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New Radiation Detector Technologies for Medical Imaging

Session chair: Gonzalez Martinez , Antonio J. (Consejo Superior Investigaciones Cientificas (CSIC), Institute for Instrumentation in Molecular Imaging (i3M), Valencia, Spain); Loignon-Houle , Francis (Université de Sherbrooke, Sherbrooke Molecular Imaging Center, CRCHUS, and Department of Nuclear Medicine and Radiobiology, Sherbrooke, Canada)
Shortcut: M-03
Date: Wednesday, 20 October, 2021, 9:15 AM - 11:15 AM
Room: MIC - 1
Session type: MIC Session


Click on an contribution to preview the abstract content.

9:15 AM M-03-01

Towards a New Generation of Scintillators in TOF-PET: Heterostructure Scintillator Pixels (#377)

F. Pagano1, 2, M. Salomoni1, N. Kratochwil1, 3, M. Pizzichemi1, 2, M. Paganoni1, 2, E. Auffray1

1 European Organisation for Nuclear Research (CERN), Geneva, Genève, Switzerland
2 University of Milano Bicocca, Milan, Italy
3 University of Vienna, Vienna, Wien, Austria


To foster the next quantum leap in PET imaging towards high-sensitivity TOF-PET scanners, research efforts are directed in finding new ways to improve the time resolution of scintillating materials. Several approaches exploiting prompt photons emission (e.g. Cherenkov, cross-luminescence, nanomaterials) have been and are being investigated. Among these, the heterostructure systems, which combines high-Z scintillators (e.g. BGO, LYSO) with ultra-fast materials (e.g. fast plastic scintillating, nanocrystals), have already been demonstrated to improve the timing properties of state-of-the-art scintillators, for pixels up to 15mm long. In this work we assessed the time resolution of different heterostructure pixels composed of alternating BGO and plastic scintillator plates (EJ212, EJ232, BC422), comparing their timing performance to pure BGO samples of the same size and obtaining promising results. For instance, a Coincidence Time Resolution (CTR) of 271 ps Full-Width-Half-Maximum (FWHM) was measured for a 3x3x15mm3 bulk BGO crystal, while for a BGO+EJ232 heterostructure (BGO plates 100 μm thick, EJ232 50 μm thick) of the same size, a CTR of 205 ps was obtained (considering only 511 keV events which deposit energy in both materials). Additionally, a simulation framework has been developed to understand and optimise the heterostructure design both in terms of energy sharing and light collection.


This work was carried out in the frame of the Crystal Clear Collaboration, based on the concept initiated in the framework of the ERC Advanced Grant TICAL (grant agreement No 338953) funded by the European Research Council. It received support from the CERN Budget for Knowledge Transfer to Medical Applications.

Keywords: PET, TOF-PET, Heterostructure, 10ps, CTR
9:30 AM M-03-02

Design Considerations for 3D Position Sensitive Scintillation Detectors that Achieve 100 ps Coincidence Time Resolution (#1134)

S. Pourashraf1, Z. Zhao2, A. Gonzalez-Montoro1, J. W. Cates3, J. Y. Won4, J. S. Lee4, C. S. Levin5, 6

1 Stanford University, Radiology, Stanford, California, United States of America
2 Shanghai Jiao Tong University, School of Biomedical Engineering, Shanghai, China
3 Lawrence Berkeley National Laboratory, Applied Nuclear Physics Program, Berkeley, California, United States of America
4 Seoul National University, Nuclear Medicine and Biomedical Sciences, Seoul, Republic of Korea
5 Stanford University, Molecular Imaging Program, Stanford, California, United States of America
6 Stanford University, Radiology, Bioengineering, Physics, and Electrical Engineering, Stanford, California, United States of America


We have studied design considerations for three dimension positioning sensitive (3DPS) scalable TOF- PET detectors that achieve 100 ps coincidence time resolution (CTR). These scintillation detectors are based on 3×3×10 mm³ “LGSO” crystals side- coupled to arrays of 3×3 mm² SiPMs. A CPLD is used to provide 3 mm positioning resolution along the length of each crystal element. Revised mixed-signal front-end electronics decreased the power dissipation by 200% and increased the detector packing fraction while preserving the CTR performance. A low timing jitter FPGA-based TDC was used for data acquisition of the timing channels. Measured CTR was 102.2 ± 1.3 ps FWHM using a 22Na point source.

AcknowledgmentThis work was supported in part by NIH research grants 5R01CA21466903, 1R01EB02512501, and by NRF- 2016R1A2B3014645 National Research Foundation of Korea. Andrea Gonzalez- Montoro is partially supported by VALi+d Program for Researchers in Postdoctoral Phase of the Ministry of Labor and Social Economy (Generalitat de Valencia) and the European Social Fund. We also thank Xilinx University Program, providing us with Kintex- 7 FPGA kit and associated licenses. In addition, we would like to thank Mr. Takeyama Toshinori- NYKSCHM, Marubeni America Corporation, and Oxide Corporation for providing fast LGSO scintillation crystals.
Keywords: Scintellation Detectors, TOF-PET, 100 ps CTR, 3D Positioning, Sensitivity
9:45 AM M-03-03

Using Neural Networks for Impact Position Estimation in Glued Monolithic Crystals (#410)

M. Freire1, S. Echegoyen1, K. Vidal1, A. Gonzalez-Montoro1, F. Sanchez1, A. J. Gonzalez1

1 Institute for Instrumentation in Molecular Imaging (i3M), Valencia, Spain


Conventional Positron Emission Tomography (PET) and Single Photon Emission Tomography (SPECT) scanners are usually built of multiple detectors placed in a cylindrical geometry with both transaxial and axial gaps between them. These undesired gaps decrease sensitivity and degrade spatial resolution towards the edges of the detector. To avoid gaps, we have experimentally investigated the possibility of gluing the lateral sides of two LYSO crystals (n=1.81) of 33x25.4x10 mm3 using a high refractive index compound named Meltmount (n=1.7) and coupled to a 12x12 SiPM array. The same configuration was tested but keeping the air gap in between the two blocks (air configuration). The photon interaction positions were estimated using a Neural Network after acquiring calibration dataset for each configuration. The calibration dataset were split into train, evaluation and test. The detector performance was evaluated using the predicted values of the test dataset. Mean average error (MAE) for the central region of the detector of 0.5x0.2 mm and 0.3x0.1 mm was obtained for the air and Meltmount configurations, respectively. The R75 parameter reported 0.7x0.3 mm and 0.4x0.1 mm for each case. The results show that gluing scintillation crystals with high index refraction compound and using NN for impact position determination reduce edge effects with almost no gaps between scintillator detectors. 

Keywords: Glued Crystals, Monolithic detectors, Neural Network, Position estimation
10:00 AM M-03-04

Optical Photon Time-of-Flight for Depth-Encoding in Prism-PET Using Tapered Scintillator Crystals (#950)

A. LaBella1, X. Cao2, X. Zeng2, W. Zhao1, A. H. Goldan1

1 Stony Brook University, Department of Radiology, Stony Brook, New York, United States of America
2 Stony Brook University, Department of Electrical and Computer Engineering, Stony Brook, New York, United States of America


Optical photon time-of-flight (TOF) has been shown to be correlated with gamma ray depth of interaction (DOI) in scintillator crystals in PET detector modules. However, this correlation is weakened by optical photon leak to pixels adjacent to the primary pixel caused by imperfect crystal-readout pixel coupling. In this work, we show how using scintillator crystals with smaller cross-sections at the readout interface strengthens the correlation between DOI and optical photon TOF. We acquired depth-collimated data at 19 different depths (1-19 mm in steps of 1 mm) in a 4-to-1 coupled Prism-PET module. A 3 MBq Na-22 point source (1 mm active diameter) was placed in a lead cylinder with a 1 mm diameter pinhole positioned adjacent to a depth-aligned Prism-PET module consisting of a 16 x 16 array of 1.4 x 1.4 x 20 mm LYSO crystals coupled 4-to-1 to 3 x 3 mm SiPM pixels with a prismatoid light guide array on the opposite end. All crystals were tapered down to a 1.2 x 1.2 mm cross-section starting 5 mm from the crystal-readout interface. Energy weighted average method was used for energy-based DOI estimation. Timing-based DOI was estimated 2 different ways: (1) subtracting the first nearest-neighbor timestamp from the primary timestamp, and (2) taking the average difference of the primary timestamp and the 3 nearest-neighbor timestamps from the pixels coupled to the same prismatoid light guide. Linear regression analysis was used to determine how correlated energy-weighted DOI was with TOF-weighted DOI. We observed a strong correlation between the energy and timing-based DOI metrics across all depths. The estimated DOI resolution of the Prism-PET module based on each method was 2.22 mm full-width at half-maximum (FWHM) for the energy-weighted method, 7.38 mm FWHM for the TOF-weighted method with 1 timestamp, and 5.38 mm FWHM for the TOF-weighted method with 3 timestamps. Our tapered crystal design is a practical method to improve DOI and timing performance with Prism-PET.

Keywords: PET, DOI, Prism-PET, TOF, Light Guide
10:15 AM M-03-05

Characterization of a PET Detector based on Semi-Monolithic Crystals (#380)

J. Barrio1, N. Cucarella1, M. Freire1, E. Lamprou1, J. M. Benlloch1, A. J. González1

1 Institute for Instrumentation in Molecular Imaging (i3M-CSIC), Valencia, Spain


A semi-monolithic scintillator crystal consists of a monolithic crystal segmented in one direction in different pieces called slabs. Semi-monolithic crystals are intended to overcome the limitations of monolithic and pixelated crystals usually employed in PET detectors, offering accurate time of flight (TOF) and depth of interaction (DOI) information, as well as good spatial resolution. In this work, the characterization of a semi-monolithic detector has been carried out. The detector consists of an array of 24 LYSO slabs of 25.4×12×0.95 mm3 each, with all faces polished and ESR covered. The semi-monolithic crystal is coupled to a matrix of 8×8 SiPMs of 3×3 mm2 active area each with 50 μm cell size and 3.2 mm pitch. The 64 individual signals are read out by the TOFPET2 ASIC. The influence of introducing light guides of different thicknesses between the crystal and the photodetector has been assessed. The energy resolution obtained is below 20% FWHM at 511 keV for all cases and shows almost no dependence on the light guide thickness. Regarding timing, the coincidence time resolution (CTR) of two equally detectors is 336 ps FWHM after applying skew-time and walk-time corrections when no light guide is employed. This value improves to 276 ps FWHM if energy weighted averaging of the timestamps belonging to the same event is applied. When using light guides, these values worsen along with the thickness. Regarding spatial resolution, a neural network for positioning decoding is being implemented.

AcknowledgmentThis work was supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement No. 695536).
Keywords: DOI, PET, semi-monolithic crystal, SiPMs, TOF
10:30 AM M-03-06

Design Considerations and Optimization for BGO-based TOF-PET Detectors (#1375)

J. W. Cates1, W. - S. Choong1

1 Lawrence Berkeley National Laboratory, Applied Nuclear Physics, Berkeley, California, United States of America


We are developing large area, bismuth germanate (BGO)-based time-of-flight positron emission tomography (TOF-PET) detectors. With the development of photosensors capable of higher ultra-violet photon detection efficiency, low-noise and fast electronic readout, and novel time-pickoff methods, BGO has received significant attention in recent years as a low-cost (4-5x lower material cost compared to LSO), high performance alternative for TOF-PET systems. The moderate Cherenkov photon yield in BGO provides prompt signatures for estimation of 511 keV photon time of interaction. In benchtop, single pixel coincidence time resolution (CTR) <300 ps FWHM can be achieved for high aspect ratio crystals, 20 mm in length. By leveraging the combination of higher 511 keV photon detection efficiency in combination with TOF capabilities, there is opportunity for BGO-based TOF-PET detectors to compete with state-of-the-art systems in effective sensitivity. We have performed a systematic study of achievable CTR, energy resolution, and sensitivity to investigate performance limits, which includes a parametric study of the influence of SiPM choice, crystal surface treatment, and electronic readout strategies. This baseline study indicates that multiplexed detector modules capable of <300 ps FWHM and <13% energy resolution at 511 keV are feasible. We have further produced multichannel, low noise and high frequency electronic readout to develop and evaluate multiplexing strategies in large area crystal arrays and their influence on performance metrics. We find that 20 mm BGO crystals with chemically processed surfaces can have same effective sensitivity as 20 mm length LSO detectors capable of ~325 ps FWHM. Therefore, BGO-based TOF-PET detectors provide a pathway for state-of-the-art PET system performance at reduced cost, or alternatively provide avenues for ultra-sensitive PET systems to be more economical.

Keywords: TOF-PET, BGO
10:45 AM M-03-07

Physical considerations for Cherenkov radiation-based coincidence time resolution measurements in BGO (#988)

A. Gonzalez-Montoro1, S. Pourashraf1, J. W. Cates2, S. Merzi3, A. Gola3, G. Borghi3, C. S. Levin1, 4

1 Stanford University, Radiology, Stanford, California, United States of America
2 Lawrence Berkeley National Laboratory, Applied Nuclear Physics, Berkeley, California, United States of America
3 Fondazione Bruno Kessler, Trento, Italy
4 Stanford University, Physics and Electrical Engineering, Stanford, California, United States of America


Exploiting the Cherenkov luminescence from 511 keV photoelectric interactions is a potential solution to re-introduce BGO scintillators in time-of-flight positron emission tomography (TOF-PET). Recent improvements in vacuum- and near- ultra-violet high density (VUV- and NUV-HD) silicon photomultiplier (SiPM) technology combined with efficient data post-processing methods, make it possible to access timing information from the relatively few Cherenkov photons emitted. To achieve good coincidence time resolution (CTR) also requires low noise and fast readout electronics with small effective capacitance, which is possible by employing bootstrapping techniques.

In this summary, we report the CTR evaluation of the new VUV-HD and NUV-HD enhanced 3x3 mm2 SiPMs coupled to matching width BGO crystals. Preliminary results using a standard evaluation board (not optimized for timing measurements), reporting the best value of 177.21.6 ps CTR FWHM for the Cherenkov component of the overall signal, thus demonstrating the excellent performance of new SiPM technology.

AcknowledgmentThis work was supported by NIH grants R01CA214669 and R01EB025125.
Andrea Gonzalez-Montoro is partially supported by VALi+d Program for Researchers in Postdoctoral Phase of the Ministry of Labor and Social Economy (Generalitat de Valencia) and the EU Social Fund.
Keywords: BGO, Chrenkov, SiPM, TOF-PET, CTR
11:00 AM M-03-08

Study on the Timing Limits of High Refractive Index TlBr and TlCl Cherenkov Radiators (#856)

G. Terragni1, 2, M. Pizzichemi1, 2, G. Ariño-Estrada3, E. Roncali3, J. Glodo4, K. Shah4, S. Cherry3, E. Auffray1, A. Ghezzi2, N. Kratochwil1, 5

1 European Organization for Nuclear Research (CERN), Geneva, Genève, Switzerland
2 University of Milano-Bicocca, Milano, Italy
3 University of California, Department of Biomedical Engineering, Davis, California, United States of America
4 Radiation Monitoring Devices, Inc., Watertown, Massachusetts, United States of America
5 University of Vienna, Vienna, Wien, Austria


Combining charge induction readout of semiconductor detectors with the possibility of utilizing prompt Cherenkov photons can lead to an outstanding detector performance in energy, timing and spatial resolution and detection efficiency. Energy resolutions as good as 3% at 1.3 MeV have been reported recently for a thallium bromide (TlBr) detector with 1.7 mm x 1.7 mm pixels. The high refractive index of such materials (n ≈ 2.3 - 2.6) leads to a good Cherenkov photon yield but, at the same time, it can complicate the photon extraction, potentially affecting the time performance. In this work, we assess the timing properties of TlBr and thallium chloride (TlCl) crystals with different geometries. By using an optimized setup with high-frequency electronics, coincidence time resolution (CTR) values down to 167 ps full width at half maximum (FWHM) are measured including all the events, regardless of the number of triggered single-photon avalanche diodes (SPADs). To understand the timing differences between TlBr/TlCl and low refractive index Cherenkov radiators like PbF2, a simulation toolkit based on Geant4 is developed. Despite the increase of the number of produced Cherenkov photons with the refractive index, the CTR deteriorates due to the lower light transfer efficiency and increased photon time spread. We present measured and simulated depth-of-interaction (DOI) collimated results and discuss paths to improve the CTR toward 100 ps for PET sized geometries, eg. by double-sided silicon photomultiplier (SiPM) readout.

AcknowledgmentThis work was performed in the framework of the Crystal Clear Collaboration. The authors thank FBK (Alberto Gola, Maria Ruzzarin) for the SiPM samples used for this work. The authors want to express their highest gratitude to Stefan Gundacker for setting up the CTR experimental setup with its HF electronics and all the knowlwege transfer.
Keywords: Cherenkov, TlBr, TlCl, SiPM, TOF-PET

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