Huini Du

A Prototype PET Scanner with DOI-Encoding Detectors

Detectors with depth-encoding allow a PET scanner to simultaneously achieve high sensitivity and high spatial resolution. Methods: A prototype PET scanner, consisting of depth-encoding detectors constructed by dual-ended readout of lutetium oxyorthosilicate (LSO) arrays with 2 position-sensitive avalanche photodiodes (PSAPDs), was developed. The scanner comprised 2 detector plates, each with 4 detector modules, and the LSO arrays consisted of 7 × 7 elements, with a crystal size of 0.9225 × 0.9225 × 20 mm and a pitch of 1.0 mm. The active area of the PSAPDs was 8 × 8 mm. The performance of individual detector modules was characterized. A line-source phantom and a hot-rod phantom were imaged on the prototype scanner in 2 different scanner configurations. The images were reconstructed using 20, 10, 5, 2, and 1 depth-of-interaction (DOI) bins to demonstrate the effects of DOI resolution on reconstructed image resolution and visual image quality. Results: The flood histograms measured from the sum of both PSAPD signals were only weakly depth-dependent, and excellent crystal identification was obtained at all depths. The flood histograms improved as the detector temperature decreased. DOI resolution and energy resolution improved significantly as the temperature decreased from 20°C to 10°C but improved only slightly with a subsequent temperature decrease to 0°C. A full width at half maximum (FWHM) DOI resolution of 2 mm and an FWHM energy resolution of 15% were obtained at a temperature of 10°C. Phantom studies showed that DOI measurements significantly improved the reconstructed image resolution. In the first scanner configuration (parallel detector planes), the image resolution at the center of the field of view was 0.9-mm FWHM with 20 DOI bins and 1.6-mm FWHM with 1 DOI bin. In the second scanner configuration (detector planes at a 40° angle), the image resolution at the center of the field of view was 1.0-mm FWHM with 20 DOI bins and was not measurable when using only 1 bin. Conclusion: PET scanners based on this detector design offer the prospect of high and uniform spatial resolution (crystal size, ∼1 mm; DOI resolution, ∼2 mm), high sensitivity (20-mm-thick detectors), and compact size (DOI encoding permits detectors to be tightly packed around the subject and minimizes number of detectors needed).

Continuous depth-of-interaction encoding using phosphor-coated scintillators.

We investigate a novel detector using a lutetium oxyorthosilicate (LSO) scintillator and YGG (yttrium-aluminum-gallium oxide:cerium, Y(3)(Al,Ga)(5)O(12):Ce) phosphor to construct a detector with continuous depth-of-interaction (DOI) information. The far end of the LSO scintillator is coated with a thin layer of YGG phosphor powder which absorbs some fraction of the LSO scintillation light and emits wavelength-shifted photons with a characteristic decay time of approximately 50 ns. The near end of the LSO scintillator is directly coupled to a photodetector. The photodetector detects a mixture of the LSO light and the light emitted by YGG. With appropriate placement of the coating, the ratio of the light converted from the YGG coating with respect to the unconverted LSO light can be made to depend on the interaction depth. DOI information can then be estimated by inspecting the overall light pulse decay time. Experiments were conducted to optimize the coating method. 19 ns decay time differences across the length of the detector were achieved experimentally when reading out a 1.5 x 1.5 x 20 mm(3) LSO crystal with unpolished surfaces and half-coated with YGG phosphor. The same coating scheme was applied to a 4 x 4 LSO array. Pulse shape discrimination (PSD) methods were studied to extract DOI information from the pulse shape changes. The DOI full-width-half-maximum (FWHM) resolution was found to be approximately 8 mm for this 2 cm thick array.

Tapered LSO Arrays for Small Animal PET

By using detectors with good depth encoding accuracy (∼2 mm), an animal PET scanner can be built with a small ring diameter and thick crystals to simultaneously obtain high spatial resolution and high sensitivity. However, there will be large wedge-shaped gaps between detector modules in such a scanner if traditional cuboid crystal arrays are used in a polygonal arrangement. The gaps can be minimized by using tapered scintillator arrays enabling the sensitivity of the scanner to be further improved. In this work, tapered lutetium oxyorthosilicate (LSO) arrays with different crystal dimensions and different combinations of inter-crystal reflector and crystal surface treatments were manufactured and their performance was evaluated. Arrays were read out from both ends by position-sensitive avalanche photodiodes (PSAPDs). In the optimal configuration, arrays consisting of 0.5 mm LSO elements could be clearly resolved and a depth of interaction resolution of 2.6 mm was obtained for a 20 mm thick array. For this tapered array, the intrinsic spatial is degraded from 0.67 to 0.75 mm compared to a standard cuboidal array with similar dimensions, while the increase in efficiency is 41%. Tapered scintillator arrays offer the prospect of improvements in sensitivity and sampling for small-bore scanners, without large increases in manufacturing complexity.

Measurements of wavelength shifting (WLS) fibre readout for a highly multiplexed, depth-encoding PET detector

We are attempting to develop a laboratory PET scanner for mouse imaging that utilizes far fewer detectors and channels of electronics, thus reducing cost, whilst retaining state-of-the-art performance. The detectors are based on LSO arrays read out by wavelength shifting (WLS) fibres placed on the top and the bottom of the arrays. Depth of interaction information will be obtained from the ratio of the signals at either end of the array. For acceptable performance, it is critical to maximize collection of light photons from the ends of the fibres and to minimize the optical crosstalk between adjacent fibres. Factors which can affect the light collection and crosstalk were studied, including coupling materials between fibres and crystals, reflectors wrapped around the fibre sides and ends, fibre size and shape, and number of layers of fibre cladding. Properties of WLS fibres such as the transmission attenuation and transverse absorption were also studied. The light yield from 2 x 2 x 10 mm(3) LSO crystals collected from the end of a 2 x 2 x 30 mm(3) WLS fibre was up to 24% (typical values 16-20%) of that obtained by direct coupling of the LSO crystal. This light collection efficiency appears to be sufficient for decoding interaction position in these detectors.

Comparison of four depth-encoding PET detector modules with wavelength shifting (WLS) and optical fiber read-out

We propose detectors for a laboratory positron emission tomography scanner specific for mouse imaging that utilizes fewer detectors and channels of electronics compared with existing designs. The detectors are based on lutetium oxyorthosilicate arrays, read out by orthogonal optical fibers placed on the top and bottom of the arrays. Depth of interaction (DOI) information is obtained from the ratio of the signals at either end of the array. Four different detector modules were evaluated, using different reflector materials and two types of optical fibers (wavelength shifting (WLS) fibers and clear optical fibers). The modules were compared in terms of flood histograms, energy resolution, DOI resolution and timing resolution. Energy resolution for single crystals at one irradiation depth was around 65% full-width half-maximum (FWHM). A DOI resolution of approximately 6 mm was obtained for the modules. Timing resolution was in the range of 5.1-7.8 ns. An array assembled in the laboratory and coupled with WLS fibers had the best DOI resolution; the same array with clear fibers had the best timing resolution and a commercially manufactured array and coupled with WLS fibers had the best energy resolution.

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.

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.