Depth Of Interaction Detector Research Articles

Development of a four-layer DOI PET detector using long crystals segmented by subsurface laser engraving

Since the spatial resolution and sensitivity of positron emission tomography (PET) scanners can be improved by using scintillation detectors with depth-of-interaction (DOI) capability, an original four-layer DOI detector using a pixelated crystal array has been developed in our group. For DOI identification, reflectors have been inserted between the crystals in each layer with a specific arrangement in order to cause light sharing between the layers of the crystal array. However, the precise fabrication of pixelated crystal arrays was challenging and expensive compared to monolithic arrays. In the current study, a four-layer DOI detector composed of long crystals segmented into four segments by applying subsurface laser engraving (SSLE) was proposed. LuYSiO5 (LYSO) crystals of 1 × 1× 20 mm3 were segmented into four segments in the largest dimension using SSLE. A 6 × 6 crystal array using the segmented crystals was prepared and mounted on a 4 × 4 channel multi-pixel photon counter (MPPC, 2 × 2 mm active area). The performance of the proposed detector was investigated and compared with that of original four-layer DOI detector composed of discrete crystals of 1 × 1× 5 mm3 in terms of the crystal identification, layer separation and energy resolution. The 2D position maps of the detectors were obtained by the Anger-logic calculation, and the crystal responses were clearly separated for both detectors. Average energy resolutions of 10.0% ± 1.0%, and 10.5% ± 0.9% at 511 keV were obtained for the proposed detector, and the original detector composed of the discrete crystals, respectively. The layer separation performance was evaluated using separation ratio, which was the ratio of true counts in a specific irradiated crystal in a layer to the total counts in all four crystals stacked on top of each other. The responses of all four layers were clearly separated with a separation ratio of over 0.89 for the proposed and original detectors, respectively. The performance of the proposed detector was well comparable to the original detector composed of the discrete crystals, while the fabrication of the segmented detector was precise and simple.

Improvement of Crystal Identification Accuracy for Depth-of-Interaction Detector System with Peak-to-Charge Discrimination Method

In positron emission tomography (PET), parallax errors degrade spatial resolution. The depth of interaction (DOI) information provides the position in the depth of the scintillator interacting with the γ-rays, thus reducing parallax errors. A previous study developed a Peak-to-Charge discrimination (PQD), which can separate spontaneous alpha decay in LaBr3:Ce. Since decay constant of GSO:Ce depends on Ce concentration, the PQD is expected to discriminate GSO:Ce scintillators with different Ce concentration. In this study, the PQD-based DOI detector system was developed, which can be processed online and implemented in PET. A detector was composed of four layers of GSO:Ce crystals and a PS-PMT. The four crystals were obtained from both the top and bottom of ingots with a nominal Ce concentration of 0.5 mol% and 1.5 mol%. The PQD was implemented on the Xilinx Zynq-7000 SoC board with 8ch Flash ADC to gain real-time processing, flexibility, and expandability. The results showed that the mean Figure of Merits in 1D between four scintillators are 1.5, 0.99, 0.91 for layers between 1st–2nd, 2nd–3rd, and 3rd–4th respectively, and the mean Error Rate in 1D between four scintillators are 3.50%, 2.96%, 13.3%, and 1.88% for layers 1, 2, 3, and 4, respectively. In addition, the introduction of the 2D PQDs resulted in the mean Figure of Merits in 2D greater than 0.9 and the mean Error Rate in 2D less than 3% in all layers.

Single-Ended Readout Depth-of-Interaction Measurements Based on Random Forest Algorithm

The depth-of-interaction (DOI) information can reduce parallax errors and improve the spatial resolution of positron emission tomography (PET) imaging. Current DOI detectors, with either single-ended readout design or dual-ended readout design, usually require substantial changes to the crystal arrays and/or readout electronics. In this work, we proposed a single-ended readout DOI estimation approach with minimal changes in the detector hardware design, using a supervised machine learning algorithm, random forest (RF). The energy value measured from one end of the crystal, combined with sample points of the pulse signal, were used as inputs to the DOI estimation algorithm, which provided a DOI performance close to that of the dual-ended readout design. We compared the proposed single-ended RF method with the classic dual-ended energy ratio method and a dual-ended version of the RF method, using positioning error and resolution as performance metrics for DOI estimation. All three methods achieved average absolute DOI positioning errors of < 1.00 mm and average DOI resolutions of < 2.20 mm full width at half maximum (FWHM). More specifically, we achieved the averaged DOI resolutions of 2.15, 1.80, and 1.93 mm FWHM for the single-ended RF method, dual-ended RF method, and dual-ended energy ratio method, respectively. In conclusion, the single-ended RF method achieved comparable performance without extra photodetectors and readout electronics and may provide a feasible solution for DOI estimation.

Axial scalable add-on PET/MRI prototype based on four-layer DOI detectors integrated with a RF coil

We developed an add-on PET which was a PET insert based on 4-layered depth-of-interaction (DOI) detectors integrated with a birdcage RF-coil. The PET detectors were placed close to subjects to achieve high sensitivity. The DOI capability suppress degradation of spatial resolution at the edge of the field of view (FOV). Previously, we developed an first prototype which had a short axial FOV. In order to extend the axial FOV, we developed new scalable add-on PET prototype. The constructed RF-coil of the new prototype was combined with 24 DOI-PET detector units Each detector unit with two PET detectors was shielded with a carbon fiber shielding box. Each PET detector consisted of a 14 × 14 × 4-layer array of lutetium fine silicate crystals (1.9 mm × 1.9 mm × 4.0 mm), an 8 × 8 multi-pixel photon counter (MPPC, S12641PA-050) array and a readout circuit board with an ASIC. The RF coil was tuned for a 3T clinical MRI (MAGNETOM Verio, Siemens). We evaluated performance the scalable add-on PET prototype in simultaneous PET/MRI measurements. We obtained spatial resolutions of 2.0 mm at the center of the FOV and better than 3.3 mm in the whole FOV. The energy resolution of 12.4 % was obtained at 511 keV. In addition, we observed no degradation of PET performance caused by the MRI measurement. The signal-to-noise ratio (SNR) of the MRI image was 186.2 in simultaneous measurements with the PET. The maximum ΔB0 and maximum difference of the secondary magnetic field due to the eddy current effect were smaller than 0.2 ppm and ±1.0μT, respectively. It meant that the influence of the PET detectors on the static magnetic field and secondary magnetic field did not cause problems.

Development of crosshair light sharing PET detector with TOF and DOI capabilities using fast LGSO scintillator

Objective. Time-of-flight (TOF) and depth-of-interaction (DOI) are well recognized as important information to improve PET image quality. Since such information types are not correlated, many TOF-DOI detectors have been developed but there are only a few reports of high-resolution detectors (e.g. 1.5 mm resolution) for brain PET systems. Based on the DOI detector, which enables single-ended readout by optically coupling a pair of crystals and having a loop structure, we have developed the crosshair light sharing (CLS) PET detector that optically couples the four-loop structure, consisting of quadrisected crystals comparable in size to a photo-sensor, to four photo-sensors in close proximity arranged in a windmill shape. Even as a high-resolution detector, the CLS PET detector could obtain both TOF and DOI information. The coincidence resolving time (CRT) of the CLS PET detector needs to be further improved, however, for application to the brain PET system. Recently, a fast LGSO crystal was developed which has advantages in detection efficiency and CRT compared to the GFAG crystal. In this work, we developed the CLS PET detector using the fast LGSO crystal for the TOF-DOI brain PET system. Approach. The crystals were each 1.45 × 1.45 × 15 mm3 and all surfaces were chemically etched. The CLS PET detector consisted of a 14 × 14 crystal array optically coupled to an 8 × 8 MPPC array. Main results. The fast LGSO array provided 10.1% energy resolution at 511 keV, 4.7 mm DOI resolution at 662 keV, and 293 ps CRT with the energy window of 440–620 keV. Significance. The developed CLS PET detector has 290% higher coincidence sensitivity, 30% better energy resolution, and 32% better time resolution compared to our previous CLS PET detector.

Initial results of a mouse brain PET insert with a staggered 3-layer DOI detector

Objective. Small animal positron emission tomography (PET) requires a submillimeter resolution for better quantification of radiopharmaceuticals. On the other hand, depth-of-interaction (DOI) information is essential to preserve the spatial resolution while maintaining the sensitivity. Recently, we developed a staggered 3-layer DOI detector with 1 mm crystal pitch and 15 mm total crystal thickness, but we did not demonstrate the imaging performance of the DOI detector with full ring geometry. In this study we present initial imaging results obtained for a mouse brain PET prototype developed with the staggered 3-layer DOI detector. Approach. The prototype had 53 mm inner diameter and 11 mm axial field-of-view. The PET scanner consisted of 16 DOI detectors each of which had a staggered 3-layer LYSO crystal array (4/4/7 mm) coupled to a 4 × 4 silicon photomultiplier array. The physical performance was evaluated in terms of the NEMA NU 4 2008 protocol. Main Results. The measured spatial resolutions at the center and 15 mm radial offset were 0.67 mm and 1.56 mm for filtered-back-projection, respectively. The peak absolute sensitivity of 0.74% was obtained with an energy window of 400–600 keV. The resolution phantom imaging results show the clear identification of a submillimetric rod pattern with the ordered-subset expectation maximization algorithm. The inter-crystal scatter rejection using a narrow energy window could enhance the resolvability of a 0.75 mm rod significantly. Significance. In an animal imaging experiment, the detailed mouse brain structures such as cortex and thalamus were clearly identified with high contrast. In conclusion, we successfully developed the mouse brain PET insert prototype with a staggered 3-layer DOI detector.

Development of dual-ended depth-of-interaction detectors using laser-induced crystals for small animal PET systems

Sensitivity and spatial resolution of positron emission tomography (PET) scanners can be improved by using thicker scintillation crystals with depth-of-interaction (DOI) encoding. Subsurface laser engraving (SSLE) can be used to segment crystals of a scintillation detector in order to fabricate a DOI detector. We previously applied SSLE to crystal bars of 3 × 3 × 20 mm3 and 1.5 × 1.5 × 20 mm3 and developed two dual-ended detectors with DOI segments of 3 mm and 1.5 mm, respectively. To further improve the DOI resolution, our SSLE detector design can be used with smaller pitch crystal bars, making them excellent detector candidates for small animal PET scanners with submillimetre resolution. In the present study, three small crystal bars of 1 × 1 × 20 mm3, 2 × 1 × 20 mm3, and 2 × 1 × 40 mm3 were laser engraved to 12, 20 and 40 segments, respectively, by applying SSLE in their height directions. The segmented crystal bars were characterised in three prototype detector arrangements. First, the 1 × 1 × 20 mm3 crystal bars were characterised in an 8 × 8 crystal array designed for DOI encoding along crystal height in a conventional small animal PET design. Second, a 4 × 8 crystal array of 2 × 1 × 20 mm3 crystal bars was characterised for using the DOI information for crystal interaction positioning along the axial axis of a small animal PET scanner. Finally, the third part of the study was performed on a single 2 × 1 × 40 mm3 crystal bar with 40 segments to investigate the feasibility of DOI estimation in longer crystals for application in a system with extended axial length. We evaluated the capability of segment identification and energy resolution of theses detectors. The 3D position maps of the detectors were obtained using the Anger-type calculation and the crystal identification performance was evaluated for each detector. Clear segment separation was obtained for the crystal arrays with 12 (segment pitch of 1.67 mm) and 20 (segment pitch of 1 mm) segments. Mean energy resolutions of 8.8% ± 0.4% and 9.6% ± 0.8% at 511 keV were obtained for the segments in the central regions of the 8 × 8 array with 12 segments and the 4 × 8 array with 20 segments, respectively. Clear segment identification was found to be difficult for the detector with 40 segments, especially for the segments at the middle of the crystal. Energy and interaction positioning characterisation results suggest that both prototype detectors with 12 and 20 segments are well suited for small animal PET scanners with high spatial resolution.

Performance evaluation of a staggered three-layer DOI PET detector using a 1 mm LYSO pitch with PETsys TOFPET2 ASIC: comparison of HAMAMATSU and KETEK SiPMs

In this study, we propose a staggered three-layer depth-of-interaction (DOI) detector with a 1 mm crystal pitch and 19.8 mm total crystal thickness for a high-resolution and high-sensitivity small animal in-beam PET scanner. A three-layered stacked LYSO scintillation array (0.9 × 0.9 × 6.6 mm3 crystals, 23 × 22 mm2 surface area) read out by a SiPM array (8 × 8 channels, 3 × 3 mm2 active area/channel and 50 μm microcell size) with data acquisition, signal processing and digitization performed using the PETsys Electronics Evaluations kit (based on the TOFPET v2c ASIC) builds a DOI LYSO detector block. The performance of the DOI detector was evaluated in terms of crystal resolvability, energy resolution, and coincidence resolving time (CRT). A comparative performance evaluation of the staggered three-layer LYSO block was conducted with two different SiPM arrays from KETEK and HAMAMATSU. 100% (KETEK) and 99.8% (HAMAMATSU) of the crystals were identified, by using a flood irradiation the front- and back-side. The average energy resolutions for the 1st, 2nd, and 3rd layers were 16.5 (±2.3)%, 20.9(±4.0)%, and 32.7 (±21.0)% (KETEK) and 19.3 (±3.5)%, 21.2 (±4.1)%, and 26.6 (±10.3)% (HAMAMATSU) for the used SiPM arrays. The measured CRTs (FWHM) for the 1st, 2nd, and 3rd layers were 532 (±111) ps, 463 (±108) ps, and 447 (±111) ps (KETEK) and 402 (±46) ps, 392 (±54) ps, and 408 (±196) ps (HAMAMATSU). In conclusion, the performance of the staggered three-layer DOI detector with 1 mm LYSO pitch and 19.8 mm total crystal thickness was fully characterized. The feasibility of a highly performing readout of a high resolution DOI PET detector via SiPM arrays from KETEK and HAMAMATSU employing the PETsys TOFPET v2c ASIC could be demonstrated.

Optimization of the surface area of laser-induced layers for PET detectors with depth-of-interaction

Detectors with depth-of-interaction (DOI) information are utilized in positron emission tomography (PET) scanners to improve the sensitivity of radiation detection and the uniformity of spatial resolution. We recently developed a series of dual-ended detectors using crystal bars segmented by applying the subsurface laser engraving (SSLE) technique to 7, 13 and 20 DOI segments. It is crucial to achieve the submillimeter level spatial resolution for our detector as the narrow crystal bars are highly fragile after applying the SSLE technique. In this work, we focused on optimizing the surface area of the SSLE-induced layer to the narrow crystal bars in order to resolve the issue of fragility while maintaining sufficient crystal segment separation. The SSLE layers were induced to the middle of the cross section of the five crystal bars with dimensions of 1.5 ×1.5× 20 mm3 with a distance of 0.1, 0.2, 0.3, 0.4 or 0.5 mm from the two opposite lateral edges of each crystal bar. All crystals were segmented into four DOI segments and the performances of the five sets of dual-ended DOI detectors consisting of one crystal bar were evaluated from the viewpoints of optimum crystal segment identification and energy resolution while mitigating the fragility issue. The 3D position maps of the detectors were obtained by the Anger-type calculation, and the crystal identification performance was evaluated. Clear separation of the segments with a distance-to-width ratio (DWR) larger than 7 was obtained for the detectors in which the distances of the SSLE-induced layer from the two opposite lateral edges of the crystal were 0.1, 0.2 and 0.3 mm. The DWR for the other two detectors was less than 4.4. Average energy resolutions of 11.8 ± 0.2%, 10.6 ± 0.2% and 8.8 ± 0.2% were obtained for the detectors in which the SSLE layers were induced with 0.1, 0.3 and 0.5 mm distances from the two opposite lateral edges of the crystal, respectively. There was a compromise between energy resolution and crystal segment identification. The optimum surface area of the SSLE-induced layer was accepted as a trade-off between crystal segment identification and fragility. The crystal segmented by the SSLE-induced layer at the middle of the cross section with a distance of 0.3 mm from the two opposite lateral edges was considered to be a good candidate with acceptable performance and fragility.

A staggered 3-layer DOI PET detector using BaSO4 reflector for enhanced crystal identification and inter-crystal scattering event discrimination capability

The spatial resolution of small animal positron emission tomography (PET) scanners can be improved by the use of crystals with fine pitch and rejection of inter-crystal scattering (ICS) events, which leads to a better quantification of radiopharmaceuticals. On the other hand, depth-of-interaction (DOI) information is essential to preserve the spatial resolution at the PET field-of-view (FOV) periphery while keeping the sensitivity. In this study we proposed a novel staggered 3-layer DOI detector using BaSO4 reflector material for an enhanced crystal identification performance as well as ICS event rejection capability over those of ESR reflector based DOI detectors. The proposed staggered 3-layer DOI detector had 3-layer staggered LYSO crystal arrays (crystal pitch = 1 mm), an acrylic light guide, and a 4 × 4 SiPM array. The 16 SiPM anode signals were read out by using a resistive network to encode the crystal position and energy information while the timing signal was extracted from the common cathode. The crystal map quality was substantially enhanced by using the BaSO4 reflector material as compared to that of the ESR reflector due to the low optical crosstalk between the LYSO crystals. The ICS events can be rejected with BaSO4 by using simple pulse height discrimination thanks to the light collection efficiency difference that depends on the crystal layers. As a result, the total number of events was decreased around 26% with BaSO4 as compared to that of ESR. The overall energy resolution and coincidence timing resolution with BaSO4 were 19.7 ± 5.6% and 591 ± 160 ps, respectively which were significantly worse than 10.9 ± 2.2% and 308 ± 23 ps values of ESR because of the relatively low light collection efficiency with BaSO4 (1057 ± 308 ADC) compared to that of ESR (1808 ± 118 ADC). In conclusion, we found the proposed staggered 3-layer DOI detector using the BaSO4 reflector material with ICS event rejection capability can be a cost-effective solution for realizing high resolution and highly sensitive small animal PET scanners while minimizing the complexity of the SiPM readout circuit.

Detector Design of PET Measuring Depth of Interaction through Energy Spectrum Analysis

A depth of interaction detector was designed using three layers of the scintillation pixel array and silicon photomultiplier through DETECT2000 simulation. The upper and lower layers used diffuse reflectors, and the middle layer used specular reflectors to distinguish the light signal arising from each layer. The signals obtained at each layer were characterized by different locations at which photoelectric peaks were formed in the energy spectrum, which could be analyzed to track the layers in which gamma-rays and scintillation pixels interacted. Based on the simulation results, the basic experiment was conducted to obtain the energy spectrum. In the experiment, three photoelectric peaks were identified in the energy spectrum as in the simulation, and the layers interacted with gamma-rays, and scintillation pixels for each layer were distinguished by analyzing each photoelectric peak. It is anticipated that by applying the detector design obtained from this study to future small animal PET operating with MRI, the depth at which gamma-rays and scintillators interact will be measurable, thus improving the spatial resolution outside the FOV.

A Crosshair Light Sharing PET Detector With DOI and TOF Capabilities Using Four-to-One Coupling and Single-Ended Readout

To improve positron emission tomography (PET) image quality, depth-of-interaction (DOI) information and time-of-flight (TOF) information are key technologies. In this work, we developed the DOI-TOF detector based on our original single-ended readout scheme with the continuous layered gadolinium fine aluminum garnet (GFAG) array. The size of each GFAG crystal is 1.45×1.45×20 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . The multipixel photon counter (MPPC) used as a photo sensor has a surface area of 3.0×3.0 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . One pair of crystals coupled with a partial optical window is arranged across two MPPCs. Boundaries without the partial optical window are covered with optical reflectors. Each pair is coupled with paired MPPCs of different patterns. Crystal identification is obtained from paired MPPCs of different patterns and output rates. By limiting the spread of scintillation light, the proposed DOI detector based on the local centroid calculation was expected to improve crystal response, including the edge. For performance comparison, we prepared three discrete layers of the GFAG array with the same total thickness. For the continuous layer, the DOI resolution, the energy resolution and the timing resolution of the pair of detectors are 4.7 mm, 14%, and 402 ps, respectively. Their respective values are 29%, 20%, and 33% better than those with the three discrete layers.

Depth of Interaction Measurements Based on Rectangular Light Sharing Window Technology and Nine-Crystals-to-One-SiPM Coupling Method

Depth of interaction (DOI) decoding capability shows great application potential for positron emission tomography (PET), especially for high-resolution small-animal PET or long axial field of view PET systems. On the basis of GATE optical simulation results, we presented a novel $9\times 9$ lutetium–yttrium oxyorthosilicate array with 10-mm rectangular light-sharing windows (RLSWs) and assembly the detector with nine crystals coupled to one silicon photomultiplier (SiPM) method; this caused DOI-related position shifts in the crystal decoding spots of the flood map. Homogeneous radiation was used to establish transfer functions to convert spot shifts into DOI measurements. The conventional collimated radiation experiments were used to assess the accuracy of the DOI measurements. Flood map results show that 81 segmented crystals can be clearly resolved, including 16 crystals adjacent to two RLSW windows (M-type), 40 crystals adjacent to one RLSW window (E-type), and 25 crystals not adjacent to any RLSW window (C-type). The mean absolute error of all DOI-capable crystals, M-type crystals, and E-type crystals are 2.15 ± 0.61, 1.95 ± 0.50, and 2.23 ± 0.63 mm, respectively. The novel nine-crystals-to-one-SiPM coupling technology is a low cost approach for constructing high-performance DOI detector modules. Detectors using nine-crystals-to-one-SiPM coupling technology are good candidates for high-resolution preclinical PET and for total-body PET.

Development of Single-Ended Readout DOI Detector With Quadrisected Crystals

In this article, we developed the novel single-end readout depth-of-interaction (DOI) detector with quadrisected crystals comparable in size to a single photo sensor. The proposed DOI detector consists of 784 gadolinium aluminum gallium garnet (GAGG) crystals, which are arranged in four layers of $14\times14$ arrays coupled with an $8\times8$ multipixel photon counter (MPPC) array. The size of each GAGG crystal is $1.45\times 1.45\times4.5$ mm3. Also, each MPPC has a surface area of $3.0\times3.0$ mm2. One pair of four-layered crystals coupled with a partial optical window is arranged across two MPPCs. Boundaries without the partial optical window are covered with optical reflectors. Light sharing is done between a pair of four-layered crystals optically coupled at one end. Each pair is coupled with paired silicon photomultipliers of different patterns. Crystal identification from each pair is obtained from the output rate of two MPPCs. From 511-keV uniform irradiation, responses of all crystal elements in the four layers can be separated clearly on a 2-D position histogram after applying the Anger calculation. Also, DOI information can be mapped to each cluster on curved lines. The energy resolution for all crystals is 9.1% after event reduction of intercrystal scatter.

Wavelength discrimination (WLD) TOF-PET detector with DOI information

Depth-of-interaction (DOI) encoding can contribute to improving spatial resolution uniformity and sensitivity in positron-emission-tomography (PET) scanners. In addition, time-of-flight (TOF) PET scanners with DOI encoding have received considerable interest because of their potential for improving the spatial resolution, sensitivity, and image quality of the overall system. In this study, a new DOI detector configuration utilizing scintillators’ emission wavelength is proposed, and experimental results on the energy, timing, and DOI performance of the detector are provided.The DOI information from the proposed phoswich-type detector can be acquired at the detector level without complex signal processing by utilizing a single optical filter with customized optical properties. For this, we used either a short pass filter (SPF) or a long pass filter (LPF) that allows light photons of a specific wavelength to pass. The two-layered phoswich detector was configured with two scintillators with different photon-emission spectra. In this study, we used Ce:GAGG (3 mm × 3 mm × 10 mm) and LYSO:Ce (3 mm × 3 mm × 10 mm) as the top and bottom layer scintillators, respectively. A digital silicon photomultiplier (dSiPM) was used as the photosensor and for data acquisition. The phoswich detector was placed in the center of two dSiPM pixels, where one of the dSiPM pixels was covered with the optical filter, and the light guide was placed on the other pixel. The detector was tested for energy, timing, and DOI encoding performance.When an SPF was used, the energy resolutions of 16.2% and 11.8% were achieved for the Ce:GAGG (top layer) and LYSO:Ce (bottom layer) respectively without correcting for saturation effect. With a small (3 mm × 3 mm × 5 mm) LYSO crystal as the reference detector, CRTs (coincidence-resolving times) of 338 ps and 244 ps were recorded for the top and bottom layers respectively. The detector configuration also provides an excellent DOI-separation figure-of-merit (FoM) value of 1.9.In the case of LPF, the energy resolutions of 12.0% and 12.9% were achieved for the Ce:GAGG (top layer) and LYSO:Ce (bottom layer), respectively. CRTs (coincidence resolving times) of 314 ps and 263 ps were recorded for the top and bottom layers, respectively. The DOI-separation FoM value of 1.5 was achieved in this setup.Results show that the proposed method can provide excellent discrete DOI positioning accuracy without compromising the timing performance of the detector.

Design study of a brain-dedicated time-of-flight PET system with a hemispherical detector arrangement

Time-of-flight (TOF) is now a standard technology for positron emission tomography (PET), but its effective use for small diameter PET systems has not been studied well. In this paper, we simulated a brain-dedicated TOF-PET system with a hemispherical detector arrangement. We modeled a Hamamatsu TOF-PET module (C13500-4075LC-12) with 280 ps coincidence resolving time (CRT), in which a 12 × 12 array of multi pixel photon counters (MPPCs) is connected to a lutetium fine silicate (LFS) crystal array of 4.1 × 4.1 mm2 cross section each, based on one-to-one coupling. On the other hand, spatial resolution degradation due to the parallax error should be carefully addressed for the small diameter PET systems. The ideal PET detector would have both depth-of-interaction (DOI) and TOF capabilities, but typical DOI detectors that are based on light sharing tend to degrade TOF performance. Therefore, in this work, we investigated non-DOI detectors with an appropriate crystal length, which was a compromise between suppressed parallax error and decreased sensitivity. Using GEANT4, we compared two TOF detectors, a 20 mm long non-DOI and a 10 mm long non-DOI, with a non-TOF, 4-layer DOI detector with a total length of 20 mm (i.e. 5 × 4 mm). We simulated a contrast phantom and evaluated the relationship between the contrast recovery coefficient (CRC) and the noise level (the coefficient of variation, COV) for reconstructed images. The 10 mm long non-DOI, which reduces the parallax error at the cost of sensitivity loss, showed better imaging quality than the 20 mm long non-DOI. For example, the CRC value of a 10 mm hot sphere at COV = 20% was 72% for the 10 mm long non-DOI, which was 1.2 times higher than that of the 20 mm long non-DOI. The converged CRC values for the 10 mm long non-DOI were almost equivalent to those of the non-TOF 4-layer DOI, and the 10 mm long non-DOI converged faster than the non-TOF 4-layer DOI did. Based on the simulation results, we evaluated a one-pair prototype system of the TOF-PET detectors with 10 mm crystal length, which yielded the CRT of 250 ± 8 ps. In summary, we demonstrated support for feasibility of the brain-dedicated TOF-PET system with the hemispherical detector arrangement.

Design of a Depth of Interaction Detector Module for Use in a Magnetic Resonance Imaging System

In this study, we designed a depth of interaction (DOI) detector module for use in a magnetic resonance imaging (MRI) system. The detector uses a semiconductor photosensor that is unaffected by a magnetic field. The detector module consists of two layers of a crystal array that have different light sharing characteristics. To verify the performance of the detector module, simulations of the optical photon transport in the crystal array were performed with a DETECT2000 tool. Various combinations of the parameters, such as the reflector used and the treatment of crystal surfaces, were simulated and the characteristics were evaluated through a reconstructed flood image. In certain combinations of the simulation conditions, all of the crystal pixels could be identified along all directions. Thus, the proposed DOI detector module could extract the three-dimensional gamma ray interaction position from the two-dimensional flood image.

Feasibility of adjustable-gantry PET system using advanced DOI method

Small-animal positron emission tomography (PET) systems are useful functional imaging systems for pathology and pharmacology research. However, conventional small-animal PET systems show degradation with regard to spatial resolution and sensitivity when small objects are image with a scanner having large diameter. In this study, to overcome this limitation, we propose a small-animal PET using an adjustable gantry and a depth-of-interaction (DOI) detector. The adjustable gantry allows the field of view (FOV) to be changed depending on object size to increase the solid-angle coverage and improve sensitivity. In addition, detector modules are arranged in a square shape to fill gaps between the detector module as well as in the corners to achieve higher sensitivity. To overcome the parallax error that occurs in the square-shaped gantry with long and thin crystal elements, a DOI detector is employed using a depth-dependent reflector pattern. The adjustable-gantry PET (AGPET) system is designed by GATE Monte Carlo simulation and evaluated by comparison with conventional small-animal PET systems, such as Inveon and PETbox, in terms of spatial resolution and sensitivity. The DOI detector blocks were composed of two 8 × 8 SiPM arrays coupled to two blocks of 12 × 12 lutetium-yttrium oxyorthosilicate (LYSO) crystals of size 2.1 ×2.1× 20 mm3. The signals of the detector blocks are fed to a data acquisition system based on field-programmable gate array, and the position, energy, and time information were recorded. The detector module performance is analyzed by the peak-to-valley ratio (PVR), distance-to-width ratio (DWR), energy resolution, and timing resolution. The results demonstrate that the proposed AGPET potentially provide higher sensitivity and resolution for small animal imaging.

Experimental evaluation of depth-encoding absorber designs for prompt-gamma Compton imaging in proton therapy

Prompt-gamma Compton imaging (PGCI) has been presented as a promising in-vivo method for proton range verification. An accurate estimation of the Bragg-peak position can potentially be achieved by imaging high-energy prompt-gamma rays (in the range of several MeV). Therefore, scintillation detectors with thick and high Z crystals are mostly used as Compton absorbers for high-energy gamma-ray detection. However, an absorber using thick crystals degrades the angular resolution unless the absorber can provide continuous depth-of-interaction (DOI) measurement. The study investigated various detector configurations using thick crystals for developing a Compton absorber with DOI resolving capability without compromising energy performance. Two groups of lutetium yttrium oxyorthosilicate (LYSO) arrays with dimensions of 1.8 × 1.8 × 50 mm3 and 1.8 × 1.8 × 20 mm3, and each with four different surface treatments (combining crystal surface finishing and type of reflector coverage), were constructed for the study. The DOI detector utilized the dual-ended readout of pixelated scintillator arrays for depth encoding. The results revealed that the influences of type of the reflector coverage and crystal surface roughness on the performance of the DOI detectors for the 50- and 20-mm-thick LYSO differed greatly. Moreover, the combined use of the proposed partial inter-crystal reflector coverage and the unpolished surface finishing on the 50-mm-thick crystals were shown to improve both the DOI resolution and the energy performance but degraded the flood map quality. These results provide useful guidance for selecting an applicable Compton DOI absorber design for developing a high-performance PGCI system.

A depth encoding PET detector using four-crystals-to-one-SiPM coupling and light-sharing window method.

Depth of interaction (DOI) decoding capability is of great importance for positron emission tomography (PET) requiring high resolution. In this study, we presented a novel low-cost DOI detector design with four crystals coupling to one SiPM, based on the method of rectangular light-sharing window (RLSW). A prototype detector was constructed, calibrated, and assessed using the methods of homogeneous radiation and flood map analysis. The DOI detector was constructed with a 4×4 array of lutetium-yttrium oxyorthosilicate (LYSO) crystals (2.95mm×2.95mm×20mm3 ), barium sulfate (BaSO4 ) reflectors, and optical glues. A RLSW 7mm in height was deployed in the BaSO4 reflectors. A non-DOI detector with identical dimensions and without RLSW was also constructed for comparison. The light-output surface of the detector was air-coupled with a 4×4 array of SiPMs (3mm×3mm2 ). The signals generated from the 16 SiPMs were read out by a custom-designed electronic system, and the signals from four adjacent 3mm SiPMs were summed into one signal to emulate a 2×2 array of 6mm SiPMs. The RLSW caused the DOI-related position shifts of the crystal spots in the flood map. A homogeneous radiation method was used to establish the transfer functions to convert the spot shifts measured from the flood map into DOI measurements. The accuracy of the DOI measurements was assessed with data acquired using the conventional collimated radiation method. All 16 crystals are distinctly separated from each other in the flood map. Twelve crystals, including four central crystals and eight edge crystals, have the DOI capability. The fullwidthhalfmaximum (FWHM) of the DOI measurements of the central crystals and the edge crystals are 3.06±0.08 and 3.79±0.15mm, respectively, for the configuration with four crystals coupling to one SiPM. By contrast, the FWHMs (3.98±0.16 and 5.12±0.38mm, respectively) are slightly worse for the configuration with one crystal coupling to one SiPM. The average and standard deviation (STD) of the FWHM energy resolutions of the DOI detector and non-DOI detector were 10.2%±0.7% and 10.7%±1.7%, respectively. Their FWHM coincidence timing resolutions were 197.0±9.6 and 206.4±13.3ps, respectively. The RLSW had no significant impact on the energy resolutions and timing resolutions of the DOI detector. The novel four-crystals-to-one-SiPM coupling technology is a cost-efficient approach to construct high-performance detector modules with DOI capability. The methods of homogeneous radiation and flood map analysis are easy to perform and of good performance. Those methods can be adapted in the clinic PET scanners to enable the capability of DOI measurements.