- [Show abstract] [Hide abstract] ABSTRACT: The main focus of our study is to investigate how the performance of digital timing methods is affected by sampling rate, anti-aliasing and signal interpolation filters. We used the Nyquist sampling theorem to address some basic questions such as what will be the minimum sampling frequencies? How accurate will the signal interpolation be? How do we validate the timing measurements? The preferred sampling rate would be as low as possible, considering the high cost and power consumption of high-speed analog-to-digital converters. However, when the sampling rate is too low, due to the aliasing effect, some artifacts are produced in the timing resolution estimations; the shape of the timing profile is distorted and the FWHM values of the profile fluctuate as the source location changes. Anti-aliasing filters are required in this case to avoid the artifacts, but the timing is degraded as a result. When the sampling rate is marginally over the Nyquist rate, a proper signal interpolation is important. A sharp roll-off (higher order) filter is required to separate the baseband signal from its replicates to avoid the aliasing, but in return the computation will be higher. We demonstrated the analysis through a digital timing study using fast LSO scintillation crystals as used in time-of-flight PET scanners. From the study, we observed that there is no significant timing resolution degradation down to 1.3 Ghz sampling frequency, and the computation requirement for the signal interpolation is reasonably low. A so-called sliding test is proposed as a validation tool checking constant timing resolution behavior of a given timing pick-off method regardless of the source location change. Lastly, the performance comparison for several digital timing methods is also shown.
- [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.
- [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.
- [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 has, however, a complication since both LuAP and LuYAP emits in the excitation band of LSO, thus limiting the functionality. In the present paper we have looked into this and suggested different ways to overcome these drawbacks.
Conference Paper: MR-compatible blood sampler for PET[Show abstract] [Hide abstract] ABSTRACT: An MR-compatible automatic blood sampler prototype for quantifying radioactivity in blood has been built and tested. The determination of the so-called input function is necessary to investigate the kinetics of the administered radiotracer in PET studies. The counting detector consists of 2 coincident LSO blocks (50times40times 30 mm<sup>3</sup>) coupled each to a single APD (10 times 10 mm<sup>2</sup> active area). Preliminary experimental and numerical results indicate that a specific sensitivity of about 1 cps / (nCi / ml) can be achieved. Tests during bimodal simultaneous MR/PET scans will be carried out in the future with a Siemens 3 T-Trio MR scanner and a brain PET insert.
Conference Paper: Scintillation kinetics of YSO:Ce[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.
Conference Paper: RF transformer coupled multiplexing circuits for APD PET detectors[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. ∼106 for PMT), and a smaller photoactive area (∼5mm × 5mm for APD vs. 10-52mm 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 × 2 APD array reading out an 8 × 8 array of Lutetium Oxyorthosilicate (LSO) crystals. Each crystal is 2mm × 2mm × 20mm. 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 2ns (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 transmission through long twisted-pair cables.
- [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: 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: 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.
- [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.
- [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.
Conference Paper: A novel, distortion-free position sensitive APD for nuclear imaging[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.
Conference Paper: Position Sensitive APDs for Small Animal Pet Imaging[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.
Conference Paper: Evaluation of Position Sensitive Avalanche Photodiodes for PET[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: 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.
- [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.
Conference Paper: Radiation hardness of high gain avalanche photodiodes[Show abstract] [Hide abstract] ABSTRACT: In this paper, we report on evaluation of radiation hardness of deep diffused, high gain avalanche photodiodes (APDs). We have performed experiments on 2 mm × 2 mm APDs to quantify the degradation in performance over a range of particle fluences. Eight APDs were irradiated at the Paul Scherrer Institut (Switzerland) with 72 MeV protons ranging from a fluence of 1 × 10<sup>8</sup> protons/cm<sup>2</sup> to 2 × 10<sup>12</sup> protons/cm<sup>2</sup> with an equivalent 1 MeV neutron fluence of ∼2 × 10<sup>8</sup> n/cm<sup>2</sup> to 5 × 10<sup>12</sup> n/cm<sup>2</sup>. The performance parameters measured include quantum efficiency (QE), gain, noise, and leakage current. Cooling and annealing measurements were also performed to explore options for reversing the effects of the damage sites within the APD. Our measurements show that our APDs have a minimal loss of performance in noise and QE up to about 1 × 10<sup>12</sup> n/cm<sup>2</sup>. Beyond this neutron fluence, we see a decrease in QE and increase in noise. We were able to reverse the performance degradation of the higher irradiated APDs with moderate cooling and with annealing at 100°C.
- [Show abstract] [Hide abstract] ABSTRACT: The use of high atomic number, room-temperature semiconductors for radiation detectors always involves making compromises to optimize the performance for specific applications. In recent years, a number of sophisticated device configurations and a variety of new read-out methods have been developed to extract the desired information from the detector signal. These approaches have significantly mitigated the effects of the inherent deficiencies found in available materials and have opened the way for promising new applications. The benefits of such approaches and their limitations are reviewed for CdTe and Cd1−xZnxTe (CZT) devices.
- [Show abstract] [Hide abstract] ABSTRACT: The authors evaluated a compact, high resolution PET detector module using avalanche photodiode (APD) arrays to replace bulky position sensitive PMTs. The newly developed APD array is a planar processed 4×4 array which has a 2×2 mm<sup>2</sup> pixel size with 0.4 mm gaps between pixels, about 60% quantum efficiency at 420 nm wavelength, and uniform high gain (>1000) across all channels. A 4×4 array of 2×2×10 mm<sup>3</sup> LSO crystals was coupled to an APD array. Different readout electronics and signal multiplexing schemes were explored. All crystals in the detector array were clearly identified in the flood source histogram, with average peak-to-valley ratios of about 12:1 using a charge sharing resistor network. The energy resolution was measured to be ~14% at 511 keV in the detector array. The measured timing resolution was 2.6 ns in coincidence with a LSO/PMT detector. By optimizing the readout electronics currently being used, it is likely that detector performance can be further improved. The authors have also determined depth-of-interaction (DOI) by reading out two APD arrays connected to the ends of a 2×2×22 mm<sup>3</sup> LSO crystal. Preliminary measurements show good DOI measurement capability with DOI positioning uncertainty between 4 and 6.5 mm
Radiation Monitoring Devices, IncWatertown, Massachusetts, United States
University of California, Los Angeles
Los Angeles, CA, United States
- Department of Molecular and Medical Pharmacology