Coincidence Response Functions Research Articles

Evaluation of a Hamamatsu TOF-PET Detector Module With 3.2-mm Pitch LFS Scintillators and a 256-Channel SiPM Array

Using time-of-flight (TOF) information in image reconstruction improves image signal-to-noise ratio and quantitative accuracy in positron emission tomography (PET). A new TOF-PET detector module with a 3.2-mm pitch lutetium fine silicate (LFS) scintillator array one-to-one coupled to a 16×16 (256-ch) silicon photomultiplier (SiPM) array was made commercially available (C13500 series, Hamamatsu Photonics K. K.). In this study, as a candidate detector module for the next-generation brain-dedicated PET, we changed the scintillator length of 20 to 10 mm according to our helmet-type PET geometry and investigated the basic performance of the PET detector module, including its energy resolution, sensitivity, coincidence response function (CRF), and coincidence timing resolution (CTR). All performance values were compared with those of the 4.2-mm pitch 144-ch detector module which was selected for our current helmet-type PET prototype. The energy resolution was 12% at 511 keV. The sensitivity was about 10% lower compared with those of the 4.2-mm pitch module. The average full width at half maximum (FWHM) of the CRFs was 1.9 mm. The CTRs had values of 235-241 ps with various energy windows after applying timing calibration. The CTR was not significantly changed (<; 10 ps) at the single count rate of 412 kcps or lower.

A study of inter-crystal scatter in dual-layer offset scintillator arrays for brain-dedicated PET scanners

A dual-layer offset (DLO) detector enables depth-of-interaction (DOI) through light sharing between two layers of scintillation arrays with a single-ended readout (SER) scheme. However, the SER scheme in DLO detectors may lead to a layer misassignment when inter-crystal scattering occurs. The aim of this work is to study inter-crystal scattering and evaluate the effects of layer misidentifications in DLO detectors on the performance of scanners suitable for a brain-dedicated PET insert. The influence of layer misidentification on the coincidence response functions (CRFs) of 3 different DLO detectors with total/front/back layer thicknesses of 15/6/9 mm, 20/8/12 mm, and 25/7.5/17.5 mm and a crystal width of about 3 mm was studied through Monte Carlo simulations. To overcome layer misidentification, we studied a practical DLO detector design in which each layer can be read out independently through a discrete-layer readout (DLR) scheme where light sharing between the layers is avoided. The CRFs of the mentioned DLO detectors assuming SER and DLR were analyzed. To evaluate the effects of layer misidentification on image quality, images of a Derenzo-like phantom were also reconstructed for all DLO and their equivalent single layer PET scanners. Our analysis showed that layer misassignments due to inter-crystal scatter in DLO detectors mainly has effect on the full-width at tenth maximum of the CRFs. According to the reconstructed images of the phantom, no significant improvements in the quality of the images were seen when SER was replaced with DLR. The results suggest that layer misidentification in DLO detectors does not play an important role in the quality of the PET images.

Geometry Optimization of a Dual-Layer Offset Detector for Use in Simultaneous PET/MR Neuroimaging

PET scanners suffer from a loss of resolution due to parallax error. Depth-of-interaction detectors, such as dual-layer offset (DLO) detectors, mitigate the loss of resolution. We are designing a DLO brain dedicated PET insert that fits into the Siemens Magnetom 7T brain MR scanner. A wide range of DLO detector geometries was evaluated through Monte Carlo simulations in this paper. The total detector thickness was varied from 10 to 30 mm and for each total thickness, various layer thickness ratios and crystal widths in the range of 0.5–4 mm were examined. The effects of each detector geometry on radial mispositioning of the incident photons, detection efficiency ratio between the two layers, coincidence response functions (CRFs), system spatial resolution, and sensitivity were studied. Optimum layer thickness ratio for each total thickness is discussed. Analysis of CRFs showed that the improvement in the CRF of a DLO PET scanner with a total detector thickness of larger than 10 mm is lower for detectors with a crystal width of smaller than 2 mm than for detectors with a larger crystal width. Estimated spatial resolution and sensitivity of a PET scanner with each suggested detector geometry are also reported.

Performance evaluation of a high-resolution brain PET scanner using four-layer MPPC DOI detectors

A high-resolution positron emission tomography (PET) scanner, dedicated to brain studies, was developed and its performance was evaluated. A four-layer depth of interaction detector was designed containing five detector units axially lined up per layer board. Each of the detector units consists of a finely segmented (1.2 mm) LYSO scintillator array and an 8 × 8 array of multi-pixel photon counters. Each detector layer has independent front-end and signal processing circuits, and the four detector layers are assembled as a detector module. The new scanner was designed to form a detector ring of 430 mm diameter with 32 detector modules and 168 detector rings with a 1.2 mm pitch. The total crystal number is 655 360. The transaxial and axial field of views (FOVs) are 330 mm in diameter and 201.6 mm, respectively, which are sufficient to measure a whole human brain. The single-event data generated at each detector module were transferred to the data acquisition servers through optical fiber cables. The single-event data from all detector modules were merged and processed to create coincidence event data in on-the-fly software in the data acquisition servers. For image reconstruction, the high-resolution mode (HR-mode) used a 1.2 mm2 crystal segment size and the high-speed mode (HS-mode) used a 4.8 mm2 size by collecting 16 crystal segments of 1.2 mm each to reduce the computational cost. The performance of the brain PET scanner was evaluated. For the intrinsic spatial resolution of the detector module, coincidence response functions of the detector module pair, which faced each other at various angles, were measured by scanning a 0.25 mm diameter 22Na point source. The intrinsic resolutions were obtained with 1.08 mm full width at half-maximum (FWHM) and 1.25 mm FWHM on average at 0 and 22.5 degrees in the first layer pair, respectively. The system spatial resolutions were less than 1.0 mm FWHM throughout the whole FOV, using a list-mode dynamic RAMLA (LM-DRAMA). The system sensitivity was 21.4 cps kBq−1 as measured using an 18F line source aligned with the center of the transaxial FOV. High count rate capability was evaluated using a cylindrical phantom (20 cm diameter × 70 cm length), resulting in 249 kcps in true and 27.9 kcps at 11.9 kBq ml−1 at the peak count in a noise equivalent count rate (NECR_2R). Single-event data acquisition and on-the-fly software coincidence detection performed well, exceeding 25 Mcps and 2.3 Mcps for single and coincidence count rates, respectively. Using phantom studies, we also demonstrated its imaging capabilities by means of a 3D Hoffman brain phantom and an ultra-micro hot-spot phantom. The images obtained were of acceptable quality for high-resolution determination. As clinical and pre-clinical studies, we imaged brains of a human and of small animals.

Evaluation of a Sub-Millimeter Resolution PET Detector With a 1.2 mm Pitch TSV-MPPC Array One-to-One Coupled to LFS Scintillator Crystals and Inter-Crystal Scatter Studies With Individual Signal Readout

To develop a small animal positron emission tomography (PET) scanner for whole body imaging of mice, a sub-millimeter resolution PET detector has been constructed and its performance evaluated. This PET detector is composed of a 1.2 mm pitch multi-pixel photon counter (MPPC) $ {8\times 8}$ array with through-silicon via (TSV) structure one-to-one coupled to Lutetium Fine Silicate (LFS) scintillator crystals of size 1.13 mm $ {\times \,\,1.13}$ mm $ {\times \,\,10}$ mm each. To evaluate the intrinsic spatial resolution and the energy resolution in coincidence events, the coincidence response functions (CRFs) of the detector pair, located facing each other at a distance of 80 mm, were measured by scanning a 0.25 mm diameter 22Na point source in 0.1 mm steps. The data were acquired by the computer automated measurement and control (CAMAC) system. The average full width at half maximum (FWHM) of the CRFs was 0.75 mm without point source size corrections. The average FWHM of the energy resolution was 16% at 511 keV. The coincidence timing resolution (CTR) was also evaluated by facing the detector and a fast photomultiplier tube (PMT) coupled with a BaF2 crystal. The expected average FWHM of the CTR between the detectors was 770 ps. Additionally, to improve detector efficiency while suppressing the positioning errors, we propose an interaction-order scheme based on Compton kinematics and adapt it to the same data. We also perform Monte Carlo simulations using GEANT4 10.01 to evaluate the accuracy of the proposed positioning scheme and compare it to that of a centroid calculation scheme. As a result, we demonstrate that the accuracy of the proposed positioning scheme is better than that of a centroid calculation scheme when inter-crystal scatter events occur.

Evaluation of High Density Pixellated Crystal Blocks With SiPM Readout as Candidates for PET/MR Detectors in a Small Animal PET Insert

Arrays of silicon photo-multipliers (SiPMs) are good candidates for the readout of detectors in PET MR inserts due to their high packing density, efficiency, low bias voltage and insensitivity to magnetic fields. In this study we report the readout performance of SensL SiPM arrays in terms of their ability to resolve all elements of pixellated lutetium oxyorthosilicate (LYSO) crystals their energy resolution, and coincidence response function. Two SensL SPMarray-4 were used as light sensors. Two L YSO crystal blocks consisting of a arrays of 8 × 8 1.2 mm × 1.2 mm × 6.0 mm crystals on the lower layer and 7 × 7 1.2 mm × 1.2 mm × 4.0 mm crystals on the upper layer (which is offset by Yz the crystal width) were mounted on the SensL arrays. A second study was performed with single layer arrays of 8 × 8 1.2 mm × 1.2 mm × 10.0 mm crystals. All of the crystals in the dual layer block were easily identified with a peak to valley ratio of 7.2 while the single layer blocks had a peak to valley ratio of 11.2. The crystal dots were more uniformly spaced in the dual layer arrays suggesting that the inter-layer connection affords some useful light sharing. The average energy resolution for all 113 crystals was 16.3 ± 2.3%. The coincidence response to a 0.25 mm Na-22 source in plastic was 0.97 ± 0.12 mm for normally incident gamma rays with the detectors 120 mm apart. These results indicate that it should be possible to achieve a spatial resolution of about one millimetre near the center of the field of view.

Development of a Position-Sensitive Detector for TOF-PET

A position-sensitive detector (PS-detector) for a time-of-flight (TOF) PET system has been developed and evaluated. The PS-detector consists of a flat panel position-sensitive photomultiplier tube (PS-PMT; Hamamatsu R8400-00-M64 MOD) and 2.9 mm times 2.9 mm times 20 mm LYSO crystals arranged in a 16 times 16 array. The PS-PMT has eight stages of metal channel dynode and 8 times 8 multiple anodes in a 52.0 mm square times 14.8 mm high metal can package. In order to compensate for the non-uniform amplitude from the multiple anodes of the PS-PMT, an application specific integrated circuit (ASIC) having 64-channel variable gain amplifiers is attached to the front-end circuits. The ASIC also has a readout circuit to reduce 64-channel outputs down to four position signals and an energy signal. The performance of PS-detector was evaluated for TOF-PET application. The coincidence response functions (CRFs) were measured by scanning a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22</sup> Na point source with 0.4 mm steps across the center of a pair of the PS-detectors. The average FWHM value of the CRFs is 2.3 mm. The energy resolution corresponding to each crystal element at 511 keV is 10.9% on the average. The timing resolutions of the PS-detector at FWHM, with a BaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> reference detector, range from 421 ps to 787 ps, with an average of 505 ps. A new PS-PMT having high QE is now under development. The timing resolutions of the new PS-PMT coupled to a LaBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and a LYSO single crystal are 236 ps FWHM and 366 ps FWHM, respectively, with a BaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> reference detector. We are developing a new TOF-PET system with PS-detectors.

Performance Results of BGO Block Detectors Based on Flat Panel PS-PMT for PET

We have developed a new detector composed of a 20times20 array of 2times2times20 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> (BGO) scintillators and a novel flat panel position sensitive photomultiplier tube(FP-PS-PMT) - Hamamatsu R8400-00-M256. The performance of the block detector was evaluated in terms of applicability to positron emission tomography (PET). The energy resolution was about 23% FWHM for 511 keV gamma-rays. The spatial resolution of the detector was evaluated by measuring the detector response functions (DRFs) for each BGO segment and by measuring the coincidence response functions (CRFs). The average FWHM values of the DRFs were 2.0 mm. For the CRFs performance, the average FWHM values were 2.82 mm at 0deg and 5.20 mm at 30deg.

Design of a Mosaic BGO Detector System for Positron CT

A new scintillation detector system has been designed for application in high spatial resolution positron CT (PCT) system. Many small BGO crystals are closely packed and coupled to a position-sensitive photomultiplier tube (PMT). The detector performance and feasibility for PCT are discussed based on both calculations and experiments. The coincidence response function of approximately 3.2mm FWHM and coincidence time resolution of less than 6ns FWHM were obtained. The results promise the possibility of a new high resolution PCT system.