Adrian Ivan

Evaluation of a prototype small-animal PET detector with depth-of-interaction encoding

We report on the evaluation of a prototype small-animal positron emission tomography (PET) detector with depth-of-interaction (DOI) encoding. The detector consisted of an 8/spl times/8 array of scintillator crystals that were read out on the top and bottom by position sensitive avalanche photodiodes (PSAPD). Each scintillator crystal had dimensions of 1.65 mm/spl times/1.65 mm/spl times/22.00 mm, and the array had a pitch of 1.75 mm. The 14/spl times/4 mm PSAPDs were coupled to compact, discrete transimpedance preamplifiers. The DOI response was measured by illuminating the detector module from the side with an electronically collimated fan beam of 511 keV gamma rays. The signals from the corner contacts of the PSAPDs were used to identify the crystal of interaction, and the ratio of the total signals from the two PSAPDs was used to calculate the depth z at which the interaction took place. By stepping the crystal array in the z direction (perpendicular to the fan beam), we were able to determine the DOI resolution for each individual crystal. For measurements made at 10/spl deg/C, the average DOI resolution was better than 3 mm. Energy resolution and timing resolution (versus a fast plastic scintillator coupled to a photomultiplier tube) were made under the same operating conditions. The average energy resolution across the array was 16.6% at 511 keV, and the average timing resolution was 3.9 ns. Importantly, the detector performance was maintained all the way out to the crystals at the edges of the PSAPDs.

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.

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.

Radioactive Source Estimation Using a System of Directional and Non-Directional Detectors

We derive source parameter estimation algorithms based on maximum likelihood estimation (MLE) for a system of non-directional (scintillator) and directional (CZT based Compton) radiation detectors. For multiple non-directional detectors, the joint likelihood of registered counts is maximized to estimate the source parameters. For directional detectors, the well-known List Mode Maximum Likelihood Expectation Maximization (MLEM) algorithm is extended to fuse information from multiple detectors and locate the source in Cartesian coordinates. We then develop multi-sensor fusion algorithms for a system of non-directional and directional detectors by combining MLE and MLEM algorithms. Results are presented which illustrate the behavior and performance of our proposed approaches.

Timing resolution performance comparison for fast and standard outputs of SensL SiPM

A new silicon photomultiplier (SiPM) with a unique fast output signal feature was recently introduced by SensL. For a SiPM device with 3×3 mm2 sensitive area, the single photo electron response from the fast output is less than 2 ns wide compared with 50 ns width observed at the standard output. Using the fast readout signal, a coincidence resolving time (CRT) of 190 ps (FWHM - full width at half maximum) was measured with a 3×3×10 mm3 LYSO crystal optically coupled to the SiPM mounted on the SensL MicroFB-SMA evaluation board. Applying additional C-R high pass filters to the standard SiPM output, we shortened the SPE response from 50 ns to 3 ns width and measured CRT values similar to those obtained with fast output signals. We have studied the effect of external filtering on CRT with two SensL 3×3 mm2 devices with different microcell sizes (35 μm and 50 μm). For both devices studied, we demonstrated similar CRT performance between the fast output and standard output with optimized pulse shaping filter. However, for applications requiring simultaneous measurement of intensity and timing of light pulses, the availability of a separate fast output directly from the sensor is convenient because it reduces the need for additional circuits and provides CRT performance equal to external filters optimized for the standard outputs.

Depth of interaction effect on timing resolution in PET block detectors

We have investigated the effect of the depth of interaction (DOI) on the coincidence timing distribution of a PET block detector excited with 511 keV photons. Measurements were performed with a detector consisting of a quad photomultiplier tube (PMT) optically coupled to a 6times6 array of mixed lutetium silicate (MLS) scintillator crystals with different surface treatments. The PET detector was side-illuminated with an electronically collimated beam and the coincidence timing distribution was recorded at different DOI as the detector block was stepped through the beam. The shift of the timing distribution peak measured the variation of the average propagation time of the scintillation photons within the block. From top to bottom (30 mm distance), the average delay ranged from 120 ps for polished crystals up to 350 ps for crystals with all sides roughened. The results of this study allowed modeling the effect of DOI delay on timing performance for front-end irradiation with 511 keV photons, assuming a range of different values for the intrinsic timing resolution of the detector. The relative contribution from DOI effects to the total time coincidence resolution was found to be significant only for detectors with an intrinsic time resolution less than 250 ps FWHM

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.

Design of a Modular and Efficient CAMAC/Lab VIEW-Based Data Acquisition System for a Time of Flight PET Test-Bed

A high-speed data acquisition (DAQ) system has been designed for a time of flight PET test-bed. The requirements of the system were flexibility, data throughput and data integrity. The software is modular so that modifications and additions can be integrated easily into the existing software architecture. The program operation is driven by commands read from a script file, simplifying implementation of complex acquisition sequences. The heart of the program is the DAQ module, which efficiently transfers data from CAMAC to file. Another software module offers online or offline analysis capabilities. The software, written in LabVIEW, communicates with a novel high-speed USB2 CAMAC controller (CCUSB). The CCUSB offers significant improvements over its GPIB predecessor, supporting FIFO buffered DAQ and a variety of data readout modes. Four readout modes have been evaluated in order to maximize the DAQ rate for this particular system. A highest sustained data rate of 15.7 k events/s was achieved for approximately 60 input channels using a 22Na flood phantom. Flexibility in the software design accommodates both current and future hardware configurations without the need to edit the LabVIEW code.

A compact SPECT detector based on a quad PMT

Traditionally, the technology of general purpose SPECT cameras has been based on a large panel of NaI:Tl scintillator, optically coupled with a number of 2” or 3” PMTs [1]. With its rather good performance at low cost, the SPECT camera design has not changed essentially since Hal Anger invented it in the late 1950s. For the last decade, however, with progress in new scintillators and photosensors, there have been renewed efforts to improve spatial and energy resolutions. A series of new compact detectors have been mostly designed for small animal or organ specific SPECT cameras. In this paper, we present a concept of a compact SPECT detector design using a NaI:Tl crystal array and a quad anode PMT. By using a crystal array and a light guide, we demonstrate that all individual crystals can be identified without any dead area at the edge of the detector. This proves the possibility of a compact SPECT detector having high spatial resolution using an array of regular types of PMTs, for example, a 2×2 or 3×3 array of PMTs. Also, we show that this block structure design enables very small gaps at the edge of the detectors resulting in a more compact geometry when packed in full size SPECT cameras.