Our goal is development of an innovative brain-PET with effective sensitivity (8X) and higher spatial resolution vs. current advanced brain-PET systems by implementation of advanced ultrafast SiPM/readout systems coupled to thin monolithic crystals arranged in “onion ring” geometry with small air-gaps between the rings enabling accurate tracking of Compton Scatter (CS) events followed by photoelectric absorption (PE) events, forming “triplets” (PE =CS-PE). We performed Monte Carlo simulations of four concentric rings with diameters 250, 270, 290, 310 mm, and 508 mm axial length with monolithic 3-mm-thick LYSO thin-slab detector modules. The brain was simulated by a water sphere containing F-18. We considered only true-coincidence (PE=PE) and triplet (PE =CS-PE) events. For triplets, back-to-forward scatter ratio is 0.26. The triplet-to-true-coincidence events ratio is 0.30. Inclusion of triplets in addition to true-coincidence events allows sensitivity increase by ~30%. Because the point-of-first interaction is well defined, the improved spatial resolution is anticipated.
In this study, we re-evaluated the attainable coincidence time resolution (CTR) performance for 3×3×3 mm3 LYSO crystals coupled to matched 3×3 mm2 SiPMs. This work was motivated by potential increased sensitivity in brain positron emission tomography (PET) detector blocks that would be enabled with ultrashort CTR (<100 ps). The recent progress in silicon photomultiplier (SiPM) technology, high-frequency read-out circuits, and optimized data processing is expected to lead directly to improved performance. The 3×3×3 mm3 LYSO crystals, with all sides polished to optical quality, were optically coupled to SiPMs designed and fabricated by Fondazione Bruno Kessler (FBK). An improved high frequency read-out circuit was designed and fabricated. CTR was measured using a 22Na positron source (<10 μCi) sandwiched between two identical LYSO/SiPM/read-out circuit stacks. Our studies show that a CTR of less than 80 ps, which, to the best of our knowledge, is the shortest reported CTR for 3×3×3 mm3 LYSO crystals. The results demonstrate, for the first time, that CTR performance in 3×3×3 mm3 LYSO crystals coupled to a 3mm×3mm2 SiPMs is comparable to CTRs achieved for ultra-small LYSO crystals (2×2×3 mm3) coupled to large 4 × 4 mm2 SiPMs. These results prove that an array of 3×3×3 mm3 LYSO/SiPM can be used to build a next generation high performance detector block with very high packing fraction, enabling ultimately very high gamma ray detection efficiency and very high system sensitivity.
There is a need to lower mass of scintillators in PET imagers. However, the tradeoff between scintillator thickness and axial field of view is not obvious, particularly if the total mass of the scintillator is a limitation. In this work, we developed fast analytical methods to assess performance of PET as a function of scintillator thickness. Calculation of the photopeak detection efficiency (PDE) is complicated by the fact that most incident 511 keV gamma rays first undergo Compton scattering in the scintillator resulting in a partial deposition of energy, as well as the production of a lower energy secondary gamma ray. The PDE is dependent on scintillator geometry and source position and must be recalculated when either changes. We compare Monte Carlo and our analytical lower and upper bound estimation of PDE for thin slab LSO scintillators as a function of its thickness assuming normal incidence. We show that that our analytical method achieves excellent agreement with the results obtained by time-consuming Monte Carlo approach. This is important because application of our fast method enables preliminary system optimization prior to time-consuming, complex Monte Carlo modeling.
Most positron emission tomography (PET) systems use an (almost) cylindrically symmetric detector geometry that acquires data in step-wise or continuous fashion. The National Electrical Manufacturers Association (NEMA) has developed performance standards (NEMA NU2-2018) to evaluate the performance of these systems. However, many Brain PET scanners no longer use a cylindrically symmetric detector arrangement; instead favoring unconventional, asymmetric spatial distributions of detectors to improve the geometric efficiency. The comparison of these systems with cylindrical devices is difficult because the NEMA standards may not be directly compatible with these non-cylindrical detector geometries. The incompatibility is due to both the source geometry and use of single-slice-rebinning (SSR). In this study, we extended the standard cylindrical polyethylene phantom used for the noise equivalent count rate (NECR) and scatter fraction (SF) measurements in NEMA NU2-2018 by adding a 20 cm diameter polyethylene sphere with a line-source channel. To avoid the use of SSR in NECR, SF and sensitivity tests, which can incorrectly assign slice locations in non-cylindrical tomographs, we instead propose a different method that uses the known positions of the line-source and the detection points of the line-of-response (LOR). Axial position can be determined from the minimum of the distance between the LOR and the line-source. These correctly binned counts were compared to cylindrical and spherical cap PET geometries using the well-validated GATE Monte Carlo code to estimate performance. The results show that our proposed modifications provide a means to estimate a non-cylindrical tomograph’s NECR, SF,and sensitivity that is consistent with the NEMA methodology.
A new challenge for time-of-flight (TOF) Positron Emission Tomography (PET) is achieving 10 ps Coincidence Timing Resolution (CTR). Such a short CTR would enable a 20-fold higher TOF-related effective sensitivity gain (TOF-gain) and direct reconstruction in PET imaging. Ultrashort CTR greatly benefits brain PET imaging because owing to the relatively small size of human head, TOF-gain only begins to be significant for CTR < 150 ps. The Brain PET (BET) consortium evaluates the potential for achieving 10 ps CTR using an updated Monte Carlo modeling program (MCPET3). This new version includes the ability to set a constant refractive index at each scintillator segment face to model the effects of optical index coupling glues. In addition, the new version provides a simple method for evaluating the effect of Cherenkov photons on the CTR. The latest modeling results are compared to recent world-record experimental CTR with good agreement and only a few adjustable parameters. The results indicate that 50 ps CTR is likely to be attained in the near future, but achieving 10 ps CTR will require a number of substantial improvements in PET detector blocks technology. Based on our simulations, we estimate that in order to achieve the 10 ps CTR a 20-fold increase in scintillator intensity (photons/ps) is required, along with additional improvements in single photon timing resolution.
Single photon avalanche diode arrays have achieved extraordinary performance and are beginning to replace vacuum tube photomultipliers in almost every application. While silicon based single photon avalanche diode arrays are a rapidly maturing technology, similar arrays in compound semiconductors have met with only limited success. This is partly due to the intrinsic high defect densities in compound semiconductors and partly due to the immaturity of the fabrication techniques available. Newly developed planar processing technologies hold the potential to substantially improve the performance of compound semiconductor SPAD arrays, including decreasing dark count rates, increasing single photon detection efficiencies, and increasing dynamic range. These new techniques have been applied to GaInP SPAD arrays, enabling the SPAD array pitch to be decreased to five microns and 40,000 SPADs/mm. The performance characteristics of these GaInP SPAD arrays will be described.
Purpose: Time-of-flight (TOF) been successfully implemented in whole body PET, significantly improving clinical performance. However, for dedicated brain PET systems, TOF has not been a priority due the relatively small size of the human head, where coincidence timing resolution (CTR) below 200 ps is necessary to arrive at substantial performance improvements. The Brain PET (BET) consortium is developing a dual-ended PET detector block concept with ultrafast CTR, high sensitivity and high spatial resolution (X, Y, depth-of-interaction, DOI) that provides a pathway to significantly improved brain PET. Methods: We have implemented analytical and Monte Carlo models of scintillation photons transport in scintillator segments with arbitrary trans-axial cross-section dimensions. Results: Timing performance is independent of trans-axial cross-section as long as there is a gap between the scintillator and reflector wrapping. Intimate contact between the wrapping with the scintillator decreases the percentage of total internally reflected photons, degrading CTR performance. Excellent CTR performance can be achieved using simple fixed voltage thresholding techniques to determine the arrival times at the top and bottom SiPM. The average of the top and bottom arrival time corresponds to the time of gamma ray absorption, while the difference in arrival time corresponds to DOI. A simple algorithm to use the difference in arrival time to compensate for gamma ray transit time and optical photon transit achieves performance within 20% of the Cramer-Rao lower bound. We established that the advanced silicon photomultiplier designs with high single photon detection efficiency (QE=80%) and high single photon timing resolution (SPTR) ~50 ps are critical for achieving ultrafast TOF-PET performance with CTR ~50 ps and ~4 mm DOI resolution.
Purpose: Time-of-flight (TOF) been successfully implemented in whole body PET, significantly improving clinical performance. However, TOF has not been a priority in development of dedicated brain PET systems due the relatively small size of the human head, where coincidence timing resolution (CTR) below 200 ps is necessary to arrive at substantial performance improvements. The Brain PET (BET) consortium is developing a PET detector block with ultrafast CTR, high sensitivity and high spatial resolution (X, Y, depth of interaction, DOI) that provides a pathway to significantly improved brain PET. Methods: We have implemented analytical and Monte Carlo models of scintillation photons transport in scintillator segments with the trans-axial cross-section equal or smaller than 3x3 mm2 . Results: The signal amplitude and timing of W mm x W mm x L mm scintillators (1 mm<W<3 mm, 5 mm <L< 30 mm) are strongly influenced by sidewall surface polish and external reflector. Highly polished surfaces provide nearly perfect total internal reflection (TIR), enabling the ultrafast timing performance to be relatively independent of scintillator crosssection. The signal amplitude in such a configuration does not depend on DOI. However, the differential signal from top and bottom SiPM in the dual-ended readout can be used to determine DOI. Using TIR alone, the average of the photon detection times at the top and bottom SiPMs provides a good estimation of the gamma ray absorption time. Averaging ~10 photons starting from 3rd photon produces the shortest CTR for SPTR=50 ps. Conclusions: We established that the advanced silicon photomultiplier designs with high single photon detection efficiency (QE=60%) and high single photon timing resolution (SPTR =50 ps) are critical for achieving ultrafast TOF-PET performance with CTR ~50 ps and ~4 mm DOI resolution.
Aggregated compound semiconductor single photon avalanche diode (SPAD) arrays are emerging as a viable alternative to the silicon photomultiplier (SiPM). Compound semiconductors have the potential to surpass SiPM performance, potentially achieving orders of magnitude lower dark count rates and improved radiation hardness. New planar processing techniques have been developed to enable compound semiconductor SPAD devices to be produced with pixel pitches of 11 – 25 microns, with thousands of SPADs per array.
Single photon avalanche diodes (SPADs) are revolutionizing ultra-sensitive photodetection applications, providing single photon sensitivity, high quantum efficiency and low dark noise at or near room temperature. When aggregated into arrays, these devices have demonstrated the ability to operate as photon number resolving detectors with wide dynamic range, or as single-photon imaging detectors. SPAD array performance has reached a point where replacing vacuum tube based MCP and PMT photodetectors for most applications is inevitable. Compound semiconductor SPAD arrays offer the unique proposition to tailor performance to match application specific wavelength, speed and radiation hardness requirements. We present a theoretical framework describing performance limits to compound semiconductor SPAD arrays and our latest experimental results detailing the performance of GaAs SPAD arrays. These devices achieve nanosecond rise and fall times, excellent photon number resolving capability, and low dark count rates. Single photon number resolving is demonstrated with 4% single photon detection efficiency at room temperature with dark count rates below 7 Mcps/mm2. Compound semiconductor SPAD arrays have the opportunity to provide orders of magnitude improvement in dark count rate and radiation hardness over silicon SPAD arrays, as well as the ability to detect wavelengths where silicon is blind.
High efficiency dynamic holography at 1.55 microns is achieved on a broad-area InP based multiple quantum well devices. The quantum well cavity is sandwiched between a DBR and amorphous mirror, and consists of a number of wells. High energy pulsed writing beams at 1.06 microns generate free carrier gratings which are probed by a 1.55 micron tunable laser in a four wave mixing configuration. Diffraction efficiency into a single order of 30% has been achieved by contribution of a phase grating, mode pulling and asymmetric Fabry-Perot reflection.
Arsenides, such as GaAs and AlGaAs, contain a dispersion of metallic As clusters in a high-quality single crystal semiconductor matrix. These composite materials exhibit interesting and useful properties, including a large electro- optic effect, and a combination of properties that make it useful as a high-speed photoconductor.
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