Kent C. Burr

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.

A new modular and scalable detector for a Time-of-Flight PET scanner

We describe a detector design for a new low-cost Time-of-Flight PET scanner. Each independent detector module is two-side buttable in the transaxial direction. To maximize the optical collection efficiency, the design uses two different-sized circular PMT types with concave photocathodes and each type is placed at a different distance from the scintillator array. The detector consists of multiple axial segments. Each axial segment is read-out by four PMTs arranged in a rectangular pattern, and neighboring segments share two PMTs in order to reduce channel count and improve optical collection efficiency. To reduce deadtime and pile-up effects, each axial segment is essentially optically isolated from its neighbors. The detector length is scalable in the axial direction by changing the number of segments. In addition to the detector design, we present timing resolution measurements as a function of singles rate.

Timing calibration for time-of-flight PET using positron-emitting isotopes and annihilation targets

Adding time-of-flight (TOF) technology has been proved to improve the image quality in PET. In order for TOF information to significantly reduce the statistical noise in reconstructed PET images, good timing resolution is needed across the scanner field of view (FOV). This work proposes an accurate, robust and practical crystal-based timing calibration method using FDG positron-emitting sources together with a spatially separated annihilation target. We calibrated a prototype Toshiba TOF PET scanner using this method and then assessed its timing resolution at different locations in the scanner FOV.

An algorithm for automatic flood histogram segmentation for a PET detector

We describe a new algorithm for automatic segmenting crystal position map (flood histogram) for PET scintillation detectors. The algorithm naturally reproduces the distortion patterns that are observed in flood histograms generated using Anger logic. It ensures that the correct number of regions is always identified so that irregularities in the flood histogram, such as shifted or merged peaks can be properly handled. Our detector design utilizes two types of photosensors with different dimensions. The mix of these two sizes of photosensors leads to non-uniform and non-symmetric flood histograms. The algorithm determines maps of the signal distribution from each crystal to all of the acquisition channels. The algorithm starts with a standard distribution template. For each flood histogram to be segmented, the same template is first adjusted on a global scale (equivalent to adjusting the gain on each channel), and then on a local scale (accounting for local variations). The boundaries for segmenting individual crystals are estimated by applying Anger logic to values interpolated from the distribution maps. An objective function is defined to quantify the quality of the segmentation, and the distribution maps are modified to minimize the objective function. The same algorithm can be used not only on our detector designs but also can be applied to segmenting all Anger-logic-generated flood histograms.

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

We report on the evaluation of a prototype PET detector with depth of interaction (DOI) encoding. The detector consisted of an 8/spl times/8 array of mixed lutetium oxyorthosilicate (MLS) crystals that were read out on the top and bottom by position sensitive avalanche photodiodes (PSAPD). Each 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 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, while the summed signals from the two PSAPDs were 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 2.9 mm. Energy resolution and timing resolution (vs. 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 edge of the PSAPDs.

Quality control algorithms studies for a pmt-based time-of-flight PET system

We developed a modular and scalable PMT-based detector for a Time-of-Flight PET system. The detector is divided into 5 segments. The position within each segment is determined by applying Anger logic to 4 PMTs which read out the segment and using a crystal look-up table (LUT) to convert the position to a specific crystal location. Routine quality controls (QC) are necessary to keep the system performance stable over time. Our approach is to keep the crystal LUT unchanged while compensating gain change for each PMT. We evaluated two algorithms for compensating gain changes. Algorithm 1 adjusts PMT gains to minimize the energy resolution for all crystals within each segment. Algorithm 2 requires reference data when the PET system is freshly calibrated (including photopeaks of crystals in the center of PMTs, normalized counts-per-crystal and position map value on crystal segmentation boundaries). During QC, PMT gains are adjusted to minimize the difference between new data and reference data. These two algorithms were validated by applying simulated gain changes to PMT signals measured using a calibration acquisition mode. They are evaluated by the accuracy in estimating simulated gain changes for each PMT channel under three conditions: moderate and large gain changes that are equivalent to or larger than what might occur in a real system; extreme gain changes to test algorithm robustness. Then the estimated gain compensation was applied to each PMT and the resulting system performance (energy resolution and total energy-qualified counts) was calculated. We evaluated performance in 5 cases: 1) from initial data, 2) after simulated PMT gain changes, 3-5) after simulated gain changes and applying a simple energy drift correction, Algorithms 1 and 2, respectively. The two algorithms are similar in terms of gain change estimation accuracy and recovering system performance for moderate to large gain changes. However, when PMTs have extreme gain changes, Algorithm 2 clearly outperforms Algorithm 1.