Silicon-photomultiplier TOF-PET Detector (#3422)
S. Cho1, R. A. Mintzer1, J. Breuer1, M. Aykac1, M. Loope1, J. Valenta1, J. L. Corbeil1, J. C. Arnott1, D. Binkley1, M. Roknsharifi1, S. Yuan1
1 Siemens Medical Solutions USA, Inc, Molecular Imaging, Knoxville, Tennessee, United States of America
Silicon photomultipliers and application specific integrated circuits are enabling development of compact time-of-flight scintillation detectors exhibiting significantly better timing than PMT based PET block detectors, while promising long term stability and reliability. We have fabricated prototype detectors using integrated circuits of our own design, and report their basic performance characteristics. The detector comprises eight pixelated-LSO/SiPM-array miniblocks in a 2×4 array. Each miniblock is optically isolated by reflective 3M ESR wrapping, and comprises a 5×5 array of 3.2×3.2×20 mm3 LSO crystals coupled to a 16 mm square, 4x4 channel low crosstalk SiPM array fabricated by Hamamatsu Photonics. A single circuit board attaches the SiPM arrays on one side to readout circuitry on the other. The detector module outputs are a set of composited position and energy signals, along with miniblock leading-edge discriminator outputs. A common cathode readout concept is used to separately derive timing and energy/position signals from the optically coupled pixels within each miniblock. This architecture requires only a single high bandwidth channel without summing to extract timing information, while reading all 16 anode signals for position and energy at much lower bandwidth. Two integrated circuits have been developed: an RF amplifier for timing signal amplification/filtering, and an ASIC comprising anode buffer amplifiers and position/energy signal generation, along with a leading-edge discriminator (LED). This ASIC has an I2C interface to set LED arming and leading edge thresholds, low voltage SiPM bias and other operating parameters. Eighteen prototype detectors with 23 degree C ambient temperature control were characterized using a reference detector in coincidence. The detectors exhibit excellent average crystal energy resolution of 10% and very good average crystal coincidence timing resolution of 220 ps FWHM with 4.5 V SiPM over-voltage.
Keywords: PET, SiPM, TOF-PET, SiPM Readout ASIC
Edge-Readout Detectors for PET (#4202)
X. Li1, M. Ruiz-Gonzalez1, L. R. Furenlid1
1 University of Arizona, College of Optical Science, Tucson, Arizona, United States of America
We have developed a novel edge-readout detector design for PET, which can easily provide sub-millimeter inplane spatial resolution together with DOI information with resolution determined by the thickness of the crystal layers. Our simulation shows that for 511 keV photons, a slab of LYSO crystal of dimensions 50.8 mm X 50.8 mm X 3 mm with 20 SiPMs coupled to its four edges can provide an in-plane spatial resolution of 0.62 mm (FWHM) when patterned with optical barriers. Our experiments show that for 662 keV photons, a slab of CsI(Tl) crystal of dimensions 27.4 mm X 27.4 mmX 3 mm with 16 SiPMs can provide an in-plane spatial resolution of 0.35 mm (FWHM). We are investigating using LYSO crystals in our experiment to analyze the timing performance of this detector geometry.
Keywords: PET, Edge-Readout, DOI, Optical Barrier
A prototype MR insert PET detector with strip-line readout (#1263)
H. Kim1, Y. Hua2, D. Xi2, N. Eclov1, C. - T. Chen1, H. T. Chen1, Q. Xie2, C. - M. Kao1
1 University of Chicago, Radiology, Chicago, Illinois, United States of America
We are developing a MR insert PET detector using SiPM for small animal imaging. Our detector design is based on the strip-line signal readout that we have developed to efficiently handle large amount signals for SiPM-based TOF PET. The strip-line readout has an advantageous feature suitable for building PET integrated within MR. In strip-line readout, the signal digitization is accomplished remotely from the detection module, thereby the front-end electronics of the detector within MR is minimal; the interference between two systems, e.g., magnetic field distortion due to PET, can be minimized accordingly. The feasibility of the strip-line readout for PET/MR has been demonstrated in our previous study. The current prototype MR insert PET is comprised of 8 detector modules, which are installed in a plastic cylindrical supporting structure. Each detector module consists of 8x4 LYSO arrays (3x3x10 mm$^3$ each) and two Hamamatsu S13361-3050NE-04 MPPC arrays (4x4, 3.2 mm pitch); 32 SiPM signals are routed in 4 strip-lines so that 8 LYSO/SiPM signals in a row are connected to each strip-line. The signals from strip-lines are fed to a 72-ch multi-voltage threshold (MVT) digitizer, which digitizes the input signals by TDCs implemented in FPGA. Experiments have been carried out to evaluate the detector performance, and image reconstruction also has been performed. In the paper, the detail on the development of the MR insert PET are explained, and preliminary test results are presented.
Keywords: Positron Emission Tomograpy, strip-line readout, MR insert PET
Maximum likelihood pulse shape discrimination for DOI and event positioning in a PET block detector (#2218)
E. Berg1, B. Patel1, E. Roncali1, S. R. Cherry1
1 University of California, Davis, Biomedical Engineering, Davis, United States of America
Maximum likelihood (ML) methods were developed for depth-of-interaction (DOI) and crystal-of-interaction encoding in a PET block detector. We previoiusly developed an ML framework for pulse shape discrimination to estimate DOI with single phosphor-coated crystals. Here, we extend these methods to a block detector containing an array of phosphor-coated crystals and investigate the feasibility of these methods with digitizing electronics typically used for PET. A joint likelihood was developed that makes use of each photodetector waveform and uses both the temporal and spatial properties of the photoelectrons to simultaneously estimate DOI and crystal-of-interaction in a single log likelihood. The joint ML method provided the best DOI encoding (2.1 mm positioning error), 36% better than delayed charge integration (DCI), but only slightly better than ML estimation using the summed waveform. This DOI encoding is in close agreement to that measured with single crystals, demonstrating the robust ML estimation with lower light collection. We also investigated the effect of sampling rate on DOI encoding with a free-running ADC. Sampling rates between 50 MS/s and 200 MS/s were implemented in a virtual free-running ADC applied to the raw waveforms (sampled at 5 GS/s). DOI estimation was nearly constant for all sampling rates after applying a correction to the ADC samples based on the position of the timing pick-off relative to the ADC clock cycle. Lastly, we compared crystal-of-interaction accuracy using the joint ML method to conventional Anger positioning. A collimated pencil beam was used to irradiate each crystal in the array so that the true crystal-of-interaction is known. There was no significant difference in crystal positioning between Anger and ML positioning (56% accuracy for both). However, ML-based DOI estimation provided a 23% increase in detection sensitivity, resulting from the recovery of accurate depth-dependent energy estimation in phosphor-coated crystals.
Keywords: Depth-of-interaction, Maximum likelihood, Signal processing, Pulse shape discrimination
Hybrid scintillators/Cherenkov radiators as cost-effective TOF-PET detectors: BSO versus BGO (#3443)
S. E. Brunner1, F. Dachs1, M. Gruber2, M. Gupta1, 3, A. Hirtl2, D. R. Schaart1
1 Delft University of Technology, Radiation Science & Technology, Delft, Netherlands
Compared to L(Y)SO:Ce, BGO offers a higher stopping power, higher photoelectric-fraction, no intrinsic radiation and cost-effectiveness. Until recently, however, BGO was considered unsuitable for time-of-flight detection due to its low light yield and long scintillation decay time.
TOF-PET detectors exploiting the fast nature of the Cherenkov effect have been proposed throughout the last decade. However, the low Cherenkov emission yield in the order of ∼10-20 photons per 511 keV annihilation photon complicates energy discrimination, impeding the application of Cherenkov PET detectors in clinical PET systems.
Using time-correlated single-photon counting (TCSPC), we recently observed a radioluminescence response in the 100 ps domain upon excitation of BGO with 511 keV photons, likely due to Cherenkov emission. Based on this observation we developed a hybrid Cherenkov/scintillation detector concept enabling TOF-detection using Cherenkov photons while maintaining energy discrimination based on scintillation photons in BGO.
Here, we show how the timing kernel of BGO can be further improved by optimizing the crystal surface finishing. We present an improvement of the full-width-at-tenth-maximum (FWTM) by almost 50%, ranging from 0.6 ns to 1.65 ns for 3 mm to 20 mm long crystals, respectively.
Furthermore, we investigate Bi4Si3O12 (BSO) as an alternative material for hybrid scintillation/Cherenkov detection. We show that the Cherenkov emission in BSO allows the FWHM of the timing kernel to be improved by up to 20% compared to BGO. The best results obtained so far using BSO cubes with 3 mm edge-lengths are a FWHM of 192 ps and a FWTM of 725 ps.
Temperature-dependent coincidence timing results will be presented for BGO and BSO crystals with polished surfaces and with surfaces optimized for coincidence timing. Furthermore, Monte Carlo simulations are presented for a better understanding of the influences of the Cherenkov emission and the surface finishing on the timing kernel.
Keywords: Cherenkov effect, BGO, BSO, TOF-PET, scintillator
Ionizing photon interactions in a material induce modulation of its optical properties at a femtosecond time scale: A new direction to improve timing for ToF-PET (#3408)
L. Tao1, R. Coffee2, C. S. Levin1
1 Stanford University, Radiology, Stanford, California, United States of America
We have recently been exploring the modulation of optical properties in a material as a new mechanism for ionizing radiation detection that can potentially be exploited to advance coincidence time resolution (CTR) for time-of-flight positron emission tomography (ToF-PET). In this paper, we take insight from the nascent field of ultrafast X-ray science using free electron lasers (xFELs) to prove the ultrafast nature of the ionizing radiation photon-induced refractive index modulation process. At the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory, the arrival time of X-ray pulses with photon energies between 0.5-10 keV are detected with 10 fs time resolution using an optical readout method. In this work, we used a similar interferometric method with spectrum encoded timing information to observe the ionization charge-induced refractive index modulation process in yttrium aluminum garnet (YAG), lutetium-yttrium oxyorthosilicate (LYSO) and bismuth silicon oxide (BSO) crystal samples. By using ultrafast X-ray pulses with 50 fs duration from LCLS, we have proven that the rise time of the refractive index modulation process induced by ionizing radiation photons is less than 50 fs for all crystal samples. This rise time (or response time of the interaction crystal) is determined by the duration of the electron cascade process after ionizing photon interactions, and by the ionizing photon energy, but not by the pulse duration. This shows the potential of adopting the interferometric detection method and using the mechanism of refractive index modulation for annihilation photon detection, with a long term goal to dramatically improve PET CTR. Since our proposed method does not depend on optical scintillation as the readout mechanism, we are free to explore non-scintillation and non-traditional detection materials.
Keywords: ToF-PET, ultrafast timing, ionizing radiation detection, refractive index modulation, LYSO, BSO, LCLS