Abstract
This disclosure describes an imaging radiation detection module with novel configuration of the scintillator sensor allowing for simultaneous optimization of the two key parameters: detection efficiency and spatial resolution, that typically cannot be achieved. The disclosed device is also improving response uniformity across the whole detector module, and especially in the edge regions. This is achieved by constructing the scintillation modules as hybrid structures with continuous (also referred to as monolithic) scintillator plate(s) and pixellated scintillator array(s) that are optically coupled to each other and to the photodetector. There are two basic embodiments of the novel hybrid structure: (1) the monolithic scintillator plate is at the entrance for the incoming radiation, preferably gamma rays, and the pixellated array placed behind the plate, all in optical contact with the photodetector, (2) the order of the scintillator components is reversed with the pixellated scintillation plate placed in front of the monolithic plate.
Claims
1. A hybrid scintillation module for the detection of radiation comprising a combination of: at least a first pixellated scintillation array (1) with a multiplicity of scintillators, and at least a first monolithic scintillation plate (2).
2. The hybrid scintillation module according to claim 1, wherein the pixellated scintillation array (1) is at the front of the hybrid scintillation module and the monolithic plate (2) is behind the pixellated scintillation array (1).
3. The hybrid scintillation module according to claim 1, wherein the monolithic plate (2) is at the front of the hybrid scintillation module and the pixellated scintillation array (1) is behind the monolithic plate (2).
4. The hybrid scintillation module according to claim 1, wherein the monolithic scintillator plate (2) has a trapezoidal shape.
5. The hybrid scintillation module according to claim 1, wherein the monolithic scintillation plate (2) is split into at least two stacked layers (8).
6. The hybrid scintillation module according to claim 1 wherein the pixellated scintillation array (1) is split into at least two stacked layers (8).
7. The hybrid scintillation module according to claim 1, that further comprises at least a light spreader window.
8. The hybrid scintillation module according to claim 1, that further comprises at least a refractive optical coupling compound which has a refractive index n, within the range 1.4<n<1.8.
9. The hybrid scintillation module according to claim 1, wherein the monolithic scintillation plate (2) and the pixellated scintillation array are made of the same scintillation material or of different scintillation materials.
10. The hybrid scintillation module, according to claim 2 that comprises a pixellated scintillation array (1) in front of a monolithic scintillation plate, wherein the thickness of the pixellated scintillation array (1) is lower than the thickness of the monolithic scintillation plate (2).
11. The hybrid scintillation module according to claim 2 that comprises pixellated scintillation arrays (1) which are arranged as a stack of at least two shifted arrays.
12. The hybrid scintillation module according to claim 11 wherein the pixellated scintillation arrays (1) are arranged as a stack of at least two shifted arrays, on top of each other.
13. The hybrid scintillation module according to claim 1 wherein the scintillation array is rough-cut and not polished.
14. The hybrid scintillation device, comprising a hybrid scintillation module comprising a combination of: at least a first pixellated scintillation array (1) with a multiplicity of scintillators, and at least a first monolithic scintillation plate (2); and and at least a photodetector (4).
15. The hybrid scintillation device according to claim 14 further comprising means for extracting 3D information on the position of the radiation conversion event from the planar 2D distribution of the scintillation light cone at the photodetector surface.
16. The hybrid scintillation device according claim 15, wherein said means are a 3D spatial differentiation algorithm that extracts the 3D information on the position of the radiation conversion event from the planar 2D distribution of the scintillation light cone at the photodetector surface.
17. The hybrid scintillation module, according to claim 1 wherein the radiation is gamma radiation.
18. The hybrid scintillation device, according to claim 14 wherein the radiation is gamma radiation.
19. The hybrid scintillation device, according to claim 14 that further comprises fiberoptic lightguide (9) between the hybrid scintillation module and the photodetector (4) to transport light further away from the scintillator sensor.
20. A method of using the hybrid scintillation module defined in claim 1 in nuclear medicine imaging comprising obtaining images from a radiation emitting object.
21. The method according to claim 20 wherein nuclear medicine imaging comprises Single-Photon Emission Computed Tomography, SPECT, or Positron Emission Tomography, PET.
22. A method of using the hybrid scintillation device defined in claim 14 in nuclear medicine imaging, comprising obtaining images from a radiation emitting object.
23. The method according to claim 22 wherein nuclear medicine imaging comprises Single-Photon Emission Computed Tomography, SPECT, or Positron Emission Tomography, PET.
24. The method according to claim 20 comprising building a gamma or PET detector built out of one to many such hybrid scintillation modules to obtain radiation emission images from an emitting object, the modules operating as single units or sets of single units, and being arranged in a ring(s) or in planar arrays.
25. The method according to claim 22 comprising building a gamma or PET detector built out of one to many such hybrid scintillation modules to obtain radiation emission images from an emitting object, the modules operating as single units or sets of single units, and being arranged in a ring(s) or in planar arrays.
26. The method according to claim 24 comprising: using a hybrid scintillation module with a pixellated scintillation array (1) in front of a monolithic scintillation plate, wherein the thickness of the pixellated scintillation array (1) is lower than the thickness of the monolithic scintillation plate (2) and producing three images as follows: one image corresponding to a high resolution detector that produces a high resolution image, a moderate resolution image produced by a moderate resolution detector only and a full resolution image produced by the full module resolution by combining the two components.
27. The method according to claim 25 comprising: using a hybrid scintillation module with a pixellated scintillation array (1) in front of a monolithic scintillation plate, wherein the thickness of the pixellated scintillation array (1) is lower than the thickness of the monolithic scintillation plate (2); and producing three images as follows: one image corresponding to a high resolution detector that produces a high resolution image, a moderate resolution image produced by a moderate resolution detector only and a full resolution image produced by the full module resolution by combining the two components.
Description
BRIEF DESCRIPTION OF THE FIGURES
[0067] FIG. 1: Concept of the Depth-of-Interaction (DOI) 10 in two generic examples of radiation, preferably gamma, detection modules. Radiation, preferably gamma rays, 5 typically arrive at some angle and interact at the different depths. Light from scintillation modules 1-2 is detected in photodetectors 4. Example of the first preferred embodiment with the pixellated scintillation array(s) placed upfront facing the incident ray, preferably gamma ray beam, and, thus, on top of the monolithic plate(s) with the photodetector on the other side of the monolithic plate and receiving scintillation light from both components of the scintillator module.
[0068] Readout for one component (left) and two-component (right) scintillation module.
[0069] FIG. 2: Particular example of the first embodiment: a pixellated scintillation component in front (1), is followed by the monolithic scintillation component (2), the spreader window or light guide (3) and the photodetector (4).
[0070] FIG. 3: The second preferred embodiment of the hybrid scintillator module with reversed order of the pixellated and monolithic components: the monolithic scintillator plate is at the entrance and the pixellated array placed behind the plate in optical contact with the photodetector.
[0071] FIG. 4: A variant of the first preferred embodiment of the hybrid scintillator module with a tapered monolithic scintillator.
[0072] FIG. 5: A variant of the second preferred embodiment of the hybrid scintillator module with a tapered monolithic scintillator.
[0073] FIG. 6: Simplified representation of scintillation light propagation (6) in the first embodiment with radiation, preferably gamma rays (5) converting in both scintillator components.
[0074] FIG. 7: Simplified representation of scintillation light propagation in the second hybrid module embodiment.
[0075] FIG. 8: Sketch of scintillation light propagation mechanism's; in the rough surface pixels (7) it produces wider opening angle scintillation light cone entering into the monolithic scintillator component where it propagates with the same angle opening (in the case of the same scintillator material in both components) until it reaches the multi-element photodetector. The light cone with a larger imprint on the photodetector produces more precise (less image non-uniformity) definition of the centre of gravity of the scintillation light spot and by this it provides better separation of the detection events produced in the neighbouring scintillation pixels.
[0076] FIG. 9: shows a particular example of the second embodiment in which the pixellated scintillator array is split into two stacked layers, (1) and (8).
[0077] FIG. 10: shows a fiberoptic lightguide (9) that can be used between the scintillator module and the photodetector in some situations requiring stand-off distance between the scintillator and electronics (to minimize mechanical edge effects or to avoid electromagnetic interference).
[0078] FIG. 11: shows modules arranged in a PET detector ring according to the first embodiment of the scintillation module.
[0079] FIG. 12: shows the tapered shape of the monolithic section from FIG. 4 and how it enables higher packing fraction of the modules placed on a ring and how it minimizes the physical gaps between the modules.
[0080] FIG. 13: shows the first reduction to practice of the first preferred embodiment using SensL (Cork, Ireland) Silicon Photomultiplier (SiPM) array. The monolithic in between the crystal array and the photodetector has 12 mm thickness. Top: Sketch, 2D contour plot of the crystal array at 511 keV where most pixels of 2 mm size are shown well separated, and energy spectrum showing both monolithic and pixellated contributions. Bottom: Profiles of one row and one column of the pixellated contribution.
[0081] FIG. 14: First reduction to practice as in FIG. 13 but using 1.5 mm×1.5 mm crystal pixels.
[0082] FIG. 15: Second reduction to practice of the second embodiment using SensL SiPM array. Raw image and plot of the section of the detector corresponding to the 2 mm×2 mm×10 mm pixellated LYSO at 511 keV. A 2 mm pitch pixellated LYSO array was inserted between the 12 mm thick monolithic LYSO plate and the SiPM array photodetector. 2 mm pixels are seen very well separated. To enhance the signal from the array, strong F18 radioactive source of annihilation photons was placed at the back of the module, behind the photodetector. Top, from left to right, 2D contour plots for the full energy spectra, 511 keV pixellated contribution and 511 keV monolithic contribution.
[0083] FIG. 16: Demonstration of the importance of surface treatment of the pixellated component of the hybrid scintillator module with the pixellated scintillation array being on top of the monolithic scintillation plate. From two otherwise identical 10×10 LYSO arrays of 1.5×1.5×10mm pixels from Proteus, one had side walls of the “as-cut” quality, while the second had all sides polished. The one with “as-cut” surfaces had about factor two higher signal response in the MPPC (Multi-Pixel Photon Counter) photodetector and the individual pixels were distinguished in the raw image (center) while in the all-sides-polished case the pixels were not separated (right) even at a higher energy of 1274 keV. In this measurement the LYSO plate of 12 mm was used demonstrating the limitation of the hybrid approach with greater scintillator thickness. (Measurement was performed at temperature of 22 degrees C.).
[0084] FIG. 17: Generic drawing of a ring of modules showing scintillation blocks A (one or many layers) coupled to the photodetector modules with electronics B, through the fiberoptic light guides C. The fiberoptic light guides built from arranged arrays of fibers or individual optical channels are image-preserving and their purpose is to transport in parallel fashion scintillation light as it is emerging from the scintillation module, to the photodetector placed at the other end of the light guide. Implementation of the fiberoptic light guide provides stand-off distance between the scintillator modules and the photodetector modules and in typical situations will minimize the dead regions—breaks between the scintillation modules. Considering the application of the PET insert in MRI, a relatively thin additional optical window may be inserted between the scintillator and the fiberoptic lightguide, that can also serve as a component of the RF shield D between the photodetector/electronics modules and the RF field of the MRI. This window will be based on special materials that provide RF shield and are optically transparent to the scintillation light.
[0085] FIG. 18 shows an example of a scintillator module structure with the pixellated array 1 split into two pixellated arrays placed on top of each other and shifted sideways, and then coupled to the monolithic scintillator plate 2. Scintillation light from the top pixellated array propagates into bottom pixellated array and then continues into the scintillation plate 2 and to the photodetector 4 via lightguide 3. The scintillation light cones 6 from the two sub-arrays are shifted relatively to each other at the photodetector surface and this allows to differentiate the gamma interactions in the top array from the interactions in the bottom array.
[0086] List of references shown in the figures with comments: [0087] 1) Pixel crystal array. A pixel crystal array is a scintillation block made out of many small crystal scintillation crystals-elements, merged (glued) together, typically with all walls and faces polished. The dimensions of each crystal pixel will depend on the resolution goal. The outer dimensions of the pixel array will depend on the selected configuration and system geometry. [0088] 2) Monolithic crystal. This is a single, continuous, block of scintillation material. As shown in FIGS. 3-5 the shape of this crystal can vary depending on the chosen configuration. Rectangular and trapezoidal shapes are the most suggested, but others could also be possible. [0089] 3) Spreader window or light guide. Spreader windows are typically made out of acrylic or glass material. Their thickness can vary from few tenths of a millimetre to few millimeters. [0090] 4) Photodetector. Several types of photodetectors can be employed, such as position sensitive photomultipliers (PSPMTs), photodiodes with and without gain (avalanche photodiodes, APDs), and silicon photomultipliers (SiPMs). Their common feature is that they are position sensitive and provide spatial information on the scintillation light distribution. The preferred option for the photodetector for our concept is array of Silicon Photomultipliers (SiPMs). [0091] 5) Impinging rays, preferably gamma rays. The photons, preferably gamma photons, are emitted by the objects that were injected or otherwise inserted mechanically or chemically with the radioactive compounds. In the case of Positron Emission Tomography (PET) typically the positron emitting radolabeled compounds are injected in patients and the emitted annihilation 511 keV photons are emitted from the uptake sites in the patient body. The purpose of the detector modules is to stop and detect these photons, preferably gamma photons. Annihilation photons can come (impinge) at different angles to the front of the detector module. Typically the angle range is within +/−45 deg measured from the vertical to the detector module. [0092] 6) Scintillation light distribution, light cones. The scintillation light comes out of the scintillator in a cone-like shape, more regular in the scintillator centre and distorted—compressed at the edges. Measurement of this light distribution to back-calculate the original conversion point of the impinging photon inside the scintillation module is the main purpose of the photodetector and the associated electronics. [0093] 7) Pixel crystal where the lateral faces are “as-cut” in contrast to polished. [0094] 8) Pixel crystal array with different scintillation material compared to (1). [0095] 9) Additional light guide, such as fiberoptic light guide to transport light further away from the scintillator module to minimize edge effects or remove photodetector with front-end electronics further from the object and for example out of the central MRI magnetic and radiofrequency field.
EXAMPLES
Example of the Hybrid Scintillation Module According to the First Embodiment
[0096] According to the first embodiment, the scintillator module comprises a pixellated scintillation array (1), a scintillator plate (2), and can also comprise an additional light guide (3). The whole assembly can be seen in FIGS. 2 and 4, where also the incoming photons (5), preferably gamma photons, and the photodetector based on a SiPM array (4) are shown. FIG. 12 shows a set of modules arranged on a ring forming a PET detector ring, also according to the first embodiment of the scintillator module.
[0097] According to the example of the first preferred embodiment of the hybrid scintillation module as shown in FIGS. 2 and 4, there is a pixellated array on top of the monolithic scintillator. The scintillation light from the array on its way to the photodetector passes through the monolithic crystal acting here as an optical window. At the same time, the monolithic plate scintillator is also functioning as an active material detecting a fraction of the incoming radiation, preferably gamma radiation, that traversed the pixellated array without producing interactions.
Example of the Functioning of the Hybrid Scintillation of the Invention According to the First Embodiment (Reduction to Practice)
[0098] Small LYSO array of 1.5 mm step and 10 mm long pixels was placed on top of a 10 mm thick monolithic plate of LYSO scintillator. On the other side of the monolithic plate a Silicon Photomultiplier photodetector was placed composed of a 4×4 array of 3 mm MPPC sensors from Hamamatsu. Na22 source of annihilation 511 keV photons (and 1274 keV gammas) was placed above the LYSO array.
[0099] FIGS. 13 and 14 show examples of operation of the embodiment from FIGS. 2 and 4. They show the first reduction to practice of the first preferred embodiment using SensL SiPM array. At the top raw, images of the two components of the hybrid scintillator can be seen, both at 511 keV: 1.6 mm pitch pixellated LYSO array coupled in front of the 12 mm thick monolithic LYSO plate. 1.5 mm pixels are seen well separated. At the bottom one can see only the image obtained at 511 keV from the pixellated component, plus the profile through one pixel row. Most pixels are shown well separated.
[0100] The plot shows the raw image obtained at the energy of 511 keV, clearly indicating that the individual LYSO pixels are well separated in the image. In addition, an example of the individual energy spectrum obtained from one of the 1.5 mm×1.5 mm×10 mm LYSO pixels is shown at right. After correcting for the energy non-linearity present when using the special diode based 4 ch charge division readout (from AiT Instruments), the energy resolution of ˜16% FWHM@511 keV is extracted from the data. While not yet optimized, the pilot results clearly demonstrate that the concept of the array-monolithic hybrid scintillator works.
Example of the Second Embodiment
[0101] FIGS. 3 and 5 show an example of the second preferred embodiment. The pixellated scintillation array performs two functions: fiberoptic light guide for the top plate scintillator and additional scintillator layer to increase light stopping power for incoming radiation, preferably gamma, for the scintillation module. The pixellated scintillation array could be further vertically split into a stack of two or even more shifted arrays to improve DOI resolution of the detector module, as illustrated in FIG. 18. With the pixellated scintillation array as a stack of two shifted arrays on top of each other, typically both arrays will have the same pixel pitch and the shift is half a pitch in both X-Y planar coordinates. FIG. 5 shows a variant with the scintillator plate in front having a tapered shape to (1) minimize the optical edge effects in the continuous scintillator component and to (2) minimize the physical gaps between the detector modules when placed in a ring.
REFERENCES
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