annihilation photons from the first scintillation crystal and configured to determine depth-of-interaction (DOI) information from the determined energy spectra.

The apparatus of claim 8, wherein the controller is configured to process the energy spectra to obtain DOI information for between 2-6 bins.

The apparatus of claim 1, wherein the apparatus comprises a scintillation detector.

The apparatus of claim 1, wherein the apparatus comprises a positron emission tomography (PET) scanner, and wherein the apparatus further comprises a plurality of scintillation crystals configured to contact a plurality of sensors.

An apparatus, comprising: a first scintillating crystal comprising a first edge and a second edge opposite the first edge, wherein the second edge comprises a slanted surface; a second scintillating crystal comprising a third edge and a fourth edge opposite the third edge, wherein the fourth edge comprises a slanted surface, wherein the fourth edge of the second scintillating crystal abuts a second edge of the first scintillating crystal to form an approximately rectangular shape; a first detector attached to the first edge of the first scintillating crystal and configured to collect a first amount of scintillation light from the first scintillating crystal; and a second detector attached to the third edge of the second scintillating crystal and configured to collect a second amount of scintillation light from the second scintillating crystal.

13. The apparatus of claim 12, wherein the first scintillating crystal and the second scintillating crystal comprise polished BGO crystal.

14. The apparatus of claim 12, further comprising a controller coupled to the first detector and to the second detector, wherein the controller is configured to determine depth-of-interaction information (DOI) based, at least in part, on the first amount of scintillation light from the first scintillating crystal and the second amount of scintillation light from the second scintillating crystal.