Study of Cerenkov Light Emission in the Semiconductors TlBr and TlCl for TOF-PET.

Thallium bromide (TlBr) and thallium chloride (TlCl) are semiconductor materials with high transparency to visible light, high index of refraction, and high detection efficiency for gamma rays and annihilation photons. This manuscript reports on measurements of the light intensity and timing response of Cerenkov light emitted in one 3 mm × 3 mm × 5 mm slab of each of these materials operated in coincidence with a lutetium fine silicate (LFS) crystal with dimensions of 3 mm × 3 mm × 20 mm. A 22Na radioactive source was used. The measured average number of detected photons per event was 1.5 photons for TlBr and 2.8 photons for TlCl when these materials were coupled to a silicon photomultiplier. Simulation predicts these results with an overestimation of 12%. The best coincidence time resolution (CTR) for events in TlBr and TlCl were 329 ± 9 ps and 316 ± 9 ps, respectively, when events with 4 photons and >7 photons were selected. Simulation showed the CTR degraded from 120 ps to 405 ps in TlCl, and from 160 ps to 700 ps in TlBr when the first or second Cerenkov photon were selected. Results of this work show TlCl has a stronger Cerenkov light emission compared to TlBr and a greater potential to obtain the best timing measurements. Results also stress the importance of improving detection efficiency and transport of light to capture the first Cerenkov photon in timing measurements.

Energy and electron drift time measurements in a pixel CCI TlBr detector with 1.3 MeV prompt-gammas.

Assessing the position of the Bragg peak (BP) in hadron radiotherapy utilizing prompt-gamma imaging (PGI) presents many challenges in terms of detector physics. Gamma detectors with the capability of extracting the best energy, timing, and spatial information from each gamma interaction, as well as with high detection efficiency and count rate performance, are needed for this application. In this work we present the characterization of a pixel Cerenkov charge induction (CCI) thallium bromide (TlBr) detector in terms of energy and and electron drift time for its potential use in PGI. The CCI TlBr detector had dimensions of 4 × 4 × 5 mm3 and one of its electrodes was segmented in pixels with 1.7 mm pitch. A silicon photomultiplier (SiPM) was optically coupled to one of the faces of the TlBr slab to read out the Cerenkov light promptly emitted after the interaction of a gamma ray. The detector was operated stand-alone and the 1.275 prompt gammas from a 22Na radioactive source were used for the study. The electron drift time was obtained by combining the Cerenkov and charge induction signals and then used as a measure of the depth of interaction. The electron mobility in TlBr was estimated as ∼27 cm2 V-1 s-1. Energy resolutions between 3.4% and 4.0% at 1.275 MeV were obtained after depth-correction. These values improved to 3.0%-3.3% when events with drift times of 3-6 µs were selected. These results show the potential of pixel CCI TlBr detectors to resolve gamma interactions in the detector with mm-like accuracy in 3D and with excellent energy resolution. Previous studies with CCI TlBr devices have shown a timing resolution of <400 ps full width at half maximum when detecting 511 keV gamma rays, therefore, the timing accuracy is expected to improve with the increased energy of the gamma rays in PGI. While other important detector characteristics such as count rate capability remain to be studied, results from this work combined with other preliminary data show pixel CCI detectors can simultaneously provide excellent energy, timing, and spatial resolution performance and are a very promising option for PGI in hadron therapy.

First Cerenkov charge-induction (CCI) TlBr detector for TOF-PET and proton range verification.

Thallium bromide (TlBr) is a semiconductor material and, simultaneously, a good Cerenkov radiator. The performance of a TlBr detector that integrates two different readouts, the charge induction readout and the detection of Cerenkov light, was evaluated. A TlBr detector with dimensions of 4 × 4 × 5 mm3, with a monolithic cathode and an anode segmented into strips, was manufactured. One of the bare and polished 4 × 4 mm2 faces of the detector was coupled to a silicon photomultiplier (SiPM) to read out the Cerenkov light. Simultaneous timing and energy resolutions of <400 ps full width at half maximum (FWHM) and ~8.5% at 511 keV were measured using the Cerenkov detection and charge induction readouts, respectively. A coincidence time resolution of 330 ps was obtained when selecting Cerenkov events with amplitudes above 70 mV. The combination of both readouts showed the potential to resolve the depth-of-interaction (DOI) positioning, based on the improvement of energy resolution when selecting events with similar electron drift times. This manuscript sets the stage for a new family of semiconductor detectors that combine charge induction readout with the Cerenkov light detection. Such detectors can provide, simultaneously, outstanding timing, energy, and spatial resolution, and will be an excellent fit for applications that require the detection of high-energy gamma photons with high timing accuracy, such as time-of-flight positron emission tomography (TOF-PET) and prompt gamma imaging (PGI) to assess the particle range in hadron therapy.

Development of TlBr detectors for PET imaging.

Thallium bromide (TlBr) is a promising semiconductor detector material for positron emission tomography (PET) because it can offer very good energy resolution and 3D segmentation capabilities, and it also provides detection efficiency surpassing that of commonly used scintillators. Energy, timing, and spatial resolution were measured for thin (<1 mm) TlBr detectors. The energy and timing resolution were measured simultaneously for the same planar 0.87 mm-thick TlBr device. An energy resolution of (6.4 ± 1.3)% at 511 keV was achieved at -400 V bias voltage and at room temperature. A timing resolution of (27.8 ± 4.1) ns FWHM was achieved for the same operating conditions when appropriate energy gating was applied. The intrinsic spatial resolution was measured to be 0.9 mm FWHM for a TlBr detector with metallic strip contacts of 0.5 mm pitch. As material properties improve, higher bias voltage should improve timing performance. A stack of thin detectors with finely segmented readout can create a modular detector with excellent energy and spatial resolution for PET applications.

Towards time-of-flight PET with a semiconductor detector.

The feasibility of using Cerenkov light, generated by energetic electrons following 511 keV photon interactions in the semiconductor TlBr, to obtain fast timing information for positron emission tomography (PET) was evaluated. Due to its high refractive index, TlBr is a relatively good Cerenkov radiator and with its wide bandgap, has good optical transparency across most of the visible spectrum. Coupling an SiPM photodetector to a slab of TlBr (TlBr-SiPM) yielded a coincidence timing resolution of 620 ps FWHM between the TlBr-SiPM detector and a LFS reference detector. This value improved to 430 ps FWHM by applying a high pulse amplitude cut based on the TlBr-SiPM and reference detector signal amplitudes. These results are the best ever achieved with a semiconductor PET detector and already approach the performance required for time-of-flight. As TlBr has higher stopping power and better energy resolution than the conventional scintillation detectors currently used in PET scanners, a hybrid TlBr-SiPM detector with fast timing capability becomes an interesting option for further development.

Performance Comparison of Different Readouts for Position-Sensitive Solid-State Photomultiplier Arrays.

A thorough comparison of five different readouts for reading out a 2 × 2 array of 5 mm × 5 mm position-sensitive solid-state photomultipliers (PS-SSPM) was undertaken. The five readouts include reading out the 20 signals (16 position and 4 timing) individually, two signal multiplexing readouts, and two position decoding readouts. Flood histogram quality, signal-to-noise ratio (SNR) and energy resolution were compared at different bias voltage (27.0 V to 32.0 V, at 0.5 V intervals) and at a fixed temperature of 0 °C by coupling a 6 × 6 array of 1.3 mm × 1.3 mm × 20 mm polished LSO crystals to the center of the PS-SSPM array. The timing resolution was measured at a bias voltage of 31.0 V (optimal bias voltage in terms of flood histogram quality). The best flood histogram quality value and signal-to-noise were 7.3 ± 1.6 and 33.5 ± 3.1, respectively, and were obtained by shaping and digitizing the 16 position signals individually. The capacitive charge-division readout is the simplest readout among the five evaluated but still resulted in good performance with a flood histogram quality value of 3.3 ± 0.4 and a SNR of 18.3 ± 1.3. The average energy resolution and the average timing resolution were 15.2 ± 1.2 % and 8.4 ± 1.6 ns for individual signal readout and 15.9 ± 1.2 % and 8.8 ± 1.3 ns by using the capacitive charge-division readout method. These studies show that for an ultra-high spatial resolution applications using the 2 × 2 PS-SSPM array, reading out the 20 signals individually is necessary; whilst the capacitive charge-division readout is a cost-effective readout for less demanding applications.

A Prototype High-Resolution Small-Animal PET Scanner Dedicated to Mouse Brain Imaging.

UNLABELLED: We developed a prototype small-animal PET scanner based on depth-encoding detectors using dual-ended readout of small scintillator elements to produce high and uniform spatial resolution suitable for imaging the mouse brain. METHODS: The scanner consists of 16 tapered dual-ended-readout detectors arranged in a 61-mm-diameter ring. The axial field of view (FOV) is 7 mm, and the transaxial FOV is 30 mm. The scintillator arrays consist of 14 × 14 lutetium oxyorthosilicate elements, with a crystal size of 0.43 × 0.43 mm at the front end and 0.80 × 0.43 mm at the back end, and the crystal elements are 13 mm long. The arrays are read out by 8 × 8 mm and 13 × 8 mm position-sensitive avalanche photodiodes (PSAPDs) placed at opposite ends of the array. Standard nuclear-instrumentation-module electronics and a custom-designed multiplexer are used for signal processing. RESULTS: The detector performance was measured, and all but the crystals at the very edge could be clearly resolved. The average intrinsic spatial resolution in the axial direction was 0.61 mm. A depth-of-interaction resolution of 1.7 mm was achieved. The sensitivity of the scanner at the center of the FOV was 1.02% for a lower energy threshold of 150 keV and 0.68% for a lower energy threshold of 250 keV. The spatial resolution within a FOV that can accommodate the entire mouse brain was approximately 0.6 mm using a 3-dimensional maximum-likelihood expectation maximization reconstruction. Images of a hot-rod microphantom showed that rods with a diameter of as low as 0.5 mm could be resolved. The first in vivo studies were performed using (18)F-fluoride and confirmed that a 0.6-mm resolution can be achieved in the mouse head in vivo. Brain imaging studies with (18)F-FDG were also performed. CONCLUSION: We developed a prototype PET scanner that can achieve a spatial resolution approaching the physical limits of a small-bore PET scanner set by positron range and detector interaction. We plan to add more detector rings to extend the axial FOV of the scanner and increase sensitivity.

Estimation of Fano factor in inorganic scintillators.

The Fano factor of an integer-valued random variable is defined as the ratio of its variance to its mean. Correlation between the outputs of two photomultiplier tubes on opposite faces of a scintillation crystal was used to estimate the Fano factor of photoelectrons and scintillation photons. Correlations between the integrals of the detector outputs were used to estimate the photoelectron and photon Fano factor for YAP:Ce, SrI2:Eu and CsI:Na scintillator crystals. At 662 keV, SrI2:Eu was found to be sub-Poisson, while CsI:Na and YAP:Ce were found to be super-Poisson. An experiment setup inspired from the Hanbury Brown and Twiss experiment was used to measure the correlations as a function of time between the outputs of two photomultiplier tubes looking at the same scintillation event. A model of the scintillation and the detection processes was used to generate simulated detector outputs as a function of time for different values of Fano factor. The simulated outputs from the model for different Fano factors was compared to the experimentally measured detector outputs to estimate the Fano factor of the scintillation photons for YAP:Ce, LaBr3:Ce scintillator crystals. At 662 keV, LaBr3:Ce was found to be sub-Poisson, while YAP:Ce was found to be close to Poisson.

A Study of Position-Sensitive Solid-State Photomultiplier Signal Properties.

We present an analysis of the signal properties of a position-sensitive solid-state photomultiplier (PS-SSPM) that has an integrated resistive network for position sensing. Attractive features of PS-SSPMs are their large area and ability to resolve small scintillator crystals. However, the large area leads to a high detector capacitance, and in order to achieve high spatial resolution a large network resistor value is required. These inevitably create a low-pass filter that drastically slows what would be a fast micro-cell discharge pulse. Significant changes in the signal shape of the PS-SSPM cathode output as a function of position are observed, which result in a position-dependent time delay when using traditional time pick-off methods such as leading edge discrimination and constant fraction discrimination. The timing resolution and time delay, as a function of position, were characterized for two different PS-SSPM designs, a continuous 10 mm × 10 mm PS-SSPM and a tiled 2 × 2 array of 5 mm × 5 mm PS-SSPMs. After time delay correction, the block timing resolution, measured with a 6 × 6 array of 1.3 × 1.3 × 20 mm3 LSO crystals, was 8.6 ns and 8.5 ns, with the 10 mm PS-SSPM and 5 mm PS-SSPM respectively. The effect of crystal size on timing resolution was also studied, and contrary to expectation, a small improvement was measured when reducing the crystal size from 1.3 mm to 0.5 mm. Digital timing methods were studied and showed great promise for allowing accurate timing by implementation of a leading edge time pick-off. Position-dependent changes in signal shape on the anode side also are present, which complicates peak height data acquisition methods used for positioning. We studied the effect of trigger position on signal amplitude, flood histogram quality, and depth-of-interaction resolution in a dual-ended readout detector configuration. We conclude that detector timing and positioning can be significantly improved by implementation of digital timing methods and by accounting for changes in the shape of the signals from PS-SSPMs.

Kinetics of Schottky defect formation and annihilation in single crystal TlBr.

The kinetics for Schottky defect (Tl and Br vacancy pair) formation and annihilation in ionically conducting TlBr are characterized through a temperature induced conductivity relaxation technique. Near room temperature, defect generation-annihilation was found to take on the order of hours before equilibrium was reached after a step change in temperature, and that mechanical damage imparted on the sample rapidly increases this rate. The rate limiting step to Schottky defect formation-annihilation is identified as being the migration of lower mobility Tl (versus Br), with an estimate for source-sink density derived from calculated diffusion lengths. This study represents one of the first investigations of Schottky defect generation-annihilation kinetics and demonstrates its utility in quantifying detrimental mechanical damage in radiation detector materials.

A Simple Capacitive Charge-Division Readout for Position-Sensitive Solid-State Photomultiplier Arrays.

A capacitive charge-division readout method for reading out a 2 × 2 array of 5 mm × 5 mm position-sensitive solid-state photomultipliers (PS-SSPM) was designed and evaluated. Using this analog multiplexing method, the 20 signals (16 position, 4 timing) from the PS-SSPM array are reduced to 5 signals (4 position, 1 timing), allowing the PS-SSPM array to be treated as an individual large-area PS-SSPM module. A global positioning approach can now be used, instead of individual positioning for each PS-SSPM in the array, ensuring that the entire light signal is utilized. The signal-to-noise ratio (SNR) and flood histogram quality at different bias voltages (27.5 V to 32.0 V at 0.5 V intervals) and a fixed temperature of 0 °C were evaluated by coupling a 6 × 6 array of 1.3 mm × 1.3 mm × 20 mm polished LSO crystals to the center of the PS-SSPM array. The timing resolution was measured at a fixed bias voltage of 31.0 V and a fixed temperature of 0 °C. All the measurements were evaluated and compared using capacitors with different values and tolerances. Capacitor values ranged from 0.051 nf to 10 nf, and the capacitance tolerance ranged from 1% to 20%. The results show that better performance was achieved using capacitors with smaller values and better capacitance tolerance. Using 0.2 nf capacitors, the SNR, energy resolution and timing resolution were 24.3, 18.2% and 8.8 ns at a bias voltage 31.0 V, respectively. The flood histogram quality was also evaluated by using a 10 × 10 array of 1 mm × 1 mm × 10 mm polished LSO crystals and a 10 × 10 array of 0.7 mm × 0.7 mm × 20 mm unpolished LSO crystals to determine the smallest crystal size resolvable. These studies showed that the high spatial resolution of the PS-SSPM was preserved allowing for 0.7 mm crystals to be identified. These results show that the capacitive charge-division analog signal processing method can significantly reduce the number of electronic channels, from 20 to 5, while retaining the excellent performance of the detector.

Comparison of large-area position-sensitive solid-state photomultipliers for small animal PET.

This paper evaluates the performance of two large-area position-sensitive solid-state photomultipliers (PS-SSPM) for use in small animal PET detector designs. Both PS-SSPM device designs are 1 cm² in area, the first being a 2 × 2 tiled array of 5 mm × 5 mm PS-SSPMs and the second being a 10 mm × 10 mm continuous PS-SSPM. Signal-to-noise measurements were performed to investigate the optimal operating parameters for each device and to compare the performance of the two PS-SSPM designs. A maximum signal-to-noise ratio of 29.3 was measured for the 5 mm PS-SSPM array and 15.1 for the 10 mm PS-SSPM, both measurements were made at 0 °C and at the optimal bias voltage. The best energy resolution measured with an array of 1.3 mm polished LSO crystals was 16% for the 5 mm PS-SSPM array and 18% for the 10 mm PS-SSPM. The timing properties of both devices were similar, with a best timing resolution (in coincidence with an LSO/PMT detector) of 6.8 ns (range 6.8-8.9 ns) and 7.1 ns (range 7.1-9.6 ns) for the 5 mm PS-SSPM and 10 mm PS-SSPM respectively. The 2 × 2 array of 5 mm PS-SSPMs was able to visually resolve the elements in an 0.5 × 0.5 × 20 mm LYSO scintillator array (unpolished, diffuse reflector) with an average peak-to-valley ratio in the flood histograms of ∼11 indicating clear separation of the crystals. Advantages and drawbacks of PET detector designs using PS-SSPM photodetectors are addressed and comparisons to other small-animal PET detector designs using position-sensitive avalanche photodiodes are made.

The defect and transport properties of acceptor doped TlBr: role of dopant exsolution and association.

The role of acceptor dopants (S and Se) in controlling the ionic conductivity of single crystal TlBr, grown by the vertical Bridgman method, was examined as a function of temperature with the aid of impedance spectroscopy. Several features in the conductivity were identified and related to acceptor dopant-Br vacancy association, acceptor dopant exsolution, and Br vacancy mobility. The corresponding enthalpies for these processes were extracted from the data and were found to be equal to H(a) = 0.42 ± 0.07 eV, H(sol) = 1.55 ± 0.18 eV and H(m,Br) = 0.31 ± 0.02 eV respectively, the latter consistent with earlier studies on donor doped and undoped TlBr. A long term conductivity decay in the extrinsic region, attributed to S or Se exsolution, was observed. The time constant associated with exsolution was found to be thermally activated with an activation energy of 0.47 ± 0.1 eV. Estimates for Se solubility at different temperatures are provided.

Signal and noise properties of position-sensitive avalanche photodiodes.

After many years of development, position-sensitive avalanche photodiodes (PSAPDs) are now being incorporated into a range of scintillation detector systems, including those used in high-resolution small-animal PET and PET/MR scanners. In this work, the signal, noise, signal-to-noise ratio (SNR), flood histogram and timing resolution were measured for lutetium oxyorthosilicate (LSO) scintillator arrays coupled to PSAPDs ranging in size from 10 to 20 mm, and the optimum bias voltage and working temperature were determined. Variations in the SNR performance of PSAPDs with the same dimensions were small, but the SNR decreased significantly with increasing PSAPD size and increasing temperature. Smaller PSAPDs (10 mm and 15 mm in width) produced acceptable flood histograms at 24 °C, and cooling lower than 16 °C produced little improvement. The optimum bias voltage was about 25 V below the break down voltage. The larger 20 mm PSAPDs have lower SNR and require cooling to 0-7 °C for acceptable performance. The optimum bias voltage is also lower (35 V or more below the break down voltage depending on the temperature). Significant changes in the timing resolution were observed as the bias voltage and temperature varied. Higher bias voltages provided better timing resolution. The best timing resolution obtained for individual crystals was 2.8 ns and 3.3 ns for the 10 mm and 15 mm PSAPDs, respectively. The results of this work provide useful guidance for selecting the bias voltage and working temperature for scintillation detectors that incorporate PSAPDs as the photodetector.

Tapered LSO arrays for small animal PET.

By using detectors with good depth encoding accuracy (∼2 mm), an animal PET scanner can be built with a small ring diameter and thick crystals to simultaneously obtain high spatial resolution and high sensitivity. However, there will be large wedge-shaped gaps between detector modules in such a scanner if traditional cuboid crystal arrays are used in a polygonal arrangement. The gaps can be minimized by using tapered scintillator arrays enabling the sensitivity of the scanner to be further improved. In this work, tapered lutetium oxyorthosilicate (LSO) arrays with different crystal dimensions and different combinations of inter-crystal reflector and crystal surface treatments were manufactured and their performance was evaluated. Arrays were read out from both ends by position-sensitive avalanche photodiodes (PSAPDs). In the optimal configuration, arrays consisting of 0.5 mm LSO elements could be clearly resolved and a depth of interaction resolution of 2.6 mm was obtained for a 20 mm thick array. For this tapered array, the intrinsic spatial is degraded from 0.67 to 0.75 mm compared to a standard cuboidal array with similar dimensions, while the increase in efficiency is 41%. Tapered scintillator arrays offer the prospect of improvements in sensitivity and sampling for small-bore scanners, without large increases in manufacturing complexity.

Temperature dependent operation of PSAPD-based compact gamma camera for SPECT imaging.

We investigated the dependence of image quality on the temperature of a position sensitive avalanche photodiode (PSAPD)-based small animal single photon emission computed tomography (SPECT) gamma camera with a CsI:Tl scintillator. Currently, nitrogen gas cooling is preferred to operate PSAPDs in order to minimize the dark current shot noise. Being able to operate a PSAPD at a relatively high temperature (e.g., 5 °C) would allow a more compact and simple cooling system for the PSAPD. In our investigation, the temperature of the PSAPD was controlled by varying the flow of cold nitrogen gas through the PSAPD module and varied from -40 °C to 20 °C. Three experiments were performed to demonstrate the performance variation over this temperature range. The point spread function (PSF) of the gamma camera was measured at various temperatures, showing variation of full-width-half-maximum (FWHM) of the PSF. In addition, a 99mTc-pertechnetate (140 keV) flood source was imaged and the visibility of the scintillator segmentation (16×16 array, 8 mm × 8 mm area, 400 µm pixel size) at different temperatures was evaluated. Comparison of image quality was made at -25 °C and 5 °C using a mouse heart phantom filled with an aqueous solution of 99mTc-pertechnetate and imaged using a 0.5 mm pinhole collimator made of tungsten. The reconstructed image quality of the mouse heart phantom at 5 °C degraded in comparision to the reconstructed image quality at -25 °C. However, the defect and structure of the mouse heart phantom were clearly observed, showing the feasibility of operating PSAPDs for SPECT imaging at 5 °C, a temperature that would not need the nitrogen cooling. All PSAPD evaluations were conducted with an applied bias voltage that allowed the highest gain at a given temperature.

Effect of Microstructure on the Radioluminescence and Transparency of Ce-Doped Strontium Hafnate Ceramics.

In this paper we report on the fabrication and characterization of SrHfO(3):Ce ceramics. Powders were prepared by solid-state synthesis using metal oxides and carbonates. X-ray diffraction measurements showed that phase-pure SrHfO(3) is formed at 1200°C. Inductively coupled plasma spectroscopy confirmed the purity and composition of each batch. SrHfO(3) exhibits several phase changes in the solid, but this does not appear to be detrimental to the ceramics. Microprobe experiments showed uniform elemental grain composition, whereas aluminum added as charge compensation for trivalent cerium congregated at grain boundaries and triple points. Radioluminescence spectra revealed that the light yield decreases when the concentration of excess Sr increases. The decrease in the light yield may be related to the change of Ce(3+) into Ce(4+) ions. For stoichiometric SrHfO(3):Ce, the light yield is about four times that of bismuth germanate (BGO), the conventional benchmark, indicating great potential for many scintillator applications.

Phantom experiments on a PSAPD-based compact gamma camera with submillimeter spatial resolution for small animal SPECT.

We demonstrate a position sensitive avalanche photodiode (PSAPD) based compact gamma camera for the application of small animal single photon emission computed tomography (SPECT). The silicon PSAPD with a two-dimensional resistive layer and four readout channels is implemented as a gamma ray detector to record the energy and position of radiation events from a radionuclide source. A 2 mm thick monolithic CsI:Tl scintillator is optically coupled to a PSAPD with a 8mm×8mm active area, providing submillimeter intrinsic spatial resolution, high energy resolution (16% full-width half maximum at 140 keV) and high gain. A mouse heart phantom filled with an aqueous solution of 370 MBq (99m)Tc-pertechnetate (140 keV) was imaged using the PSAPD detector module and a tungsten knife-edge pinhole collimator with a 0.5 mm diameter aperture. The PSAPD detector module was cooled with cold nitrogen gas to suppress dark current shot noise. For each projection image of the mouse heart phantom, a rotated diagonal readout algorithm was used to calculate the position of radiation events and correct for pincushion distortion. The reconstructed image of the mouse heart phantom demonstrated reproducible image quality with submillimeter spatial resolution (0.7 mm), showing the feasibility of using the compact PSAPD-based gamma camera for a small animal SPECT system.

Advances in CMOS Solid-state Photomultipliers for Scintillation Detector Applications.

Solid-state photomultipliers (SSPMs) are a compact, lightweight, potentially low-cost alternative to a photomultiplier tube for a variety of scintillation detector applications, including digital-dosimeter and medical-imaging applications. Manufacturing SSPMs with a commercial CMOS process provides the ability for rapid prototyping, and facilitates production to reduce the cost. RMD designs CMOS SSPM devices that are fabricated by commercial foundries. This work describes the characterization and performance of these devices for scintillation detector applications. This work also describes the terms contributing to device noise in terms of the excess noise of the SSPM, the binomial statistics governing the number of pixels triggered by a scintillation event, and the background, or thermal, count rate. The fluctuations associated with these terms limit the resolution of the signal pulse amplitude. We explore the use of pixel-level signal conditioning, and characterize the performance of a prototype SSPM device that preserves the digital nature of the signal. In addition, we explore designs of position-sensitive SSPM detectors for medical imaging applications, and characterize their performance.

Experimental characterization and system simulations of depth of interaction PET detectors using 0.5 mm and 0.7 mm LSO arrays.

Small animal PET scanners may be improved by increasing the sensitivity, improving the spatial resolution and improving the uniformity of the spatial resolution across the field of view. This may be achieved by using PET detectors based on crystal elements that are thin in the axial and transaxial directions and long in the radial direction, and by employing depth of interaction (DOI) encoding to minimize the parallax error. With DOI detectors, the diameter of the ring of the PET scanner may also be decreased. This minimizes the number of detectors required to achieve the same solid angle coverage as a scanner with a larger ring diameter and minimizes errors due to non-collinearity of the annihilation photons. In this study, we characterize prototype PET detectors that are finely pixelated with individual LSO crystal element sizes of 0.5 mm x 0.5 mm x 20 mm and 0.7 mm x 0.7 mm x 20 mm, read out at both ends by position sensitive avalanche photodiodes (PSAPDs). Both a specular reflector and a diffuse reflector were evaluated. The detectors were characterized based on the ability to clearly resolve the individual crystal elements, the DOI resolution and the energy resolution. Our results indicate that a scanner based on any of the four detector designs would offer improved spatial resolution and more uniform spatial resolution compared to present day small animal PET scanners. The greatest improvements to spatial resolution will be achieved when the detectors employing the 0.5 mm x 0.5 mm x 20 mm crystals are used. Monte Carlo simulations were performed to demonstrate that 2 mm DOI resolution is adequate to ensure uniform spatial resolution for a small animal PET scanner geometry using these detectors. The sensitivity of such a scanner was also simulated using Monte Carlo simulations and was shown to be greater than 10% for a four ring scanner with an inner diameter of 6 cm, employing 20 detectors per scanner ring.