CA2700470A1 - Scintillation light detection systems and methods based on monolithic scintillators - Google Patents
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Abstract
The present invention discloses detector structure and arrangement based on monolithic scintillation materials, and discloses direct methods of resolving three-dimensional position (X, Y, Z), energy (E), and time of absorption events of high energy photons in monolithic scintillators using explicit mathematical formulas, for applications such as gamma ray detection and imaging, and positron imaging.
Description
FIELD OF THE INVENTION
[0001]The present invention relates to imaging systems based on coincident or non-coincident scintillation light detection, particularly those systems exploiting monolithic scintillators (crystalline or non-crystalline) to convert high energy photons to optical or near optical wavelengths. This disclosure describes methods of resolving the three-dimensional (3D) position, as well as the energy and time of absorption events of high energy photons in monolithic scintillators using explicit mathematical formulas. Applications include, but not limited to, radiation detectors, high energy spectroscopy systems, gamma cameras, positron emission tomography (PET) and single photon emission tomography (SPECT) imaging systems.
BACKGROUND
[00021 Scintillation light detection has numerous applications in radiation imaging, particularly in SPECT
and PET systems used for medical imaging in nuclear medicine. Traditionally, PET and SPECT detectors were constructed using a slab of scintillating material (a monolithic scintillator) coupled to an array of photomultiplier tubes (photodetectors) wired in Anger Logic configuration.
Such systems were able to resolve energy, time and X and Y position of a scintillation light generation event in the scintillator. Among major limitations of such systems, one can point out to limited spatial resolution of the system, and the limited count rate that is inversely proportional to the area of the monolithic scintillator.
[0003]Today's PET scanners are constructed using detector modules comprised of pixilated scintillating crystals (called "crystals" hereafter), i.e., a group of tiny bars of crystals coupled on one end to a number of photodetectors. Such detector modules can detect the energy and time of an absorption event and determine in which crystal the absorption event occurred. Putting all that together, they can resolve the X and Y
position with higher spatial resolution (limited to the pixilated crystal pitch), as well as the energy and time of the scintillation light generation event at a much higher count rate compared to traditional monolithic detectors.
100041Detecting where in depth of the crystal the absorption event happened, i.e., the depth of interaction (DOI) or the Z component of the position of the event, considerably improves the image resolution of PET
systems with 3D reconstruction, particularly those that have small bore diameters, or use parallel plate detectors. In pixilated detectors, extraction of DOI requires exploiting a second set of photodetectors on the other end of the detector module (dual-ended readout technique) that considerably adds to the price and complexity of the system.
[00051Because of the inherent DOI dependency of light propagation in large volumes, monolithic crystals with different configuration, with different calculation methods have been proposed for detecting 3D
position of the absorption events as well as their energy and timing information. Examples are the US
patent application 20040227091 that discloses a detector module (Fig. 1.a) based on monolithic scintillator block 110 coupled through optical spacers 120, to array of photodetectors 130 operating in dual-ended readout configuration, or the US patent application 2010/0044571 Al that discloses a detector module (Fig.
1.b) based on a monolithic scintillator block 110 coupled through optical spacer 120 to array of photodetectors 130 on one side that uses statistical methods for resolving the 3D position of absorption events. Other methods, such as resistive networks (US Patent 7,476,864 B2), and neural networks with learning capabilities have been also proposed for detection and resolving of scintillation light and results have been published in literature. These are examples of the prior arts based on monolithic scintillators, that to the best of the inventor's knowledge, lack the capability of resolving DOI
(such as the tradition PET
detectors mentioned in [0002]), or use limited size scintillators such as the above cited US patent applications, or use statistical differences in the pattern of photodetectors' signals to resolve the 3D position of the event, such as the US patent application 2010/0044571 Al, or the neural networks method.
[0006]This background information is provided for the purpose of making known information believed by the inventor and/or the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.
SUMMARY OF THE INVENTION
[0007]This summary provides a selection of simplified concepts that are further described in Detailed Description below. It is not intended to list or limit the key features of the present invention in this summary, nor it is intended to provide this section as an aid to determine the scope of the claimed subject matter.
[0008]An object of the present invention is to provide a radiation detector apparatus using a monolithic scintillating material coupled to an array of photodetectors on one side of the scintillator that detect the scintillation light generated in said scintillator using a subset of the photodetector array that are not physically grouped, or are not selected by wiring, but, they are selected, and are assigned to resolve said scintillation light based on their generated signal level.
[0009]In accordance with another aspect of the present invention, there is provided a radiation detection system base on monolithic scintillators that can detect and resolve scintillation light events simultaneously and independent of each other if the subsets of photodetectors involved in resolving those events do not share any photodetectors, which means, scaling the monolithic scintillator in X-Y plane, does not affect the detector performance in detecting and resolving the scintillation light. In such a detection system, the maximum event rate that the system can resolve is determined by size of the detector subset.
[0010]In accordance with another aspect of the present invention, there is provided a method of resolving the depth of interaction (DOI, or Z) of a scintillation light generation event in monolithic scintillators using polynomial functions of the inverse square root of the sum of scaled photodetectors' signals obtained from a subset of photodetectors that receive the largest amount of said scintillation light.
[0011]In accordance with another aspect of the present invention, there is provided a method of resolving the energy of scintillation light generation event in monolithic scintillators using the sum of scaled photodetectors' signals obtained from a subset of photodetectors that receive the largest amount of said scintillation light, corrected by the previously resolved Z component of the position of said event.
[001211n accordance with another aspect of the present invention, there is provided a method of resolving the X or Y component of the position of scintillation light generation event in monolithic scintillators using weighting functions that their parameters are adjusted according to the previously resolved Z component of the position of said event.
[001311n accordance with another aspect of the present invention, there is provided a method of detecting events of a specific energy using a two-dimensional filtering technique based on resolved energy and resolved Z component of the position of the event. Events are selected if their resolved energy and their resolved Z are within a desired range. Or, events are discarded if their resolved energy, or their resolved Z, is outside a desired range.
[001411n accordance with another aspect of the present invention, there is provided a method of refining the time of a scintillation light generation event in monolithic scintillators using the resolved X, Y and Z
component of the position of said event that accounts for the propagation time that it takes for the light to travel from the position of the event to the photodetector that receives the largest amount of said scintillation light.
BRIEF DISCRIPTION OF FIGURES AND EQUATIONS
[0015]FIG. l.a. illustrates a monolithic scintillator block coupled to two arrays of photodetectors in dual-ended configuration according to the prior art.
[0016]FIG. 1.b. illustrates a monolithic scintillator block coupled an array of photodetectors on one side, for resolving events using statistical methods (maximum likelihood estimation) according to the prior art.
[0017]FIG. 2.a. illustrates a schematic diagram of the cross section of detector structure comprising: an X-Y
scalable monolithic scintillators coupled an array of photodetectors on one side according to an embodiment of the present invention.
[0018]FIG. 2.b. illustrates a schematic diagram of the cross section of detector structure, and assignment of a subset of photodetector array to resolve a scintillation event in the monolithic scintillator according to an embodiment of the present invention.
[0019]FIG. 2.c. illustrates a schematic diagram of the cross section of detector structure for resolving simultaneous events independent of each other in the same monolithic scintillator according to an embodiment of the present invention.
[0020]FIG. 3. illustrates a schematic diagram of top-view of part of a photodetector array and assignment of subsets of different size for resolving scintillation events in monolithic scintillators according to various embodiments and methods of the present invention.
[0021]FIG. 4. illustrates a schematic diagram of arrangement of photodetectors in an example of a 5x5 subset of photodetectors for resolving the X component of a scintillation event in monolithic scintillators according to a method of the present invention.
[0022]FIG. 5. illustrates a schematic diagram of arrangement of photodetectors in an example of a 3x3 subset of photodetectors for resolving the Y component of a scintillation event in monolithic scintillators according to a method of the present invention.
[0023]FIG. 6. illustrates a schematic diagram of arrangement of photodetectors in an example of a 2x2 subset of photodetectors for resolving the X component of a scintillation event in monolithic scintillators according to a method of the present invention.
[0024]FIG. 7. illustrates a schematic diagram of arrangement of photodetectors in an example of a 2x2 subset of photodetectors for resolving the Y component of a scintillation event in monolithic scintillators according to a method of the present invention.
[0025]FIG. 8. illustrates simulation results of the example for variations of Sz as a function of actual value of Z according to a method of the present invention.
[0026]FIG. 9. illustrates simulation results of the example for variations offz as a function of actual value of Z according to a method of the present invention.
[0027]FIG. 10. illustrates simulation results of the example for ZR, the resolved value of Z, as a function of actual value of Z according to a method of the present invention.
[0028]FIG. 11. illustrates simulation results of the example for histogram of the error in resolving Z
according to a method of the present invention.
[0029]FIG. 12. illustrates the concept of two-dimensional filtering a method of the present invention, where resolved energy of events are plotted against their ZR, or resolved Z, for simulation results of the example.
[0030]FIG. 13. illustrates simulation results of the example for plot of XR
(or YR), the resolved value of X
(or Y), as a function of the actual value of X (or Y) according to a method of the present invention.
[0031]FIG. 14. illustrates simulation results of the example for histogram the error in resolving X (or Y) according to a method of the present invention.
[0032]EQN. 1. formulates the method of calculating Z component of the 3D
position of scintillation event according to the present invention.
[00331EQN. 9. formulates the method of calculating the energy (E) of a scintillation event according to the present invention.
[0034]EQN. 12. formulates the method of calculating Xcomponent of the 3D
position of scintillation event according to the present invention.
[0035]EQN. 16. formulates the method of calculating Y component of the 3D
position of scintillation event according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Detector structure and description:
[0036]According to Fig. 2.a of the present invention, a radiation detection apparatus is presented comprising: the monolithic scintillating material 210 (such as LSO, LYSO or BGO crystals, or non-crystalline scintillators such as plastic scintillators) scalable in the X-Y
plane (Y axis is normal to X and Z
axis), the optional non-scintillating spacer 220, that optically couples the said monolithic scintillator to the array of photodetectors 230 (such as photomultiplier tubes, avalanche photodiodes, or solid-state photomultiplying photodetectors like silicon photomultipliers), that according to Fig. 2.b, the subset of photodetectors 232 near the photodetector 231 that is closest to the scintillation event 240, and generates the largest signal, detects and resolves the event 240 in the area 211 underneath the photodetector 231 generating the largest signal. According to Fig. 2.c of the present invention, other events such as 250 can be resolved simultaneously and independently of the event 240 if they are outside of the area 212, i.e., if the subsets of photodetectors involved in resolving those events do not share any photodetector with the subset 232 resolving the event 240.
[0037]According to Fig. 3 of the present invention, there is provided a schematic diagram of the top-view of part of the photodetector array, where different subsets of photodetectors for resolving the scintillation event 320 are illustrated as examples. Each element (310) of the photodetector array, called Pd;,;, and generates a signal Dv;,,j, supposedly proportional to the number of incident scintillation photons as the result of an absorption event in the monolithic scintillator. Subsets is chosen based on the photodetector 330 (Pd;MJM) that is closest to the event 320, and generates the maximum signal;
all elements of the subset are near Pd;MJM, including Pd;M,;M, and their output signals are generated within a limited time window, with the condition Dv, j > a preset threshold. Examples are given for regular NxN
subset for N = 2 or a 2x2 subset 340, N= 3 or a 3x3 sunset 350, and N= 5 or a 5x5 subset 360. For odd numbers of N, Pd;M,;M is in the center of the subset.
Resolving depth of interactions, or Z:
[0038]In accordance to a method of the present invention, the Z component of position of the absorption event (depth of interaction), is directly calculated using a polynomial function of the inverse square root of detector values:
ZR I NZ b,, C,Z-n/2 ' (1) in which ZR is the resolved value of actual Z, bõ are coefficients of the polynomial function of the order Nz, and Sz is the sum of scaled photodetectors' signals obtained from Eqn. 2:
SZ - I C,,iDvi,i (2) i C,,; are pre-defined constants, that may be used to homogenize the gain of photodetectors, or used to shape the subset. For example, in a 3x3 subset if CCU of the four corners are set to zero, the square shape subset shapes into a cross.
[0039]Next, Sz is converted tofz using a function, F, which establishes an approximately linear relationship between Sz and Z. The function F is derived based of the physics of light spread in monolithic scintillators:
fz =F(SZ)=kSZ-V2 (3) in that, k is a pre-defined constant.
[0040]Based on the physics of light spread on a rectangular area from a point at distant of Z above the center of the rectangular area,fz can be described as:
fz =k2sin-'/2(k3SZ) (4) where k, and k3 are pre-defined constants.
[0041]Because the product of k3Sz is smaller than unity, the sine function is approximated by its own argument so that Eqn. 4 reduces to Eqn. 3. There is a function, G, that best approximates the relationship betweenfz, and Z, so thatfz - G(Z). The inverse function of G, i.e., W, is used to resolve Z values (named ZR) fromfz:
ZR = G-' (fz) = (5) [0042]The inverse function, G-', can be approximated using different methods of function approximation such as polynomial functions, exponential or Tailor series. Here is a polynomial example of the order Nz :
ZR = G-' (M = In-0 anfzn = (6) [0043]The polynomial coefficients, an , are optimized by establishing a linear regression between ZR and Z.
In summary, by combining functions F and G-', resolved values of depth of interaction, ZR, are directly calculated from Sz (the scaled sum of detector values in a subset of the detector array):
ZR =G-' =F(SZ)=In?0anknSZ-n/2 (7) [0044]Egn. 7 is in fact in the form of Eqn. 1, i.e., ZR = En obn S,-n/2 , in which, bn =ankn. (8) [0045]For a practical implementation, a limited order polynomial function is considered, and the function can be simply implemented using lookup tables.
Resolving energy of interactions:
[0046]In accordance to another method of the present invention, the energy (E) of a scintillation event in the monolithic scintillator is resolved to ER from photodetectors' signals and a polynomial function calculated from the resolved depth of interaction, ZR:
ER=SE - Jnej dnZRn (9) in which, do are coefficients of the polynomial function of the order Ne, and SE is the sum of scaled photodetectors' signals in the photodetector array subset:
SE - ci,jDvi,j ' (10) i j cij are pre-defined constants, and SE and Sz (Eqn. 2) are not necessarily equal.
[0047]In a calibration process, polynomial coefficients, d, are calculated from the following equation for known values of SE measured or simulated for the desired value of the event energy:
n . (11) ed,, ZRn - SE = 0 Resolving X-Y position of interactions:
1004811n accordance to a method of the present invention, the X (or Y) position of a scintillation event is resolved to XR (or YR) using Z dependent weighting function in the subset of photodetector array:
X - xc + A(ZR) . (SxR - SxL) (12) R s B(ZR) - SXM - SxR - SxL
in which, xcs is the physical X position of the center of the subset, A and B
are pre-defined functions of resolved depth of interaction ZR.
[0049]SxL is the sum of scaled photodetectors' values on the left side of the subset along the X axis :
SxL = E Ecx,,jDv,,j . (13) i j<jM
[0050]If the number of photodetectors in the length of the subset along the X
axis is an even number (N= 2 for example), then SxM is zero, otherwise SxM is the scaled sum of detector values at jM :
SxM = ~cxi,jMDvi,jM . (14) [0051]SxR is the sum of scaled photodetectors' values on the right side of the subset along the Xaxis :
SX R = I I cxi,jDvi, j . (15) i j>jM
[00521Similar to Eqns. 12 - 15, the Y position of the interaction is resolved to YR as follow:
P A(ZR) . (SYr - SYB) (16) R - Ycs + B(ZR) - SYM - SYr - SYB
in which, ycs is the physical Y position of the center detector subset, A and B are pre-defined functions of resolved depth of interaction ZR.
[0053]Syr is the scaled sum of detector values on top side of the subset along the Y axis :
SYT = ZEcyj,jDvi,j . (17) i <iM j [00541If the number of photodetectors in the length of the subset along the Y
axis is an even number (N= 2 for example), then SyM is zero, otherwise SyM is the scaled sum of detector values at iM :
SYM = CYiM,jDv,M,j = (18) [0055]SyB is the scaled sum of detector values on bottom side of the subset along the Y axis :
SYB = j jC.yi,jDvi,j . (19) i>iM j [0056]According to a method of the present invention, use of functions A(ZR) and B(ZR) to correct the weighting functions (Eqn. 12 and 16) with the resolved value ZR, considerably improves the accuracy of the resolved position as opposed to when constant coefficients are used in place of A(ZR) and/or B(ZR) functions.
Examples of internal arrangement of subsets for resolving X and Y:
[0057]Fig. 4 provides the schematic diagram of top-view of a 5x5 example subset for resolving the X
component of the position of an absorption event detected by PdiMjM (410).
Photodetectors on the left side of the subset along the X axis (420), are used to calculate SxL, photodetectors at the center column 430 are selected to calculate SxM, and SxR is calculated using photodetectors on the right side of the subset along the X axis (440). xcs represents physical X position of the center of the 5x5 subset.
[0058]Fig. 5 provides an example of 3x3 subset for resolving Y around PdiMjM
(510). Photodetectors 520 that are on the top side of the subset along the Y axis, are used to calculate SyT, photodetectors 530 at the center row are selected to calculate SyM, and SyB is calculated using photodetectors 540 on the bottom side of the subset along the Y axis. yes represents physical Y position of the center of the 3x3 subset.
[0059]Fig. 6 provides an example of a 2x2 subset for resolving the X component of the position of an absorption event detected by any of the four photodetectors in the subset. The two photodetectors 610 on the left side of the subset are used to calculate SxL, and SxR is calculated using photodetectors 620 on the right side of the subset. xcs represents physical X position of the center of the 2x2 subset.
[0060]Fig. 7 provides an example of a 2x2 subset for resolving the Y component of the position of an absorption event detected by any of the four photodetectors in the subset. The two photodetectors 710 on the top side of the subset are used to calculate SyT, and SyB is calculated using photodetectors 720 on the bottom side of the subset. ycs represents physical Y position of the center of the 2x2 subset.
Example of Monte-Carlo simulation results:
[0061]Performance of disclosed methods in resolving 3D position and energy of scintillation events in a detector model are presented in Fig. 8 to Fig. 14. The detector model is comprised of a 100x100x10 mm3 slab of scintillating material coupled, through a 100x 100x2.5 mm3, to a 4x4 array of photodetectors (3.4 mm pitch) located at the center of the model detector, wherein, generation of scintillation light was simulated at uniformly at X-Y-Z steps of 0.8 mm. In order to evaluate performance of the innovated methods in general, properties of the scintillation material, or electronic noise of the processing circuitry were not included in the simulation. 3Td order polynomial functions were used.
[0062]Fig. 8 illustrates variations of SZ against Z, the actual depth of interaction according to Eqn. 2, Fig. 9 shows variations offz as a function of actual Z according to Eqn. 3, Fig. 10 illustrates how closely Z values are resolved using Eqn. 1, and Fig. 11 shows the error in resolved values of Z
as the result of using Eqn. 1;
the error at FWHM is -0.3 mm.
[0063]Fig. 12 shows ER (from Eq. 9) as a function of ZR (from Eqn. 1) for events of two different energies (511 keV, the desired energy to resolve, and 360 keV, scattered photons for example), exhibiting the capability of 2D filtration of unwanted events using an energy window, as well as a depth of interaction window according to a method of the present invention. For a given energy of interest, Eqn. 11 results in polynomial coefficients that make ER independent of ZR when calculated from Eqn. 9.
[0064]Fig. 13 illustrates resolved values of X (or Y)as the result of using Eqn. 12 (or Eqn. 16) against the actual value of X (or Y), and Fig. 14 illustrates the histogram of error in resolved value of X (or Y) for two cases where parameters of Eqn. 12 (or Eqn. 16), i.e. A and B are constant, compared to when they are adjusted using resolved value of Z to minimize the resolution error according a method of the present invention.
[0001]The present invention relates to imaging systems based on coincident or non-coincident scintillation light detection, particularly those systems exploiting monolithic scintillators (crystalline or non-crystalline) to convert high energy photons to optical or near optical wavelengths. This disclosure describes methods of resolving the three-dimensional (3D) position, as well as the energy and time of absorption events of high energy photons in monolithic scintillators using explicit mathematical formulas. Applications include, but not limited to, radiation detectors, high energy spectroscopy systems, gamma cameras, positron emission tomography (PET) and single photon emission tomography (SPECT) imaging systems.
BACKGROUND
[00021 Scintillation light detection has numerous applications in radiation imaging, particularly in SPECT
and PET systems used for medical imaging in nuclear medicine. Traditionally, PET and SPECT detectors were constructed using a slab of scintillating material (a monolithic scintillator) coupled to an array of photomultiplier tubes (photodetectors) wired in Anger Logic configuration.
Such systems were able to resolve energy, time and X and Y position of a scintillation light generation event in the scintillator. Among major limitations of such systems, one can point out to limited spatial resolution of the system, and the limited count rate that is inversely proportional to the area of the monolithic scintillator.
[0003]Today's PET scanners are constructed using detector modules comprised of pixilated scintillating crystals (called "crystals" hereafter), i.e., a group of tiny bars of crystals coupled on one end to a number of photodetectors. Such detector modules can detect the energy and time of an absorption event and determine in which crystal the absorption event occurred. Putting all that together, they can resolve the X and Y
position with higher spatial resolution (limited to the pixilated crystal pitch), as well as the energy and time of the scintillation light generation event at a much higher count rate compared to traditional monolithic detectors.
100041Detecting where in depth of the crystal the absorption event happened, i.e., the depth of interaction (DOI) or the Z component of the position of the event, considerably improves the image resolution of PET
systems with 3D reconstruction, particularly those that have small bore diameters, or use parallel plate detectors. In pixilated detectors, extraction of DOI requires exploiting a second set of photodetectors on the other end of the detector module (dual-ended readout technique) that considerably adds to the price and complexity of the system.
[00051Because of the inherent DOI dependency of light propagation in large volumes, monolithic crystals with different configuration, with different calculation methods have been proposed for detecting 3D
position of the absorption events as well as their energy and timing information. Examples are the US
patent application 20040227091 that discloses a detector module (Fig. 1.a) based on monolithic scintillator block 110 coupled through optical spacers 120, to array of photodetectors 130 operating in dual-ended readout configuration, or the US patent application 2010/0044571 Al that discloses a detector module (Fig.
1.b) based on a monolithic scintillator block 110 coupled through optical spacer 120 to array of photodetectors 130 on one side that uses statistical methods for resolving the 3D position of absorption events. Other methods, such as resistive networks (US Patent 7,476,864 B2), and neural networks with learning capabilities have been also proposed for detection and resolving of scintillation light and results have been published in literature. These are examples of the prior arts based on monolithic scintillators, that to the best of the inventor's knowledge, lack the capability of resolving DOI
(such as the tradition PET
detectors mentioned in [0002]), or use limited size scintillators such as the above cited US patent applications, or use statistical differences in the pattern of photodetectors' signals to resolve the 3D position of the event, such as the US patent application 2010/0044571 Al, or the neural networks method.
[0006]This background information is provided for the purpose of making known information believed by the inventor and/or the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.
SUMMARY OF THE INVENTION
[0007]This summary provides a selection of simplified concepts that are further described in Detailed Description below. It is not intended to list or limit the key features of the present invention in this summary, nor it is intended to provide this section as an aid to determine the scope of the claimed subject matter.
[0008]An object of the present invention is to provide a radiation detector apparatus using a monolithic scintillating material coupled to an array of photodetectors on one side of the scintillator that detect the scintillation light generated in said scintillator using a subset of the photodetector array that are not physically grouped, or are not selected by wiring, but, they are selected, and are assigned to resolve said scintillation light based on their generated signal level.
[0009]In accordance with another aspect of the present invention, there is provided a radiation detection system base on monolithic scintillators that can detect and resolve scintillation light events simultaneously and independent of each other if the subsets of photodetectors involved in resolving those events do not share any photodetectors, which means, scaling the monolithic scintillator in X-Y plane, does not affect the detector performance in detecting and resolving the scintillation light. In such a detection system, the maximum event rate that the system can resolve is determined by size of the detector subset.
[0010]In accordance with another aspect of the present invention, there is provided a method of resolving the depth of interaction (DOI, or Z) of a scintillation light generation event in monolithic scintillators using polynomial functions of the inverse square root of the sum of scaled photodetectors' signals obtained from a subset of photodetectors that receive the largest amount of said scintillation light.
[0011]In accordance with another aspect of the present invention, there is provided a method of resolving the energy of scintillation light generation event in monolithic scintillators using the sum of scaled photodetectors' signals obtained from a subset of photodetectors that receive the largest amount of said scintillation light, corrected by the previously resolved Z component of the position of said event.
[001211n accordance with another aspect of the present invention, there is provided a method of resolving the X or Y component of the position of scintillation light generation event in monolithic scintillators using weighting functions that their parameters are adjusted according to the previously resolved Z component of the position of said event.
[001311n accordance with another aspect of the present invention, there is provided a method of detecting events of a specific energy using a two-dimensional filtering technique based on resolved energy and resolved Z component of the position of the event. Events are selected if their resolved energy and their resolved Z are within a desired range. Or, events are discarded if their resolved energy, or their resolved Z, is outside a desired range.
[001411n accordance with another aspect of the present invention, there is provided a method of refining the time of a scintillation light generation event in monolithic scintillators using the resolved X, Y and Z
component of the position of said event that accounts for the propagation time that it takes for the light to travel from the position of the event to the photodetector that receives the largest amount of said scintillation light.
BRIEF DISCRIPTION OF FIGURES AND EQUATIONS
[0015]FIG. l.a. illustrates a monolithic scintillator block coupled to two arrays of photodetectors in dual-ended configuration according to the prior art.
[0016]FIG. 1.b. illustrates a monolithic scintillator block coupled an array of photodetectors on one side, for resolving events using statistical methods (maximum likelihood estimation) according to the prior art.
[0017]FIG. 2.a. illustrates a schematic diagram of the cross section of detector structure comprising: an X-Y
scalable monolithic scintillators coupled an array of photodetectors on one side according to an embodiment of the present invention.
[0018]FIG. 2.b. illustrates a schematic diagram of the cross section of detector structure, and assignment of a subset of photodetector array to resolve a scintillation event in the monolithic scintillator according to an embodiment of the present invention.
[0019]FIG. 2.c. illustrates a schematic diagram of the cross section of detector structure for resolving simultaneous events independent of each other in the same monolithic scintillator according to an embodiment of the present invention.
[0020]FIG. 3. illustrates a schematic diagram of top-view of part of a photodetector array and assignment of subsets of different size for resolving scintillation events in monolithic scintillators according to various embodiments and methods of the present invention.
[0021]FIG. 4. illustrates a schematic diagram of arrangement of photodetectors in an example of a 5x5 subset of photodetectors for resolving the X component of a scintillation event in monolithic scintillators according to a method of the present invention.
[0022]FIG. 5. illustrates a schematic diagram of arrangement of photodetectors in an example of a 3x3 subset of photodetectors for resolving the Y component of a scintillation event in monolithic scintillators according to a method of the present invention.
[0023]FIG. 6. illustrates a schematic diagram of arrangement of photodetectors in an example of a 2x2 subset of photodetectors for resolving the X component of a scintillation event in monolithic scintillators according to a method of the present invention.
[0024]FIG. 7. illustrates a schematic diagram of arrangement of photodetectors in an example of a 2x2 subset of photodetectors for resolving the Y component of a scintillation event in monolithic scintillators according to a method of the present invention.
[0025]FIG. 8. illustrates simulation results of the example for variations of Sz as a function of actual value of Z according to a method of the present invention.
[0026]FIG. 9. illustrates simulation results of the example for variations offz as a function of actual value of Z according to a method of the present invention.
[0027]FIG. 10. illustrates simulation results of the example for ZR, the resolved value of Z, as a function of actual value of Z according to a method of the present invention.
[0028]FIG. 11. illustrates simulation results of the example for histogram of the error in resolving Z
according to a method of the present invention.
[0029]FIG. 12. illustrates the concept of two-dimensional filtering a method of the present invention, where resolved energy of events are plotted against their ZR, or resolved Z, for simulation results of the example.
[0030]FIG. 13. illustrates simulation results of the example for plot of XR
(or YR), the resolved value of X
(or Y), as a function of the actual value of X (or Y) according to a method of the present invention.
[0031]FIG. 14. illustrates simulation results of the example for histogram the error in resolving X (or Y) according to a method of the present invention.
[0032]EQN. 1. formulates the method of calculating Z component of the 3D
position of scintillation event according to the present invention.
[00331EQN. 9. formulates the method of calculating the energy (E) of a scintillation event according to the present invention.
[0034]EQN. 12. formulates the method of calculating Xcomponent of the 3D
position of scintillation event according to the present invention.
[0035]EQN. 16. formulates the method of calculating Y component of the 3D
position of scintillation event according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Detector structure and description:
[0036]According to Fig. 2.a of the present invention, a radiation detection apparatus is presented comprising: the monolithic scintillating material 210 (such as LSO, LYSO or BGO crystals, or non-crystalline scintillators such as plastic scintillators) scalable in the X-Y
plane (Y axis is normal to X and Z
axis), the optional non-scintillating spacer 220, that optically couples the said monolithic scintillator to the array of photodetectors 230 (such as photomultiplier tubes, avalanche photodiodes, or solid-state photomultiplying photodetectors like silicon photomultipliers), that according to Fig. 2.b, the subset of photodetectors 232 near the photodetector 231 that is closest to the scintillation event 240, and generates the largest signal, detects and resolves the event 240 in the area 211 underneath the photodetector 231 generating the largest signal. According to Fig. 2.c of the present invention, other events such as 250 can be resolved simultaneously and independently of the event 240 if they are outside of the area 212, i.e., if the subsets of photodetectors involved in resolving those events do not share any photodetector with the subset 232 resolving the event 240.
[0037]According to Fig. 3 of the present invention, there is provided a schematic diagram of the top-view of part of the photodetector array, where different subsets of photodetectors for resolving the scintillation event 320 are illustrated as examples. Each element (310) of the photodetector array, called Pd;,;, and generates a signal Dv;,,j, supposedly proportional to the number of incident scintillation photons as the result of an absorption event in the monolithic scintillator. Subsets is chosen based on the photodetector 330 (Pd;MJM) that is closest to the event 320, and generates the maximum signal;
all elements of the subset are near Pd;MJM, including Pd;M,;M, and their output signals are generated within a limited time window, with the condition Dv, j > a preset threshold. Examples are given for regular NxN
subset for N = 2 or a 2x2 subset 340, N= 3 or a 3x3 sunset 350, and N= 5 or a 5x5 subset 360. For odd numbers of N, Pd;M,;M is in the center of the subset.
Resolving depth of interactions, or Z:
[0038]In accordance to a method of the present invention, the Z component of position of the absorption event (depth of interaction), is directly calculated using a polynomial function of the inverse square root of detector values:
ZR I NZ b,, C,Z-n/2 ' (1) in which ZR is the resolved value of actual Z, bõ are coefficients of the polynomial function of the order Nz, and Sz is the sum of scaled photodetectors' signals obtained from Eqn. 2:
SZ - I C,,iDvi,i (2) i C,,; are pre-defined constants, that may be used to homogenize the gain of photodetectors, or used to shape the subset. For example, in a 3x3 subset if CCU of the four corners are set to zero, the square shape subset shapes into a cross.
[0039]Next, Sz is converted tofz using a function, F, which establishes an approximately linear relationship between Sz and Z. The function F is derived based of the physics of light spread in monolithic scintillators:
fz =F(SZ)=kSZ-V2 (3) in that, k is a pre-defined constant.
[0040]Based on the physics of light spread on a rectangular area from a point at distant of Z above the center of the rectangular area,fz can be described as:
fz =k2sin-'/2(k3SZ) (4) where k, and k3 are pre-defined constants.
[0041]Because the product of k3Sz is smaller than unity, the sine function is approximated by its own argument so that Eqn. 4 reduces to Eqn. 3. There is a function, G, that best approximates the relationship betweenfz, and Z, so thatfz - G(Z). The inverse function of G, i.e., W, is used to resolve Z values (named ZR) fromfz:
ZR = G-' (fz) = (5) [0042]The inverse function, G-', can be approximated using different methods of function approximation such as polynomial functions, exponential or Tailor series. Here is a polynomial example of the order Nz :
ZR = G-' (M = In-0 anfzn = (6) [0043]The polynomial coefficients, an , are optimized by establishing a linear regression between ZR and Z.
In summary, by combining functions F and G-', resolved values of depth of interaction, ZR, are directly calculated from Sz (the scaled sum of detector values in a subset of the detector array):
ZR =G-' =F(SZ)=In?0anknSZ-n/2 (7) [0044]Egn. 7 is in fact in the form of Eqn. 1, i.e., ZR = En obn S,-n/2 , in which, bn =ankn. (8) [0045]For a practical implementation, a limited order polynomial function is considered, and the function can be simply implemented using lookup tables.
Resolving energy of interactions:
[0046]In accordance to another method of the present invention, the energy (E) of a scintillation event in the monolithic scintillator is resolved to ER from photodetectors' signals and a polynomial function calculated from the resolved depth of interaction, ZR:
ER=SE - Jnej dnZRn (9) in which, do are coefficients of the polynomial function of the order Ne, and SE is the sum of scaled photodetectors' signals in the photodetector array subset:
SE - ci,jDvi,j ' (10) i j cij are pre-defined constants, and SE and Sz (Eqn. 2) are not necessarily equal.
[0047]In a calibration process, polynomial coefficients, d, are calculated from the following equation for known values of SE measured or simulated for the desired value of the event energy:
n . (11) ed,, ZRn - SE = 0 Resolving X-Y position of interactions:
1004811n accordance to a method of the present invention, the X (or Y) position of a scintillation event is resolved to XR (or YR) using Z dependent weighting function in the subset of photodetector array:
X - xc + A(ZR) . (SxR - SxL) (12) R s B(ZR) - SXM - SxR - SxL
in which, xcs is the physical X position of the center of the subset, A and B
are pre-defined functions of resolved depth of interaction ZR.
[0049]SxL is the sum of scaled photodetectors' values on the left side of the subset along the X axis :
SxL = E Ecx,,jDv,,j . (13) i j<jM
[0050]If the number of photodetectors in the length of the subset along the X
axis is an even number (N= 2 for example), then SxM is zero, otherwise SxM is the scaled sum of detector values at jM :
SxM = ~cxi,jMDvi,jM . (14) [0051]SxR is the sum of scaled photodetectors' values on the right side of the subset along the Xaxis :
SX R = I I cxi,jDvi, j . (15) i j>jM
[00521Similar to Eqns. 12 - 15, the Y position of the interaction is resolved to YR as follow:
P A(ZR) . (SYr - SYB) (16) R - Ycs + B(ZR) - SYM - SYr - SYB
in which, ycs is the physical Y position of the center detector subset, A and B are pre-defined functions of resolved depth of interaction ZR.
[0053]Syr is the scaled sum of detector values on top side of the subset along the Y axis :
SYT = ZEcyj,jDvi,j . (17) i <iM j [00541If the number of photodetectors in the length of the subset along the Y
axis is an even number (N= 2 for example), then SyM is zero, otherwise SyM is the scaled sum of detector values at iM :
SYM = CYiM,jDv,M,j = (18) [0055]SyB is the scaled sum of detector values on bottom side of the subset along the Y axis :
SYB = j jC.yi,jDvi,j . (19) i>iM j [0056]According to a method of the present invention, use of functions A(ZR) and B(ZR) to correct the weighting functions (Eqn. 12 and 16) with the resolved value ZR, considerably improves the accuracy of the resolved position as opposed to when constant coefficients are used in place of A(ZR) and/or B(ZR) functions.
Examples of internal arrangement of subsets for resolving X and Y:
[0057]Fig. 4 provides the schematic diagram of top-view of a 5x5 example subset for resolving the X
component of the position of an absorption event detected by PdiMjM (410).
Photodetectors on the left side of the subset along the X axis (420), are used to calculate SxL, photodetectors at the center column 430 are selected to calculate SxM, and SxR is calculated using photodetectors on the right side of the subset along the X axis (440). xcs represents physical X position of the center of the 5x5 subset.
[0058]Fig. 5 provides an example of 3x3 subset for resolving Y around PdiMjM
(510). Photodetectors 520 that are on the top side of the subset along the Y axis, are used to calculate SyT, photodetectors 530 at the center row are selected to calculate SyM, and SyB is calculated using photodetectors 540 on the bottom side of the subset along the Y axis. yes represents physical Y position of the center of the 3x3 subset.
[0059]Fig. 6 provides an example of a 2x2 subset for resolving the X component of the position of an absorption event detected by any of the four photodetectors in the subset. The two photodetectors 610 on the left side of the subset are used to calculate SxL, and SxR is calculated using photodetectors 620 on the right side of the subset. xcs represents physical X position of the center of the 2x2 subset.
[0060]Fig. 7 provides an example of a 2x2 subset for resolving the Y component of the position of an absorption event detected by any of the four photodetectors in the subset. The two photodetectors 710 on the top side of the subset are used to calculate SyT, and SyB is calculated using photodetectors 720 on the bottom side of the subset. ycs represents physical Y position of the center of the 2x2 subset.
Example of Monte-Carlo simulation results:
[0061]Performance of disclosed methods in resolving 3D position and energy of scintillation events in a detector model are presented in Fig. 8 to Fig. 14. The detector model is comprised of a 100x100x10 mm3 slab of scintillating material coupled, through a 100x 100x2.5 mm3, to a 4x4 array of photodetectors (3.4 mm pitch) located at the center of the model detector, wherein, generation of scintillation light was simulated at uniformly at X-Y-Z steps of 0.8 mm. In order to evaluate performance of the innovated methods in general, properties of the scintillation material, or electronic noise of the processing circuitry were not included in the simulation. 3Td order polynomial functions were used.
[0062]Fig. 8 illustrates variations of SZ against Z, the actual depth of interaction according to Eqn. 2, Fig. 9 shows variations offz as a function of actual Z according to Eqn. 3, Fig. 10 illustrates how closely Z values are resolved using Eqn. 1, and Fig. 11 shows the error in resolved values of Z
as the result of using Eqn. 1;
the error at FWHM is -0.3 mm.
[0063]Fig. 12 shows ER (from Eq. 9) as a function of ZR (from Eqn. 1) for events of two different energies (511 keV, the desired energy to resolve, and 360 keV, scattered photons for example), exhibiting the capability of 2D filtration of unwanted events using an energy window, as well as a depth of interaction window according to a method of the present invention. For a given energy of interest, Eqn. 11 results in polynomial coefficients that make ER independent of ZR when calculated from Eqn. 9.
[0064]Fig. 13 illustrates resolved values of X (or Y)as the result of using Eqn. 12 (or Eqn. 16) against the actual value of X (or Y), and Fig. 14 illustrates the histogram of error in resolved value of X (or Y) for two cases where parameters of Eqn. 12 (or Eqn. 16), i.e. A and B are constant, compared to when they are adjusted using resolved value of Z to minimize the resolution error according a method of the present invention.
Claims (14)
1: A scintillation light detection apparatus comprising of an array of photodetectors optically coupled, using an optional non-scintillating material, to a monolithic scintillator material on one side, wherein the three-dimensional position (X, Y, Z or depth of interaction), energy and time of the scintillation light generation event as the result of interaction of a high energy photon with said scintillation material, is calculated using the signals that are generated, within a limited time window, by a subset of said array of photodetectors neighboring the photodetector that receives the largest amount of said scintillation light.
2. A scintillation light detection system, according to claim 1, that the detection of the scintillation light generation event and resolving the position, energy and time of said event is independent of the X-Y extent of the scintillating material, wherein simultaneous events can be independently detected and their position, energy and time can be independently resolved using subsets of the photodetector array that do not have any shared photodetectors.
3. A method of calculating the Z component, or depth of interaction (DOI), of the three-dimensional position of the scintillation light in monolithic scintillators, where in the said Z is directly calculated using a limited order polynomial function of the inverse square root of the sum of the scaled photodetectors' signals generated by a limited number of photodetectors receiving said scintillation light.
4. A scintillation light detection system according to claim 2, which resolves the Z component of the three-dimensional position of the scintillation light using the method of claim 3, from the signals generated by the subset of photodetectors according to claim 2.
5. A method of calculating the X and/or Y component of the three-dimensional position of the scintillation light in monolithic scintillators, where in the said X and/or Y are directly calculated using weighting functions that find the center of mass of photodetectors' signals generated by a limited number of photodetectors receiving the said scintillation light, and the parameters of the said weighting functions are adjusted using a previously resolved value of Z component of the said three-dimensional position, to minimize the difference between the actual and resolved value of the said X
and/or Y components.
and/or Y components.
6. A scintillation light detection system according to claim 4, which resolves the X and Y component of the three-dimensional position of the scintillation light using the method of claim 5, from the signals generated by the subset of photodetectors according to claim 2, and using the resolved Z
component of the position of said scintillation light according to claim 4.
component of the position of said scintillation light according to claim 4.
7. A method of calculating the energy of a scintillation light generation event in monolithic scintillators, where in the energy of said event is directly calculated from the sum of scaled photodetectors' signals generated by a limited number of photodetectors receiving said scintillation light, added to the value of a limited order polynomial function of a previously resolved Z component of the position of said event to minimize the difference between the resolved value of the energy and an desired value of an energy to detect.
8. A scintillation light detection system according to claim 6, which resolves the energy of a scintillation light generation event using the method of claim 7, from the signals generated by the subset of photodetectors according to claim 2, and using the resolved Z component of the position of said event according to claim 4.
9. A two-dimensional filtering method, based on the resolved energy and the resolved Z component of the position of scintillation light generation events, that selects the events that their resolved energy is within a desired energy range, and their resolved Z is within a desired DOI range, or rejects those events that either their resolved energy, or their resolved Z is outside a desired range.
10. A scintillation light detection system according to claim 8, which filters scintillation light generation events using the method of claim 9.
11. A radiation imaging system using scintillation light detection systems according to claims 10, 8, 6 and 4.
12. A method of calculating the time of a scintillation light generation event in monolithic scintillators, wherein the time of the event is calculated from the time-varying signal of the photodetector generating the largest signal among the photodetectors receiving said scintillation light, and the calculated time is adjusted using previously resolved three-dimensional position of said event to account for the propagation time of the scintillation light from the center of said event to the center of said photodetector.
13. A coincident radiation detection system that uses method of claim 12 for calculating the time of scintillation light generation events that their energy and three-dimensional position are detected by radiation imaging systems according to claim 11.
14. A positron imaging system using two or more coincident radiation detection systems according to the claim 13.
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| WO2021258507A1 (en) * | 2020-06-24 | 2021-12-30 | 北京卫星环境工程研究所 | Three-dimensional position calibration method and apparatus for continuous crystal gamma detector, and device |
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