[Show abstract][Hide abstract] ABSTRACT: The integration of magnetic resonance imaging (MRI) and positron emission tomography (PET) is an upcoming hybrid imaging technique. Prototype scanners for pre-clinical and clinical research have been built and tested. However, the potential of the PET part can be better exploited if the arterial input function (AIF) of the administered tracer is known. This work presents a dedicated MR-compatible blood sampling system for precise measurement of the AIF in an MR-PET study. The device basically consists of an LSO/APD-detector assembly which performs a coincidence measurement of the annihilation photons resulting from positron decays. During the measurement, arterial blood is drawn continuously from an artery and lead through the detector unit. Besides successful tests of the MR compatibility and the detector performance, measurements of the AIF of rats have been carried out. The results show that the developed blood sampling system is a practical and reliable tool for measuring the AIF in MR-PET studies.
Physics in Medicine and Biology 10/2010; 55(19):5883-93. · 2.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Silicon Photomultipliers (SiPMs) are increasingly being studied for their use in clinical and pre-clinical PET applications. Many groups have evaluated the performance of Multi-Pixel Photon Counters (MPPCs) from Hamamatsu Photonics. When coupled to PET scintillator crystals, these devices have shown promising results in terms of energy and timing resolution. The purpose of this paper is to analyze the main factors that determine the performance of SiPM based PET detectors and to provide guidelines for further optimization towards the performance levels of state-of-the-art PMT detectors. We present a statistical signal analysis that links the energy and time resolution to fundamental device characteristics, such as photon detection efficiency, cell density, secondary avalanche probability and dark rate. The trade-offs and impact of these device parameters on the overall detector performance is analyzed and discussed.
Nuclear Science Symposium Conference Record (NSS/MIC), 2009 IEEE; 12/2009
[Show abstract][Hide abstract] ABSTRACT: A way to improve the spatial resolution in positron emission tomography (PET) is to determine the depth-of-interaction (DOI) in the detector. A way to achieve this is to use the phoswich approach, a detector with two or more layers of different scintillators. The layer identification is done by using differences in scintillation decay time and pulse shape discrimination techniques. The advantages of the concept have been demonstrated in the HRRT high resolution PET system using a LSO/LYSO combination giving a high spatial resolution uniformity of around 2.5 mm within a larger part of the imaged volume. A phoswich combination that lately has received attention is LuAP/LSO or LuYAP/LSO. The suggestions come from the crystal clear collaboration and there is a patent application for its use in PET. This particular combination of phoswich may, however, have a complication since both LuAP and LuYAP emit in the excitation band of LSO, thus making the functionality more complex. In the present paper we have looked into this and suggested different ways to overcome potential drawbacks.
IEEE Transactions on Nuclear Science 03/2009; · 1.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Compared to the fast rise and exponential decay of Lu<sub>2</sub>SiO<sub>5</sub>:Ce, Y<sub>2</sub>SiO<sub>5</sub>:Ce has a slower rise time and a non-exponential decay. In an effort to understand this difference, the scintillation kinetics of YSO:Ce were investigated as a function of X-ray and gamma-ray energy as well as under alpha particle excitation. Although some influence of excitation energy and energy density on the kinetics was observed, in no case did the behavior match LSO:Ce. Therefore, a further investigation using thermoluminescence techniques probed the effect of electron traps on the rise and decay times. TL glow curves revealed several large trap populations, particularly near 100 K. The participation of the traps in the scintillation process was eliminated by making scintillation decay time measurements at 40 K, and a time profile similar to LSO:Ce was observed, possibly because the traps do not release electrons at this low temperature and only direct energy transfer to Ce luminescence centers contributes to the observed scintillation time profile.
[Show abstract][Hide abstract] ABSTRACT: Two, APD-based, PET modules have been evaluated for use in combined PET/MR imaging. Each module consists of 4 independent, optically isolated detectors. Each detector consists of an 8×8 array of 2×2×20 mm LSO crystals read out by a 2×2 array of 5×5 mm Hamamatsu S8664-55 APDs. The average crystal energy resolution and time resolution (against a plastic scintillator on a PMT) of the detectors was 17% and 1.8 ns, respectively. The modules were positioned in the tunnel of a 1.5 T Siemens Symphony MR scanner. The presence of the PET modules decreased the MR signal-to-noise ratio by about 15% but no image interference was observed. The gradient and RF pulse sequences of the MR produced adverse effects on the PET event signals. These high-frequency pulses did not affect the true PET events but did increase the dead time of the PET system. Simultaneous, artifact-free, images were acquired with the PET and MR system using a small Derenzo phantom. These results show that APD-based PET detectors can be used for a high-resolution and cost-effective integrated PET/MR system.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 12/2006; · 1.32 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: High spatial resolution is essential to image small lesions in positron emission tomography. Traditional methods suggest using small crystals to obtain high resolution. In this study, a new method is introduced to achieve high spatial resolution by using scintillators with different decay times arranged in a checkerboard pattern. Pulse shape discrimination was implemented to determine which crystal the gamma energy was deposited. In this work, a 13×13 LSO crystal array and a 13×13 LSO-GSO checkerboard crystal array using 4×4×20 mm<sup>3</sup> crystals were built and optimized using light sharing techniques. Similarly, using 2.5×2.5×19 mm<sup>3</sup> LSO and LYSO crystals, a 17×17 LSO-LYSO checkerboard crystal array was built as a high-resolution application. The average pixel energy resolution for the 13×13 LSO was measured to be 14.5%. Similarly, the average pixel resolutions for the GSO part of the 13×13 LSO-GSO crystal array and the LSO part of the same array were measured to be 13.9% and 18.5%, respectively. The average crystal energy resolutions for the LSO and LYSO parts of the 17×17 LSO-LYSO crystal array were measured to be 23.5% and 24.2%, respectively. The average peak-to-valley ratios in the position profiles were measured to be 2.1 for the 13×13 LSO crystal array, 7.2 for the LSO part of the 13×13 LSO-GSO crystal array and 4.3 for the GSO part of the 13×13 LSO-GSO crystal array. Similarly, the average peak-to-valley ratios in the position profiles were measured to be 2.5 for the LSO part of the 17×17 LSO-LYSO crystal array and 2.7 for the LYSO part of the 17×17 LSO-LYSO crystal array.
IEEE Transactions on Nuclear Science 03/2006; · 1.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Avalanche-photodiodes (APDs) as photosensors in positron emission tomography (PET) detectors have been extensively investigated in this field. Compared with conventional photosensors such as the photomultiplier tubes (PMTs), most APDs have advantages of higher quantum efficiency (~70% for APD vs. ~20% for PMT), robust packaging and very low magnetic susceptibility. However, it usually has very low gain (~200 for APD vs. ~10<sup>6</sup> for PMT), and a smaller photoactive area (~5 mm times 5 mm for APD vs. 10-52 mm diameter for PMT). The proposal described in this paper was based on a previous APD block detector design, in which each block consists of a 2 times 2 APD array reading out an 8 times 8 array of lutetium oxyorthosilicate (LSO) crystals. Each crystal is 2 mm times 2 mm times 20 mm. Due to the small block size, in order to build an APD PET system with similar axial field-of-view of a conventional PET scanner, substantially more APD detectors would be needed. Consequently, more electronics processing channels would be required. To simplify the detector electronics, we initiate a multiplexing concept based on RF transformers. This approach may reduce the signal-processing channels by a factor of 16 (from 64 channels to four). The circuits would work from both current and voltage sources, as opposed to resistor networks which map signals only from current sources. We built prototype printed-circuit-boards (PCBs) to evaluate different multiplexing schemes. The initial measurements demonstrate that the multiplexing circuits can be implemented in the detector electronics to reduce signal output channels, without increasing signal rise-time and degrading signal-to-noise ratio (SNR). The detector maintains an energy resolution of 19% and timing resolution of about 2 ns (block to single crystal). Moreover, the transformer can function as a single-ended (pseudo-differential) to true-differential converter; this would facilitate retaining signal integrity in transmissi-
on through long twisted-pair cables
[Show abstract][Hide abstract] ABSTRACT: An APD-based, light-sharing detector has been evaluated for use in PET. The detector configuration is a 2 × 2 array of 5 mm × 5 mm Hamamatsu S8664-55 APDs used to readout multi-crystal LSO blocks. Initially, a basic performance study was undertaken with a single APD coupled to a chemically etched 4 mm × 4 mm × 10 mm LSO crystal (teflon wrapped) using a custom, single-channel fast ASIC preamplifier. Timing and energy resolution measurements with a <sup>22</sup>Na source were performed using the monolithic LSO crystal coupled to the APD. The timing resolution of the APD channel in coincidence with a plastic scintillator coupled to a PMT was 870 ps FWHM. The energy resolution of the 511 keV photopeak was 12.1% FWHM. Based on these initial results, a 9 × 9 array of 2 mm × 2 mm × 20 mm LSO crystals was assembled and evaluated with a 2 × 2 APD array. The LSO block had an average energy resolution of 20.9% and a timing resolution of 2.47 ns FWHM. These results show that APDs are promising photodetectors for high-resolution and cost-effective PET systems utilizing light-sharing block detectors.
IEEE Transactions on Nuclear Science 11/2005; · 1.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: High spatial resolution is essential to image small lesions in positron emission tomography (PET). Traditional methods suggest using small crystals to obtain high resolution. In this study, a novel method is introduced to achieve high spatial resolution by using scintillators with different decay times arranged in a checkerboard pattern. Pulse shape discrimination (PSD) was implemented to determine which crystal the gamma energy is deposited. In this work, 13×13 LSO and 13×13 LSO-GSO crystal arrays using 4×4×20 mm<sup>3</sup> crystals were built and optimized using light sharing techniques. The average pixel resolution for the 13×13 LSO was measured to be 14.5%. Similarly, the average pixel resolutions for the GSO part of the 13×13 LSO-GSO crystal array and the LSO part of the same array were measured to be 13.9% and 18.5%, respectively. Due to the difference in the probability of forward scattering between GSO and LSO, the average energy resolution for LSO degraded from 14.5% to 18.5%. The average peak-to-valley ratios were measured to be 2.1 for the 13×13 LSO crystal array, 7.2 for the LSO part of the 13×13 LSO-GSO crystal array and 4.3 for the GSO part of the 13×13 LSO-GSO crystal array.
[Show abstract][Hide abstract] ABSTRACT: Localizing gamma-ray events accurately and performing pileup rejection/correction functions are desirable in positron-emission-tomography (PET) front-end electronics development. Two techniques, the traditional analog integration with charge-sensitive amplifiers and the recent digital integration by using free-running analog-to-digital converters (ADCs), are the typical methods to obtain the event energy and position information. Pileup issues have been extensively investigated in both these techniques. In this new study, a pulse-shape-restore (PSR) method for event localization is presented. From each PET scintillation detector, a photo-sensor current output signal is amplified then conditioned by a filter. Subsequently the signal is digitized with a fast sampling free-running ADC. The digitized signal is finally processed in a Field Programmable Gate Array (FPGA) by using a numerical line fitting method to restore the signal to its theoretic shape. The event energy is directly obtained from the restored pulse shape rather than from the integration calculation. With the PSR method, we may enhance the event localization accuracy and improve the signal energy resolution. Moreover, the PSR method will be implemented as a pileup rejection /correction algorithm to improve the detector count-rate ability and reduce the gamma ray mispositioning in high count-rate conditions
[Show abstract][Hide abstract] ABSTRACT: We are developing a high-resolution, high-efficiency positron emission tomography (PET) detector module with depth of interaction (DOI) capability based on a lutetium oxyorthosilicate (LSO) scintillator array coupled at both ends to position-sensitive avalanche photodiodes (PSAPDs). In this paper we present the DOI resolution, energy resolution and timing resolution results for complete detector modules. The detector module consists of a 7 x 7 matrix of LSO scintillator crystals (1 x 1 x 20 mm3 in dimension) coupled to 8 x 8 mm2 PSAPDs at both ends. Flood histograms were acquired and used to generate crystal look-up tables. The DOI resolution was measured for individual crystals within the array by using the ratio of the signal amplitudes from the two PSAPDs on an event-by-event basis. A measure of the total scintillation light produced was obtained by summing the signal amplitudes from the two PSAPDs. This summed signal was used to measure the energy resolution. The DOI resolution was measured to be 3-4 mm FWHM irrespective of the position of the crystal within the array, or the interaction location along the length of the crystal. The total light signal and energy resolution was almost independent of the depth of interaction. The measured energy resolution averaged 14% FWHM. The coincidence timing resolution measured using a pair of identical detector modules was 4.5 ns FWHM. These results are consistent with the design goals and the performance required of a compact, high-resolution and high-efficiency PET detector module for small animal and breast imaging applications.
Physics in Medicine and Biology 10/2004; 49(18):4293-304. · 2.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper, investigation of position sensitive avalanche photodiodes (PSAPDs) as optical detectors for reading out segmented scintillation arrays of LSO in high resolution PET modules is reported. PSAPDs with 8×8 mm<sup>2</sup> and 14×14 mm<sup>2</sup> area have been characterized with single LSO crystals and arrays. Energy resolution of 19% (FWHM) for 511 keV γ-rays and coincidence timing resolution of ∼3 ns (FWHM) have been recorded with PSAPD coupled to 1×1×20 mm<sup>3</sup> LSO detectors. Flood histogram studies have been successfully conducted by coupling multi-element element LSO arrays (1 mm pixels, 20 mm tall) to the PSAPDs. Finally, depth of interaction (DOI) resolution of <4.5 mm (FWHM) has been measured by coupling two PSAPDs on opposite ends of a 20 mm long LSO crystal with a 1×1 mm<sup>2</sup> cross section. Based on these results, PSAPDs appear to be promising for high resolution PET. An important advantage of these PSAPDs is significant reduction in electronic readout requirements.
IEEE Transactions on Nuclear Science 03/2004; · 1.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We investigate a prototype hybrid position sensitive avalanche photodiode (HPSAPD) that combines conventional photomultiplier tube (PMT) and solid-state photodiode technology to form a rugged, compact, high gain (∼ 10<sup>6</sup> 10<sup>7</sup>), high signal-to-noise ratio (S/N) photodetector. This detector uses a photocathode to convert incident light into photoelectrons that are accelerated to a position sensitive avalanche photodiode (PSAPD). Through impact ionization initiated by the incident accelerated photoelectrons, the PSAPD provides additional gain. The PSAPD provides an output signal used for energy and timing resolution information and 4 additional output signals for position information. Here we characterize and show the initial spectroscopic and imaging capabilities of a prototype HPSAPD that uses a GaAs photocathode and a planar PSAPD of 14 × 14 mm<sup>2</sup> area.
[Show abstract][Hide abstract] ABSTRACT: In this paper, an investigation of position sensitive avalanche photodiodes (PSAPDs) as optical detectors for reading out segmented scintillation arrays of LSO in high-resolution PET modules is reported. PSAPDs with 8 × 8 mm<sup>2</sup> have been characterized with single LSO crystals and arrays. Energy resolution of 19% (FWHM) for 511 keV γ-rays and coincidence timing resolution of ∼3 ns (FWHM) have been recorded with PSAPD coupled to 1 × 1 × 20 mm<sup>3</sup> LSO detectors. Flood histogram studies have been successfully conducted by coupling multi-element element LSO arrays (1 mm pixels, 20 mm tall) to the PSAPDs.
[Show abstract][Hide abstract] ABSTRACT: We have investigated a new position sensitive avalanche photodiode (PSAPD) for indirect and direct radiation imaging. This PSAPD exhibits minimal image distortion and still has all the attractive characteristics of our normal high gain APDs. The arc-PSAPD incorporates a resistive arc between the corner contacts which eliminates the 'pincushion' or 'barrel' effect commonly seen with four corner contact devices. Simulations have been performed to model the position distortion of such a device. Position and energy resolution have also been measured with these devices. Gamma ray imaging with various scintillator arrays and direct charged particle and low energy X energy resolution-ray images have been acquired.
[Show abstract][Hide abstract] ABSTRACT: A gamma ray detector for PET, consisting of an array of mixed lutetium oxyorthosilicate (MLS) scintillator crystals coupled to a position sensitive avalanche photodiode (PSAPD), was evaluated. The scintillator array was constructed from individual MLS crystals with dimensions of 1.5 mm×1.5 mm×15 mm. The assembled 7×7 array, including inter-crystal reflector material, had a pitch of 1.79 mm. The low noise, high gain PSAPD had dimensions of 14 mm×14 mm. Peaks associated with each of the 49 scintillator crystals were readily identifiable in flood histograms, and most of the crystals demonstrated energy resolution in the range of 15% to 20% at 511 keV. Preliminary measurements of the timing of the PSAPD in coincidence with a fast-scintillator/PMT detector indicated a timing resolution of approximately 4 ns. The operating characteristics and design attributes, such as compactness and reduced readout channel requirements, of the PSAPD make it attractive for high resolution PET applications.
[Show abstract][Hide abstract] ABSTRACT: In this paper, we report on the investigation of silicon avalanche photodiodes (APDs) for high-energy photon imaging applications. This includes a new APD design that provides X-ray and γ-ray imaging with significant reduction in electronic readout requirements. This new APD design, referred to as position-sensitive avalanche photodiode (PSAPD), involves charge sharing amongst the electrodes that enable determination of position of interaction. PSAPDs with 14 × 14 mm<sup>2</sup> area have been fabricated using planar processing. The performance of these devices has been evaluated for energy resolution, timing resolution (4 ns full-width at half-maximum), and spatial resolution (∼300 μm intrinsic spatial resolution). The potential of these APDs in high-energy physics and medical imaging is addressed.
IEEE Transactions on Nuclear Science 09/2002; · 1.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper, we report on gamma-ray and thermal neutron detection with RbGd<sub>2</sub>Br<sub>7</sub>:Ce scintillators. RbGd<sub>2</sub>Br<sub>7</sub>:Ce (RGB) is a new scintillator material that shows high light output (56000 photons/MeV) and has a fast principal decay constant (45 ns) when doped with 10% Ce. These properties make RGB an attractive scintillator for γ-ray detection. Also, due to the presence of Gd as a constituent, RGB has a high cross-section for thermal neutron absorption and can achieve close to 100% stopping efficiency with 0.5-mm-thick RGB crystals. Crystals of RGB with three different Cc concentrations (0.1, 5, and 10%) have been grown. Their basic scintillation properties such as light output, decay time, and emission spectrum have been measured. In addition, high-efficiency thermal neutron detection has been confirmed in our studies.
IEEE Transactions on Nuclear Science 09/2002; · 1.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We are developing a compact positron emission tomography (PET) detector module with a depth of interaction capability (DOI) based on a lutetium oxyorthosilicate (LSO) scintillator array coupled at both ends by avalanche photodiode (APD) arrays. This leads to a detector with high sensitivity that can provide high and uniform image resolution. We report studies on improving the DOI resolution by optimizing the crystal surface treatment. Six 2×2×20 mm LSO crystals were treated with different surface finishes along their length: raw saw-cut, polished optical finish, and chemically etched by hot anhydrous phosphoric acid (H<sub>3</sub>PO<sub>4</sub>) with etching times varying from 1 to 5 min. The ratio of the signals from the two APD arrays was used to measure DOI, and the sum of the signals to measure the total light output. Crystals finished by chemical etching for 2-3 min gave the best overall detector performance, with DOI resolutions ranging from 3.1 to 3.9 mm for events above a 150-keV threshold and uniform light output for different DOI positions. The energy resolution ranged between 14% and 18%. This detector design appears promising for PET applications requiring very high resolution and high sensitivity, for example, in small animal imaging and human breast imaging.
IEEE Transactions on Nuclear Science 07/2002; · 1.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper, development of large-area planar avalanche photodiodes (APDs) and monolithic APD arrays for X-ray and scintillation detection is discussed. Single APDs with areas as large as 10 cm/sup 2/ have been fabricated and tested with a CsI(Tl) scintillator (3.8 cm diameter, 2.5 cm height). The resolution of the 662 keV photopeak has been measured to be 9% (FWHM). The X-ray detection performance, gain, and noise of these large APDs have been characterized. Multielement APD arrays have also been fabricated in various formats, such as 4/spl times/4 to 14/spl times/14 elements (2 mm pixels), and the uniformity of gain, noise, and sensitivity has been evaluated for 4/spl times/4 arrays using an /sup 55/Fe source. Timing properties have been measured. Packaging issues related to the APD arrays are discussed.
IEEE Transactions on Nuclear Science 01/2002; · 1.46 Impact Factor