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Jan 26, 2022, 3:47:36 PM
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Experimental Assessment on Data Sufficiency Condition for WGI Compton Imaging (#647)
H. Tashima1, E. Yoshida1, S. Takyu1, F. Nishikido1, T. Nishina2, 1, M. Suga2, H. Wakizaka1, M. Takahashi1, K. Nagatsu1, T. B. Atsushi1, K. Kamada3, 4, A. Yoshikawa3, 4, K. Parodi5, T. Yamaya1
1 National Institutes for Quantum and Radiological Science and Technology, Chiba, Japan
We developed a whole gamma imaging (WGI) prototype composed of a scatterer ring insert and an absorber ring. It was the world’s first realization of a full-ring Compton camera geometry. In this study, we focused on the data sufficiency condition (DSC) for Compton image reconstruction. Contrary to PET which requires a full-ring geometry to satisfy the DSC, Compton imaging does not require a full ring theoretically. Reducing the detector-ring coverages can improve flexibility of the geometry design and reduce manufacturing costs although sensitivity is decreased. However, the DSC for reconstruction is not guaranteed in a practical situation due to actual absorber ring arrangement and detection limits in scattering angle. In this study, we experimentally assessed the DSC for the angular coverages. We evaluated image artifacts on reconstructed Compton images because, in general, images distort and exhibit artifacts when the DSC is not satisfied, degrading quantitative accuracy. We measured a cylindrical phantom filled with 89Zr solution of 10.2 MBq by the WGI prototype for 60 min and extracted 909-keV Compton events. We selected events related to the detector blocks marked as available to change detector angular coverages defined for the phantom bottom point. First, we changed the number of detectors in the scatterer ring, keeping the full-ring absorber. Next, we changed the absorber ring angular coverage by changing the number of detectors, keeping the scatterer ring at the number where the angular coverage just exceeded 180°. As a result, when the angular coverage of the scatterer detector was less than 180°, reconstructed images exhibited artifacts. The absorber ring detector with less than 180° coverage showed artifacts when used with the scatterer detector with more than 180° coverage. We concluded that angular coverages of more than 180° for both the scatterer and absorber detectors against any region on a subject are the DSC for Compton image reconstruction.
Keywords: Whole gamma imaging, Compton image reconstruction, 89Zr, data sufficiency condition
Simultaneous PET, SPECT and therapeutic nuclides imaging with Compton-PET hybrid camera (#698)
M. Uenomachi1, K. Ogane2, 3, Z. Zhong4, K. Shimazoe5, K. Kamada6, 7, H. Takahashi8, Y. Wang1, H. Haba1
1 RIKEN, Nishina Center for Accelerator-Based Science, Wako, Japan
211At is one of promising therapeutic radionuclides for targeted alpha therapy. For clinical application, simultaneous 211At imaging with diagnostic nuclides, such as PET and SPECT nuclides, is helpful to evaluate the therapeutic effect and accuracy of accumulation of new radiotherapeutic agents. However, it is difficult to integrate PET and SPECT modalities because SPECT require Pb-based collimators, which limit the gamma-rays energy for imaging. In this research, we developed a Compton-PET hybrid imaging system, which consists of eight Compton cameras. This imaging system can visualize PET and SPECT (or therapeutic) nuclides without any mechanical collimators by using both PET and Compton imaging. We demonstrated simultaneous 211At, 18F (PET nuclide) and 111In (SPECT nuclide) imaging with the developed Compton-PET hybrid camera and succeeded to visualize each nuclide. Moreover, the double photon coincidence method was applied to reduce backgrounds in Compton images of 18F and 111In, which caused by scattered photons of higher energy gamma-rays.
Keywords: PET imaging, Compton imaging, multi-nuclide imaging, Double photon coincidence method, Compton-PET hybrid camera
FlashDC project: development of a beam monitor for Flash radiotherapy (#1378)
G. Traini5, A. De Gregorio4, M. De Simoni2, 5, P. De Maria6, M. Fischetti2, 5, G. Franciosini4, 5, M. Garbini3, M. Marafini3, V. Patera5, 2, A. Schiavi2, 5, A. Sciubba2, 5, M. Toppi1, 2, A. Trigilio4, 5, A. Sarti2, 5
1 INFN, LNF, frascati, Italy
Surgery, chemotherapy and conventional radiotherapy can intervene in only 65% of cancer cases, leaving an important number of patients with untreatable tumours, close to organs at risk and/or radioresistant, without adequate therapies capable of extending the patient life expectancy and improving its quality. This landscape could significantly change: recent preclinical studies show, for radically increased dose rates (> 40 Gy/s) wrt conventional treatments, a significant toxicity reduction in healthy tissues (up to 60%), while keeping the same tumor control probability. Such FLASH effect could have an incalculable social impact, but there are many open questions to be addressed before the adoption in clinical practice. One of the greatest challenges is related to the beam intensity control as there are no available technologies at the moment for beam monitoring that can meet the clinical dosimetric and quality assurance requirements in the FLASH regime. While most conventional monitors show at high dose rates non-recoverable saturation effects and/or beam energy dependences, the FlashDC project - Flash Detector beam Counter - aims to develop an innovative and economical detection system, based on the air fluorescence physical principle, to overcome the current limitations of standard monitoring technologies. The first results obtained with a preliminary device are encouraging and will be shown in this contribution. The final goal of the project is the construction of a compact tool capable of monitoring the beam intensity in FLASH. The final detector will be able to monitor the instantaneous intensity of a typical Flash pulse with a spatial resolution ~ mm able to monitor small radiation fields. Data taking campaigns and detailed Monte Carlo simulations will be used to finalize the detector layout, evaluating the expected performances and the efficacy of the background reduction strategies.
Keywords: Beam Monitor, Flash Effect, Radiotherapy
Imaging of hydroxyl radicals (OH-) distributions using luminol water during irradiation of carbon-ions (#198)
S. Yamamoto1, T. Yabe1, T. Akagi2
1 Nagoya University, Graduate School of Medicine, Nagoya, Japan
Hydroxyl radical (OH-) plays important roles in biological effects in radiation therapy. However, the short time OH- distribution measurements to observe the dynamic changes during irradiation are not realized. To make the distribution measurements possible, we tried the imaging of the light emitted from luminol water during irradiation of carbon-ions because luminol water emits light by the productions of OH- in water. Imaging of the light emitted from luminol water was conducted using cooled charge coupled device (CCD) camera during carbon-ions irradiation to the luminol water. By the irradiations of carbon-ions, OH- reacted luminol in water and emitted light which was imaged by a cooled CCD camera. We also carried out 0.1 s interval time sequential dynamic imaging of luminol water during carbon-ions irradiation. The light intensity of the emitted light in the luminol water was more than 100 times higher than that of the luminescence of water and the range of the beam could be determined within the errors of 1 mm. In the 0.1 s interval time sequential dynamic luminol water images, decrease of the Bragg peak height was observed as the time increased at the end of the spill of the carbon-ions. We conclude that high intensity luminol water imaging was possible during irradiation of carbon-ions and it will provide new insights in OH- productions in water during FLASH radiotherapy.
Keywords: hydroxyl radical, carbon-ions, luminol water, optical imaging
Initial prototyping of a forceps-type coincidence detector for intraoperative diagnosis of lymph node metastasis (#1163)
S. Ito1, M. Takahashi2, H. G. Kang2, S. Takyu2, K. Kawamura3, F. Nishikido2, Y. Seto4, T. Yamaya2
1 Mirai-imaging corporation, Fukushima, Japan
18F-FDG is known as a good biomarker for diagnosis of lymph node metastasis, but their intraoperative application has not been studied well. In this work, we designed an intraoperative forceps-type coincidence detector to measure radioactivity concentration in each lymph node. Coincidence pairs of annihilation photons from 18F were measured by using two small scintillators that form the head of the forceps. From initial clinical data, we derived the efficiency requirement of 0.54% or more for the detector. However, the detector head had a size limitation since it had to pass through a typical trocar hole of 12 mm diameter. We made a prototype device by using rectangular BGO crystals with dimensions of 8 mm width x 10 mm length x 4 mm thickness, and we measured sensitivity profiles using a 22Na point source. The sensitivity of 1.18% was obtained for the point source when located at the center of the FOV with the distance of 5 mm between two crystals. However, in the case of a large size lymph node, higher sensitivity will be required because the sensitivity will decrease according to the distance between crystals. Therefore, we followed the experiment by GEANT4 simulation to find an appropriate crystal shape from three candidate shapes: rectangular, half-cylinder and half-shell shapes. We found that the half-shell shape crystal had the highest sensitivity.
AcknowledgmentThis research was supported by AMED under Grant Number JP21hm0102078h0002.
Keywords: PET, Forceps, Monte Carlo simulation
Ultra-Purification of Trimethylbismuth by Rectification for Use in a Charge Readout Detector (#751)
B. Gerke1, S. Peters2, K. Bolwin1, V. Hannen2, C. Huhmann2, N. Marquardt2, C. Weinheimer2, K. P. Schäfers1
1 University of Münster, European Institute for Molecular Imaging, Münster, North Rhine-Westphalia, Germany
Trimethybismuth (TMBi) is an organometallic dielectric liquid that has been proposed before as a potential material for charge readout detectors used in PET imaging. Due to the expected low number of free electrons after gamma interaction, any polar impurity in TMBi will cause electrons to recombine. Thus, TMBi has to be ultra purified for successful operation of charge readout detectors.
AcknowledgmentWe gratefully acknowledge support by Deutsche Forschungsgemeinschaft (SCHA 1447/3-1 and WE 1843/8-1).
Keywords: PET, TMBi, Detector, Purification, Distillation
Optimization of GFAG Crystal Surface Treatment for SiPM Based Clinical TOF PET Detector (#109)
H. G. Kang1, K. Kamada2, 3, A. Yoshikawa2, 3, F. Nishikido1, T. Yamaya1
1 National Institutes for Quantum and Radiological Science and Technology, Institute for Quantum Medical Science, Chiba, Japan
Coincidence timing resolution (CTR) is one of the important parameters in state-of-the-art clinical positron emission tomography (PET) scanners to increase the signal-to-noise ratio of reconstructed PET images by using time-of-flight (TOF) information. The aim of this study was to enhance the CTR and energy resolution of a silicon photomultiplier (SiPM) based clinical TOF-PET detector by optimizing the crystal surface treatment used for a 20 mm long crystal geometry. The TOF-PET detector consisted of a cerium doped gadolinium fine aluminum gallate (Ce:GFAG) scintillation crystal (3.0 × 3.0 × 20 mm3, C&A, Japan) and an SiPM (Hamamatsu, S13360-3050CS, Japan) with an effective area of 3.0 × 3.0 mm2. s with Eight different types of GFAG crystal surface treatment: mechanical polishing (M.P) of all surfaces and seven different partial saw-cuts (i.e. top, ⅟₂-side lower, ½-side upper, 1-side, 2-side, 3-side, and 4-side) were used to find the optimal treatment. The timing and energy signals were extracted by using a high-frequency SiPM readout circuit and then were digitized by using a CAMAC DAQ system. The 1-side saw-cut resulted in a 14 ps better CTR (203±3 ps) and 1.9% better energy resolution (9.3%) over those of the M.P crystal (CTR=217±4 ps, energy resolution=11.2%). The ⅟₂-side saw-cut enhanced the CTR (203±3 ps), however the energy resolution (12.5%) was degraded due to the variation in light collection efficiency along the crystal depth. In conclusion, the 1-side saw-cut was the optimal crystal surface treatment in terms of CTR and energy resolution for the SiPM based TOF PET detector.
Keywords: TOF, CTR, Crystal surface treatement, SiPM
Towards TOF improvements: Metascintillator simulation using BaF2 as fast scintillator (#354)
L. Moliner1, G. Konstantinou2, J. M. Benlloch1, P. R. Lecoq1, 2
1 Universidad Politecnica de Valencia/CSIC, I3M - Institute of Instrumentation for Molecular Imaging, Valencia, Spain
Time-of-Flight information provides an important increase on the image quality in PET. One of the most limiting factors to improve this information is the light production of the scintillator crystals which convert the gamma radiation into detectable light. This work presents a simulation study based on the concept of metascintillators that can lead to a PET TOF detector with a good tradeoff between detection efficiency and timing resolution. While in metascintillators the combination of the stopping power of a high-Z material with the photon production of fast organic scintillators has shown promise, the low stopping power of the organic scintillators limits the extent to which metascintillator functionality can be applicable in industrial contexts. If instead of organic scintillators, fast materials such as BaF2 are used, this compromise is lifted. The results of the simulations using a combination of BGO and LYSO with BaF2 in two configurations (parallel and perpendicular) show the performance and feasibility of the use of these types of meta-scintillators. Energy sharing seems to be significantly more extensive as compared to previous published works, rendering this combination attractive for experimentation and development up to system level.
Keywords: PET, TOF, Metamaterial, Scintillators
Improving Metascintillator Time Resolution Using Energy-Sharing Time-Walk Corrections (#736)
G. Konstantinou1, R. Latella1, 2, J. Barrio2, L. Moliner2, J. M. Benlloch2, A. J. Gonzalez2, P. Lecoq1, 2
1 Metacrystal S.A., Geneva, Switzerland
Metascintillators are scintillators which principal is based on sharing the energy of an impinging gamma ray between their composing materials. A dense crystal such as LYSO for helps improving the gamma stopping potential and a fast organic compound such as BC-422 for its light production kinetics. We have developed an heterostructure which combines the advantages of those constituting materials. We analyze the extent of energy sharing, as understood through experiment, in a coincidence timing resolution (CTR) measurement setup, using a 3x3x15 mm3 metascintillator, against a reference detector. The features of individual pulses allow one characterizing the photoelectric interactions in order to evaluate the timing response of the metascintillator. At the same time, the photon production characteristics of the first picoseconds are significantly affected by the energy sharing, deteriorating the statistical robustness of the time measurement. Guided by theoretical analysis of the pulse shape, we defined different populations of photoelectric interactions accordingly to the extent of energy sharing and investigated how the addition of a time-walk correction affects the timing of shared events. We demonstrated a significant improvement, in the order of 18%, for the majority of shared events, while the events with higher production in the fast emitter show a 32% improvement. Deeper understanding of the sharing mechanism and improvements in energy resolution of the system are expected to significantly improve these figures by allowing a more precise determination of the photoelectric interaction event subsets. This approach will further be applied to different metapixels (length, volume ratio, material) in an optimized environment.
This work was supported in part by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement No 695536): 4D-PET.
Keywords: PET, scintillators, DTR, TOF, metascintillators
Dual-line dynamic time-over-threshold method for simultaneous energy estimation and pulse shape discrimination (#341)
S. W. Lee1, 2, H. S. Shim3, 4, J. S. Lee1, 3
1 Seoul National University, Department of Nuclear Medicine, Seoul, Republic of Korea
The time-over-threshold (ToT) measurement method has several advantages over conventional pulse height or integrated charge measurements using analog-to-digital (ADC) converters. However, the nonlinear relation between the ToT value and the energy is the main drawback of this method. Depth of interaction (DOI) measurement in PET systems allows simultaneous achievement of high resolution and sensitivity, thus improving the image quality. One of the widely used DOI measurement methods is pulse shape discrimination, which uses scintillation crystals with different decay times. Recently, the pulse shape discrimination using the ToT method has been proposed; however, this approach requires additional pulse height measurements to provide DOI information for scattered events. Therefore, in this study, we propose a new method for estimating energy and discriminating pulse shape at the same time by modifying the dynamic ToT method, which features better energy linearity than the conventional ToT method.
Conventional dynamic ToT uses only a single voltage comparator with an adaptive dynamic that starts at a low level and increases gradually, resulting in improved energy linearity and resolution. However, in the proposed method, the low level threshold remains constant to capture the falling edge of the scintillation pulse. By subtracting the durations of the two digital pulses (one dynamic and one static) measured using time-to-digital convertors, we can acquire the decay time information.
The proposed method showed excellent energy linearity without energy resolution degradation. Unlike the original dynamic ToT, the proposed method is able to differentiate the shape of pulses, which is required for the DOI measurement. Because the proposed method does not require additional pulse height measurements based on ADC to distinguish the scattered events, the circuit complexity and cost will be relatively low.
Keywords: Time over threshold (TOT), depth of interaction (DOI), PET
Dynamic Time-Over-Threshold Readout for Improved Energy Linearity in PET Detectors that Achieve 100 ps CTR (#1143)
S. Pourashraf1, A. Gonzalez-Montoro1, J. W. Cates2, Z. Zhao3, J. Y. Won4, J. S. Lee4, C. S. Levin5, 6
1 Stanford University, Radiology, Stanford, California, United States of America
We have embedded a dynamic time-over-threshold (DynTOT) block in our scalable TOF-PET detector readout electronics to linearize the multiplexed energy spectra while keeping 100 ps FWHM Coincidence Time Resolution (CTR) performance. This DynTOT block is constructed by off-the-shelve components and consumes only 20 mW power per detector ‘layer’ in our design. Using 3×3×10 mm³ “LGSO” crystals coupled to arrays of 3×3 mm² SiPMs, energy resolution of 13.8±0.1%, 13.9±0.3%, and 17.1±0.6% were experimentally achieved for conventional pulse height, DynTOT, and conventional TOT methods, respectively. Including the DynTOT method, coincidence data were experimentally acquired using a low jitter FPGA-based TDC, and 102.2 ± 1.3 ps FWHM CTR was achieved, demonstrating the robustness of our novel and scalable timing electronic chain.
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: TOF-PET, energy Linearity, Energy Resolution, Dynamic TOT, 100 ps CTR
Pushing the Timing Limits of the TOFPET2 ASIC (#394)
V. Nadig1, S. Gundacker1, H. Radermacher1, D. Schug1, 2, B. Weissler1, 2, L. Yin1, R. Allendorf1, V. Schulz1, 2
1 RWTH Aachen University, Department of Physics of Molecular Imaging Systems, Institute for Experimental Molecular Imaging, Aachen, North Rhine-Westphalia, Germany
With the major goal of reaching a coincidence time resolution (CTR) well below 100 ps, not only scintillators and photosensors, but also front-end electronics have become the focus of latest efforts to improve the CTR in time-of-flight positron emission tomography (TOFPET). This study adapted a high-frequency (HF), baseline-shift compensating front-end circuit combined with an ASIC by PETsys Electronics S.A. to initiate the transfer of this benchtop readout technique to PET systems. Testing Hamamatsu silicon-photomultipliers (SiPMs) coupled to 3 mm x 3 mm x 20 mm Cerium-doped lutetium-oxyorthosilicate (LYSO:Ce) crystals with the combined HF and TOFPET2 ASIC readout architecture improved the CTR to 191 ps compared to a reference CTR of 200 ps acquired with the PETsys TOFPET2 Evaluation Kit. For Broadcom SiPMs and the same crystals, a CTR of 186 ps (HF) compared to 202 ps (reference) is reported. Smaller LYSO:Ce crystals of 2 mm x 2 mm x 3 mm size coupled to the same SiPMs reached CTRs of 128 ps and 135 ps (HF) compared to 135 ps and 149 ps (reference). Despite the significant improvement of the CTR by the HF readout architecture, the amplified signals provoked up to four side peaks in the coincidence time difference spectra with a periodic distance of 350 ps to 390 ps to the main peak. In-depth studies revealed that these side peaks are also visible within standard Evaluation Kit measurements. It is shown that the peak amplitude is reduced with decreased SiPM signal height and by re-configuring the trigger logic to a single-threshold trigger in the energy branch.
AcknowledgmentThe authors would like to thank Stefan Brunner from Broadcom for providing samples and Stefan Tavernier, Ricardo Bugalho and Luis Ferramacho from PETsys Electronics S.A. for sharing their expertise in our many discussions.
Keywords: time-of-flight, PET, high-frequency readout, CTR
Towards 200 ps CRT in DOI-capable Semi-Monolithic PET-Detectors for Clinical Applications (#938)
S. Naunheim1, T. Solf2, Y. Kuhl1, D. Schug1, 3, V. Schulz1, 3, F. Mueller1
1 RWTH Aachen University, Department of Physics of Molecular Imaging Systems, Institute for Experimental Molecular Imaging, Aachen, North Rhine-Westphalia, Germany
In PET imaging, timing performance is usually evaluated by coincidence resolving time (CRT) representing a key characteristic of PET detectors. Good timing resolution enables the possibility using time-of-ﬂight information, thus improving the image’s signal-to-noise ratio. Semi-monolithic detectors provide intrinsic depth of interaction (DOI) capabilities, and combine advantages of segmented and monolithic detectors, namely small read-out area and high density of scintillation photons. The used coincidence measurement setup consists of a semi-monolithic slab detector comprising 8 monolithic LYSO slabs (each 3.9 x 31.9 x 19.0 mm3) and an one-to-one coupled detector (64 segments à 3.9 x 3.9 x 19.0 mm3). Both detectors are coupled to 16 digital SiPMs (DPC 3200-22, Philips Digital Photon Counting).
Keywords: Time Calibration, Slabs, Time of Flight, Monolithic Scintillators, Coincidence Time Resolution
Detection of sub-Terahertz waves modulated by ionization-induced charge carriers using a sub-wavelength antenna (#1066)
Y. Kim1, D. Jeong1, R. Coffee2, C. S. Levin1
1 Stanford University, Radiology, Palo Alto, California, United States of America
We are currently exploring the modulation of optical properties as an alternative mechanism for ionizing radiation detection for time-of-flight positron emission tomography (TOF-PET) rather than scintillation. “Pump-probe spectroscopy” is one of the most promising approaches for ultrafast free charge carrier detection, with a goal to achieve sub-10 ps coincidence time resolution for TOF-PET. Electromagnetic waves with sub-terahertz wavelength can interact strongly with the charge carriers generated from the ionization process due to the well-matched photon energy and the intraband transition energy. However, the mismatch between the millimeter-length wavelength of the sub-terahertz waves and the size of the spatial distribution of charge carriers limit the amount of probe wave modulation. The probe waves were manipulated to be confined below the diffraction limit to maximize the light-matter interaction using a sub-wavelength antenna on a silicon substrate. The transmission through the antenna decreased 32% in a finite-difference time-domain simulation as a result of free charge carriers produced from 511 keV interactions. A high electron mobility transistor (HEMT) attached to the sample measured the intensity of the near-field transmitted wave, with triggering to focus on only the possible ionization events. The experimentally measured transmission spectrum shows a 6.3 times larger standard deviation than the noise at 1.745 GHz under irradiation with a Tl-204 beta source. We observed the probe wave was modulated by charge carriers generated as a result of the ionizing radiation absorbed in the sub-wavelength antenna. This result suggests the further investigation of this novel method of ionizing radiation detector using sub-THz probe waves.
This work is partly supported by NIH grant 5R01EB02390302, T32 CA118681, and National Research Foundation (NRF) of Korea grant 2020R1A6A3A03039918.
Keywords: ToF-PET, PET, CTR, Optical Properties Modulations, Terahertz waves