David L. McDaniel

Validation of GATE Monte Carlo simulations of the GE Advance/Discovery LS PET scanners

The recently developed GATE (GEANT4 application for tomographic emission) Monte Carlo package, designed to simulate positron emission tomography (PET) and single photon emission computed tomography (SPECT) scanners, provides the ability to model and account for the effects of photon noncollinearity, off-axis detector penetration, detector size and response, positron range, photon scatter, and patient motion on the resolution and quality of PET images. The objective of this study is to validate a model within GATE of the General Electric (GE) Advance/Discovery Light Speed (LS) PET scanner. Our three-dimensional PET simulation model of the scanner consists of 12 096 detectors grouped into blocks, which are grouped into modules as per the vendors specifications. The GATE results are compared to experimental data obtained in accordance with the National Electrical Manufactures Association/Society of Nuclear Medicine (NEMA/SNM), NEMA NU 2-1994, and NEMA NU 2-2001 protocols. The respective phantoms are also accurately modeled thus allowing us to simulate the sensitivity, scatter fraction, count rate performance, and spatial resolution. In-house software was developed to produce and analyze sinograms from the simulated data. With our model of the GE Advance/Discovery LS PET scanner, the ratio of the sensitivities with sources radially offset 0 and 10 cm from the scanners main axis are reproduced to within 1% of measurements. Similarly, the simulated scatter fraction for the NEMA NU 2-2001 phantom agrees to within less than 3% of measured values (the measured scatter fractions are 44.8% and 40.9 +/- 1.4% and the simulated scatter fraction is 43.5 +/- 0.3%). The simulated count rate curves were made to match the experimental curves by using deadtimes as fit parameters. This resulted in deadtime values of 625 and 332 ns at the Block and Coincidence levels, respectively. The experimental peak true count rate of 139.0 kcps and the peak activity concentration of 21.5 kBq/cc were matched by the simulated results to within 0.5% and 0.1% respectively. The simulated count rate curves also resulted in a peak NECR of 35.2 kcps at 10.8 kBq/cc compared to 37.6 kcps at 10.0 kBq/cc from averaged experimental values. The spatial resolution of the simulated scanner matched the experimental results to within 0.2 mm.

Time-of-Flight PET Detector Based on Multi-Pixel Photon Counter and Its Challenges

Geiger-mode multi-pixel APD is being recognized as the best alternative solid-state photo-sensor to vacuum PMT for various specific applications. Especially, its magnetic field immunity and high gain made it popular in MR/PET detector research. In this paper, we utilized its compactness, high gain and high photon detection efficiency in the design of TOF PET detector. In a typical block detector based on PMT, the full timing capability of both PMT and scintillator could not be achieved due to its light sharing for Anger logic scheme. Since Geiger-mode APD is a solid-state based technology, we can apply one-to-one coupling between a scintillator and the photo-sensor to optimize the signal-to-noise ratio. Also, the high photon detection efficiency of MPPC, Geiger mode APD from Hamamatsu, would help to improve timing resolution. So, we made a block detector based on a 4 × 4 array of 3 × 3 mm2 MPPC coupled to a 4 × 4 array of 3 × 3 × 25 mm3 LYSO crystals to evaluate its performance. We have achieved the average of 9% energy resolution and 314 ps coincidence timing resolution with very good uniformity. This block timing resolution showed no degradation in timing compared to individual single channel timing resolution as expected from one-to-one readout. On top of that, the result proves that the solid-state based photo-sensor can be used for TOF PET detector. During the development and setup of the detector, we recognized that a compact and low power electronics readout scheme is one of the biggest challenges, including its cost, for MPPC or other Geiger-mode APD to be used in products.

Evaluation of a position sensitive avalanche photodiode for PET

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 /spl times/ 1.5 mm /spl times/ 15 mm. The assembled 7 /spl times/ 7 array, including intercrystal reflector material, had a pitch of 1.79 mm. The low noise, high gain PSAPD had dimensions of 14 mm /spl times/ 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. 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.

Crystal-based coincidence timing calibration for PET scanner

A crystal-based timing calibration method is implemented and investigated on a GE PET scanner. This method first calculates block-level adjustments using a commercially available algorithm, and then, based on the calculated block-level adjustments, derives the crystal-level adjustments needed within each block. Concurrently with the time difference acquisition for block-pair adjustment, the time differences are also accumulated for all the crystals within each block. The crystal averages are calculated from the accumulated crystal time differences and represent the time differences among these crystals. The crystal-based adjustments are set to correct for both block time differences and crystal variations. A method to minimize the discrepancy in averaged correction for a block before and after discretization of the desired crystal adjustments is proposed. The performance of this crystal-based timing adjustment method is also presented.

Time-of-flight PET-MR detector development with silicon photomultiplier

Silicon photomultiplier technology based on Geigermode avalanche in p-n junction has made unforeseen progress for the last 5-6 years. With its high gain and high photon detection efficiency, it has shown that it could replace PMTs in many applications including time-of-flight PET. Also, its magnetic immunity and compactness made it very suitable for PET-MR detector. In this paper, we present time-of-flight PET-MR detector based on silicon photomultiplier from its selection, prototype test with discrete electronics, ASIC and a module design. Also, the system performance data is presented.

Sensitivity improvement of time-of-flight (ToF)-PET detector through recovery of Compton scattered annihilation photons

In PET detector designs, the scintillator material can be partitioned such that a block of crystals share timing/energy readout electronics. A fraction of the incident 511 keY photons produce simultaneous events in two adjacent block readouts due to Compton scattering followed by escape from the primary block. These inter-block Compton scatter events are typically not processed in current PET scanners. We have used radiation transport simulations to determine the fraction of inter-block Compton events for different block sizes using LYSO scintillator. The simulations showed the statistical distribution of the energy signals and time stamps in the two blocks and guided the selection of energy and time criteria for an event recovery algorithm. With a particular block size, we experimentally demonstrated that inter-block Compton events may be recovered as valid events with a corrected time stamp and estimated position of initial interaction. The results showed a significant improvement in detector sensitivity at the expense of a small degradation in the timing resolution of the detector block.

High spatial resolution detector using an 8/spl times/8 MLS crystal array and a quad anode photomultiplier

A compact and high position resolution detector for whole body PET has been developed using an 8/spl times/8 MLS scintillation crystal array and a quad-anode photomultiplier. The scintillation crystal block consists of 64 single crystals with dimensions of 4.67/spl times/4.67/spl times/30mm/sup 3/. Light sharing is controlled through the finishes of the crystal surfaces, Teflon reflective layers and a simple thin light guide. The block properties have been characterized using 511 keV gamma rays using Ge/sup 68/. Peaks in the crystal decoding map are well isolated with an average peak-to-valley ratio of 3.17. An energy resolution of 15% FWHM and a timing resolution of 1.2 ns FWHM were obtained.

Dependence of timing resolution on crystal size for TOF PET

Recently, with the prospect of great improvement in image quality, the development of time of flight technology has become an exciting topic for positron emission tomography. The excitement was further accelerated by the introduction of various fast and high light output scintillators as well as photosensors. However, the development of improved time of flight detectors is not only about the selection of crystals and photosensors, but also about how detectors are assembled to optimize their performance. For example, depending on crystal block structure, photo-sensor layout, and coupling methods, a detectors timing resolution can be drastically different. Since the effect of block structure for timing resolution is complex and less understood it is essential to first dissect the block structure and understand the impact of its basic components on timing resolution. In this paper, we will present the dependence of timing resolution on varying the dimensions of the scintillator crystals that are the main component of a block detector.

Detector characterization of Discovery ST whole-body PET scanner

A new whole-body PET scanner from General Electric (Discovery ST) is based on 6/spl times/6 BGO block detector using quad photo multipliers and utilizes a state of the art data acquisition system. The detector consists of 280 detector units in a ring structure. Each detector unit consists of a BGO block and a quad photomultiplier. The DST scanner has high sensitivity compared to other available whole-body scanners. This paper presents the characterization parameters of the detector including energy resolution, the quality of position maps, temperature dependence, and timing resolution. The data from several production samples are analyzed and compared to the design models. Stability and long-term drift measurements are provided for system sensitivity and PMT gains.

Evaluation of position sensitive avalanche photodiodes for PET

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/spl times/1.5 mm/spl times/15 mm. The assembled 7/spl times/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/spl times/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.