Bio

Bio


Joshua's research focus at Stanford has been enabling technologies and instrumentation for future and vital roles of Positron Emission Tomography (PET), including: Ultra-fast Time-of-Flight PET (TOF-PET) instrumentation, clinical PET detector technology with high sensitivity and resolving power for diagnostics in early stage cancer, high resolution MRI-compatible TOF-PET detector technologies for oncology and neurology, and devices for precise quantification of tracer kinetics in dynamic PET imaging. The ultimate goal is to introduce new PET imaging systems that advance capabilities of whole body clinical imaging in sensitivity, accuracy, and precision for oncology, neurology, and cardiology.

He is also interested in the application of instrumentation and algorithms for imaging in biomedical, high energy and applied physics, astronomy, energy, materials, nuclear security, and autonomous vehicle research.

Honors & Awards


  • Valentin T Jordanov Radiation Instrumentation Award, IEEE Foundation (2016)
  • Valentin T Jordanov Radiation Instrumentation Award, IEEE Foundation (2015)
  • Valentin T Jordanov Radiation Instrumentation Award, IEEE Foundation (2014)
  • Outstanding PhD Student, UT NE Magazine, University of Tennessee (2013)
  • Nuclear Engineering PhD Graduate Research Excellence Award, University of Tennessee (2013)
  • University of Tennessee Chancellor?s Award for Extraordinary Professional Promise, University of Tennessee (2013)
  • IEEE Nuclear and Plasma Sciences Society Graduate Scholarship Award, IEEE NPSS (2013)
  • 1st Place, Oak Ridge National Laboratory Intern Research Contest, Oak Ridge National Laboratory (2008)

Boards, Advisory Committees, Professional Organizations


  • Member, Society of Nuclear Medicine (2015 - Present)
  • Member, IEEE Nuclear and Plasma Sciences Society (2008 - Present)

Professional Education


  • Fellowship, Stanford University, Molecular Imaging Scholars Program (2016)

Research & Scholarship

Current Research and Scholarly Interests


Joshua's research focus at Stanford has been enabling technologies and instrumentation for future and vital roles of Positron Emission Tomography (PET), including: Ultra-fast Time-of-Flight PET (TOF-PET) instrumentation, clinical PET detector technology with high sensitivity and resolving power for diagnostics in early stage cancer, high resolution MRI-compatible TOF-PET detector technologies for oncology and neurology, and devices for precise quantification of tracer kinetics in dynamic PET imaging. The ultimate goal is to introduce new PET imaging systems that advance capabilities of whole body clinical imaging in sensitivity, accuracy, and precision for oncology, neurology, and cardiology.

He is also interested in the application of instrumentation and algorithms for imaging in biomedical, high energy and applied physics, astronomy, energy, materials, nuclear security, and autonomous vehicle research.

Publications

All Publications


  • Time-over-threshold for pulse shape discrimination in a time-of-flight phoswich PET detector PHYSICS IN MEDICINE AND BIOLOGY Chang, C., Cates, J. W., Levin, C. S. 2017; 62 (1): 258-271

    Abstract

    It is well known that a PET detector capable of measuring both photon time-of-flight (TOF) and depth-of-interaction (DOI) improves the image quality and accuracy. Phoswich designs have been realized in PET detectors to measure DOI for more than a decade. However, PET detectors based on phoswich designs put great demand on the readout circuits, which have to differentiate the pulse shape produced by different crystal layers. A simple pulse shape discrimination approach is required to realize the phoswich designs in a clinical PET scanner, which consists of thousands of scintillation crystal elements. In this work, we studied time-over-threshold (ToT) as a pulse shape parameter for DOI. The energy, timing and DOI performance were evaluated for a phoswich detector design comprising [Formula: see text] mm LYSO:Ce crystal optically coupled to [Formula: see text] mm calcium co-doped LSO:Ce,Ca(0.4%) crystal read out by a silicon photomultiplier (SiPM). A DOI accuracy of 97.2% has been achieved for photopeak events using the proposed time-over-threshold (ToT) processing. The energy resolution without correction for SiPM non-linearity was [Formula: see text]% and [Formula: see text]% FWHM at 511?keV for LYSO and LSO crystal layers, respectively. The coincidence time resolution for photopeak events ranges from 164.6?ps to 183.1?ps FWHM, depending on the layer combinations. The coincidence time resolution for inter-crystal scatter events ranges from 214.6?ps to 418.3?ps FWHM, depending on the energy windows applied. These results show great promises of using ToT for pulse shape discrimination in a TOF phoswich detector since a ToT measurement can be easily implemented in readout electronics.

    View details for DOI 10.1088/1361-6560/62/1/258

    View details for Web of Science ID 000391567700007

    View details for PubMedID 27991437

    View details for PubMedCentralID PMC5280037

  • Highly multiplexed signal readout for a time-of-flight positron emission tomography detector based on silicon photomultipliers. Journal of medical imaging (Bellingham, Wash.) Cates, J. W., Bieniosek, M. F., Levin, C. S. 2017; 4 (1): 011012-?

    Abstract

    Maintaining excellent timing resolution in the generation of silicon photomultiplier (SiPM)-based time-of-flight positron emission tomography (TOF-PET) systems requires a large number of high-speed, high-bandwidth electronic channels and components. To minimize the cost and complexity of a system's back-end architecture and data acquisition, many analog signals are often multiplexed to fewer channels using techniques that encode timing, energy, and position information. With progress in the development SiPMs having lower dark noise, after pulsing, and cross talk along with higher photodetection efficiency, a coincidence timing resolution (CTR) well below 200 ps FWHM is now easily achievable in single pixel, bench-top setups using 20-mm length, lutetium-based inorganic scintillators. However, multiplexing the output of many SiPMs to a single channel will significantly degrade CTR without appropriate signal processing. We test the performance of a PET detector readout concept that multiplexes 16 SiPMs to two channels. One channel provides timing information with fast comparators, and the second channel encodes both position and energy information in a time-over-threshold-based pulse sequence. This multiplexing readout concept was constructed with discrete components to process signals from a [Formula: see text] array of SensL MicroFC-30035 SiPMs coupled to [Formula: see text] Lu1.8Gd0.2SiO5 (LGSO):Ce (0.025 mol. %) scintillators. This readout method yielded a calibrated, global energy resolution of 15.3% FWHM at 511 keV with a CTR of [Formula: see text] FWHM between the 16-pixel multiplexed detector array and a [Formula: see text] LGSO-SiPM reference detector. In summary, results indicate this multiplexing scheme is a scalable readout technique that provides excellent coincidence timing performance.

    View details for DOI 10.1117/1.JMI.4.1.011012

    View details for PubMedID 28382312

  • A multiplexed TOF and DOI capable PET detector using a binary position sensitive network. Physics in medicine and biology Bieniosek, M. F., CATES, J. W., Levin, C. S. 2016; 61 (21): 7639-7651

    Abstract

    Time of flight (TOF) and depth of interaction (DOI) capabilities can significantly enhance the quality and uniformity of positron emission tomography (PET) images. Many proposed TOF/DOI PET detectors require complex readout systems using additional photosensors, active cooling, or waveform sampling. This work describes a high performance, low complexity, room temperature TOF/DOI PET module. The module uses multiplexed timing channels to significantly reduce the electronic readout complexity of the PET detector while maintaining excellent timing, energy, and position resolution. DOI was determined using a two layer light sharing scintillation crystal array with a novel binary position sensitive network. A 20?mm effective thickness LYSO crystal array with four 3?mm??×??3?mm silicon photomultipliers (SiPM) read out by a single timing channel, one energy channel and two position channels achieved a full width half maximum (FWHM) coincidence time resolution of 180??±??2?ps with 10?mm of DOI resolution and 11% energy resolution. With sixteen 3?mm??×??3?mm SiPMs read out by a single timing channel, one energy channel and four position channels a coincidence time resolution 204??±??1?ps was achieved with 10?mm of DOI resolution and 15% energy resolution. The methods presented here could significantly simplify the construction of high performance TOF/DOI PET detectors.

    View details for PubMedID 27740946

  • Analog filtering methods improve leading edge timing performance of multiplexed SiPMs. Physics in medicine and biology Bieniosek, M. F., CATES, J. W., Grant, A. M., Levin, C. S. 2016; 61 (16): N427-40

    Abstract

    Multiplexing many SiPMs to a single readout channel is an attractive option to reduce the readout complexity of high performance time of flight (TOF) PET systems. However, the additional dark counts and shaping from each SiPM cause significant baseline fluctuations in the output waveform, degrading timing measurements using a leading edge threshold. This work proposes the use of a simple analog filtering network to reduce the baseline fluctuations in highly multiplexed SiPM readouts. With 16 SiPMs multiplexed, the FWHM coincident timing resolution for single [Formula: see text] mm LYSO crystals was improved from 401??±??4 ps without filtering to 248??±??5 ps with filtering. With 4 SiPMs multiplexed, using an array of [Formula: see text] mm LFS crystals the mean time resolution was improved from 436??±??6 ps to 249??±??2 ps. Position information was acquired with a novel binary positioning network. All experiments were performed at room temperature with no active temperature regulation. These results show a promising technique for the construction of high performance multiplexed TOF PET readout systems using analog leading edge timing pickoff.

    View details for DOI 10.1088/0031-9155/61/16/N427

    View details for PubMedID 27484131

  • Achieving fast timing performance with multiplexed SiPMs. Physics in medicine and biology Bieniosek, M. F., CATES, J. W., Levin, C. S. 2016; 61 (7): 2879-2892

    Abstract

    Using time of flight (ToF) measurements for positron emission tomography (PET) is an attractive avenue for increasing the signal to noise (SNR) ratio of PET images. However, achieving excellent time resolution required for high SNR gain using silicon photomultipliers (SiPM) requires many resource heavy high bandwidth readout channels. A method of multiplexing many SiPM signals into a single electronic channel would greatly simplify ToF PET systems. However, multiplexing SiPMs degrades time resolution because of added dark counts and signal shaping. In this work the relative contribution of dark counts and signal shaping to timing degradation is simulated and a baseline correction technique to mitigate the effect of multiplexing on the time resolution of analog SiPMs is simulated and experimentally verified. A charge sharing network for multiplexing is proposed and tested. Results show a full width at half maximum (FWHM) coincidence time resolution of [Formula: see text] ps for a single 3?mm??×??3?mm??×??20?mm LYSO scintillation crystals coupled to an array of sixteen 3?mm??×??3?mm SiPMs that are multiplexed to a single timing channel (in addition to 4 position channels). A [Formula: see text] array of 3?mm??×??3?mm??×??20?mm LFS crystals showed an average FWHM coincidence time resolution of [Formula: see text] ps using the same timing scheme. All experiments were performed at room temperature with no thermal regulation. These results show that excellent time resolution for ToF can be achieved with a highly multiplexed analog SiPM readout.

    View details for DOI 10.1088/0031-9155/61/7/2879

    View details for PubMedID 26987898

  • Advances in coincidence time resolution for PET. Physics in medicine and biology Cates, J. W., Levin, C. S. 2016; 61 (6): 2255-2264

    Abstract

    Coincidence time resolution (CTR), an important parameter for time-of-flight (TOF) PET performance, is determined mainly by properties of the scintillation crystal and photodetector used. Stable production techniques for LGSO:Ce (Lu1.8Gd0.2SiO5:Ce) with decay times varying from???30-40 ns have been established over the past decade, and the decay time can be accurately controlled with varying cerium concentration (0.025-0.075 mol%). This material is promising for TOF-PET, as it has similar light output and equivalent stopping power for 511 keV annihilation photons compared to industry standard LSO:Ce and LYSO:Ce, and the decay time is improved by more than 30% with proper Ce concentration. This work investigates the achievable CTR with LGSO:Ce (0.025 mol%) when coupled to new silicon photomultipliers. Crystal element dimension is another important parameter for achieving fast timing. 20?mm length crystal elements achieve higher 511 keV photon detection efficiency, but also introduce higher scintillation photon transit time variance. 3?mm length crystals are not practical for PET, but have reduced scintillation transit time spread. The CTR between pairs of [Formula: see text] mm(3)and [Formula: see text] mm(3) LGSO:Ce crystals was measured to be [Formula: see text] and [Formula: see text] ps FWHM, respectively. Measurements of light yield and intrinsic decay time are also presented for a thorough investigation into the timing performance with LGSO:Ce (0.025 mol%).

    View details for DOI 10.1088/0031-9155/61/6/2255

    View details for PubMedID 26914187

  • Analytical calculation of the lower bound on timing resolution for PET scintillation detectors comprising high-aspect-ratio crystal elements PHYSICS IN MEDICINE AND BIOLOGY Cates, J. W., Vinke, R., Levin, C. S. 2015; 60 (13): 5141-5161

    Abstract

    Excellent timing resolution is required to enhance the signal-to-noise ratio (SNR) gain available from the incorporation of time-of-flight (ToF) information in image reconstruction for positron emission tomography (PET). As the detector's timing resolution improves, so does SNR, reconstructed image quality, and accuracy. This directly impacts the challenging detection and quantification tasks in the clinic. The recognition of these benefits has spurred efforts within the molecular imaging community to determine to what extent the timing resolution of scintillation detectors can be improved and develop near-term solutions for advancing ToF-PET. Presented in this work, is a method for calculating the Cramér-Rao lower bound (CRLB) on timing resolution for scintillation detectors with long crystal elements, where the influence of the variation in optical path length of scintillation light on achievable timing resolution is non-negligible. The presented formalism incorporates an accurate, analytical probability density function (PDF) of optical transit time within the crystal to obtain a purely mathematical expression of the CRLB with high-aspect-ratio (HAR) scintillation detectors. This approach enables the statistical limit on timing resolution performance to be analytically expressed for clinically-relevant PET scintillation detectors without requiring Monte Carlo simulation-generated photon transport time distributions. The analytically calculated optical transport PDF was compared with detailed light transport simulations, and excellent agreement was found between the two. The coincidence timing resolution (CTR) between two [Formula: see text] mm[Formula: see text] LYSO:Ce crystals coupled to analogue SiPMs was experimentally measured to be [Formula: see text] ps FWHM, approaching the analytically calculated lower bound within 6.5%.

    View details for DOI 10.1088/0031-9155/60/13/5141

    View details for Web of Science ID 000356872000014

    View details for PubMedID 26083559

  • Direct conversion semiconductor detectors in positron emission tomography MODERN PHYSICS LETTERS A Cates, J. W., Gu, Y., Levin, C. S. 2015; 30 (14)
  • The lower timing resolution bound for scintillators with non-negligible optical photon transport time in time-of-flight PET PHYSICS IN MEDICINE AND BIOLOGY Vinke, R., Olcott, P. D., Cates, J. W., Levin, C. S. 2014; 59 (20): 6215-6229
  • Significant Increases in Light Extraction From YAP:Ce Scintillators With a Uniform Surface Taper Modification at the Exit Boundary IEEE TRANSACTIONS ON NUCLEAR SCIENCE Cates, J., Hayward, J., Zhang, X. 2013; 60 (5): 3995-4001
  • Measurement of Achievable Timing Resolution With ZnO:Ga Films IEEE TRANSACTIONS ON NUCLEAR SCIENCE Cates, J. W., HAYWARD, J. P., Zhang, X. 2013; 60 (4): 3127-3133
  • Characterizing the Timing Performance of a Fast 4H-SiC Detector With an Am-241 Source IEEE TRANSACTIONS ON NUCLEAR SCIENCE Zhang, X., Cates, J. W., Hayward, J. P., Bertuccio, G., Puglisi, D., Hausladen, P. A. 2013; 60 (3): 2352-2356
  • Increased Light Extraction From Inorganic Scintillators With Laser-Etched Microstructures IEEE TRANSACTIONS ON NUCLEAR SCIENCE Cates, J. W., HAYWARD, J. P., Zhang, X. 2013; 60 (2): 1027-1032
  • Achievable Position Resolution of an Alpha Detector with Continuous Spatial Response for Use in Associated Particle Imaging 60th IEEE Nuclear Science Symposium (NSS) / Medical Imaging Conference (MIC) / 20th International Workshop on Room-Temperature Semiconductor X-ray and Gamma-ray Detectors Cates, J. W., HAYWARD, J. P., Zhang, X. IEEE. 2013
  • Timing Resolution Study of an Associated Particle Detector for Fast Neutron Imaging IEEE TRANSACTIONS ON NUCLEAR SCIENCE Cates, J. W., HAYWARD, J. P., Zhang, X., Hausladen, P. A., Dabbs, B. 2012; 59 (4): 1750-1756
  • Benchmarking the GEANT4 full system simulation of an associated alpha-particle detector for use in a D-T neutron generator APPLIED RADIATION AND ISOTOPES Zhang, X., Hayward, J. P., Cates, J. W., Hausladen, P. A., Laubach, M. A., Sparger, J. E., Donnald, S. B. 2012; 70 (8): 1485-1493

    Abstract

    The position-sensitive alpha-particle detector used to provide the starting time and initial direction of D-T neutrons in a fast-neutron imaging system was simulated with a Geant4-based Monte Carlo program. The whole detector system, which consists of a YAP:Ce scintillator, a fiber-optic faceplate, a light guide, and a position-sensitive photo-multiplier tube (PSPMT), was modeled, starting with incident D-T alphas. The scintillation photons, whose starting time follows the distribution of a scintillation decay curve, were produced and emitted uniformly into a solid angle of 4? along the track segments of the alpha and its secondaries. Through tracking all photons and taking into account the quantum efficiency of the photocathode, the number of photoelectrons and their time and position distributions were obtained. Using a four-corner data reconstruction formula, the flood images of the alpha detector with and without optical grease between the YAP scintillator and the fiber-optic faceplate were obtained, which show agreement with the experimental results. The reconstructed position uncertainties of incident alpha particles for both cases are 1.198 mm and 0.998 mm respectively across the sensitive area of the detector. Simulation results also show that comparing with other faceplates composed of 500 ?m, 300 ?m, and 100 ?m fibers, the 10-?m-fiber faceplate is the best choice to build the detector for better position performance. In addition, the study of the background originating inside the D-T generator suggests that for 500-?m-thick YAP:Ce coated with 1-?m-thick aluminum, and very good signal-to-noise ratio can be expected through application of a simple threshold.

    View details for DOI 10.1016/j.apradiso.2012.04.026

    View details for Web of Science ID 000307415800006

    View details for PubMedID 22728838

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