Qingyang Wei

A neighborhood standard deviation based algorithm for generating PET crystal position maps

Positron emission tomography (PET) is typically based on 2-D array of scintillation crystals coupled to photon detector and decoded by the Anger-logic. The decoded result is a pseudo-position of the gamma interaction. A crystal position map (CPM) generated from the flood histogram is used as a crystal look-up table (CLT) to assign each pseudo-position to a specific crystal. It is crucial that the accuracy of CPMs affects the detectors spatial resolution. In this paper, we developed a neighborhood standard deviation (NSD) based algorithm for generating CPM. We first calculated the NSD of each pixel in the flood histogram including the x and y directions. NSD maps have strips whose peaks highly correspond to the valley of the flood histogram. The peaks were identified by fitting the profiles of NSD to Gaussian mixture functions using nonlinear least-square method. Using the peaks, the CPM was generated by a scan line algorithm. The proposed algorithm was applied in an animal PET system. 115 of 120 detector blocks can be automatically segmented in ~1000 s. A hot rod phantom experiment was performed, and the reconstruction results showed that the one with CPM generated by NSD based automatic method achieved higher spatial resolution than the one with CPM generated by manual segmentation. We concluded that the proposed method is fast, robust and high accuracy.

A novel sub-millimeter resolution PET detector with TOF capability

We have developed a PET detector module based on sub-millimeter lutetium oxyorthosilicate (LSO) crystal block and prototype Silicon Photomultiplier (SiPM) arrays with additional fast timing output. The LSO array consists of 20×20 crystals of which each pixel is 0.78 × 0.78×3 mm3 and the pitch is 0.8 mm due to the reflector between pixels. The FM SiPM array and FB SiPM array are arranged 4×4 with 4×4mm2 size for each pixel and 0.2mm gap between adjacent pixels. Through dual readout of both fast and standard output signals from this new SiPM, good energy and timing performance can be achieved at the same time. Through multiplexing and summing scheme and charge division resistor network, sixteen channels of fast output and sixteen channels of standard output are reduced to one fast timing signal and four energy signals, making the detector block compact and scalable in building PET scanners. The performance of FM and FB detector module was evaluated. 16×16 LSO crystals were resolved in the flood image and the edge crystals could be identified if a light guide was used. The average energy resolution of FM detector and FB detector were 15.3% ±1.4% and 11.3% ± 0.7%,respectively, at 511 KeV. Through time pick-off method of leading edge discriminator (LED), the average coincidence resolving timing (CRT) of the block against a 3x3x20mm3 coupled with a FB30035 pixel were 401± 10 ps for FM detector and 303 ± 16 ps for FB detector. The average block-to-block CRT were estimated to be 495 ps for FM detector and 324 ps for FB detector. The FB array has demonstrated a better performance due to its much higher PDE at 420nm wavelength than FM array. We conclude that the proposed design is feasible for developing sub-millimeter resolution TOF PET detectors.

Design and sampling completeness evaluation of scanning orbits in multi-pinhole small animal SPECT imaging

For a multi-pinhole SPECT system, it is important to choose an optimal scanning orbit to fulfill the sampling completeness requirement. In this study, we propose a novel four-degree-of-freedom (4-DOF) orbit which combines a conventional helical scanning path and a superimposed periodic reciprocating 3×3-pioint shift movement in transaxial plane. An Orlov-sphere-based numerical evaluation method was used and a sampling completeness percentage (SCP) index was introduced to access the sampling completeness for the merits of scanning orbits over entire field of view (FOV) in terms of sampling completeness. By comparing distribution of sampling completeness over the FOV of conventional circular orbit, helical orbits, and 4-DOF orbit applied to an experimental small animal SPECT system, it is shown that the novel 4-DOF orbit has better sampling performance than conventional orbits. The evaluation results of different orbits and the mouse whole-body imaging experiment demonstrate that this method is feasible and applicable for guiding the design of scanning orbits and determining the merits of the orbits.

Measure PET detector performance with the intrinsic radioactivity of scintillator

One common configuration of PET detector modules is an array of scintillators, usually LYSO or its variant, coupled to multiple photomultiplier tubes (PMTs). The objective of this work was to test the feasibility of measuring the performance of such detector module with the intrinsic radioactivity from the 176Lu in the scintillator for its convenience and uniformity. The performance indices of a detector module include the individual crystal energy resolution and relative crystal light collection efficiency. They are indispensable in detector and system development. As no prominent photoelectric-peak exists in the 176Lu energy spectrum, it is not straight forward to obtain the detector performance indices from the acquired data. We developed a method to derive the crystal energy resolution and the relative crystal light collection efficiency by fitting the experiment crystal energy spectrum with simulated crystal energy spectrum. The detector module of this study consisted of a 6 by 6 L YSO scintillator array coupled to 4 PMTs. A 2-minute and a 30-minute two dimensional position map (2Dmap) were acquired for evaluating data with short and long scans. With the short acquisition data, we can generally evaluate the performance of the detector. With the long acquisition data, we segmented the 2Dmap and obtained the crystal energy resolutions to be 25% to 45% and the relative crystal light collection efficiency to be 0.43 to 1. We concluded that the 176Lu background radiation could be used to evaluate LYSO and its variant based PET detector module.

Development of multi-channel fast SiPM readout electronics for clinical TOF PET detector

Silicon photomultipliers (SiPM) are rapidly adapted in simultaneous time of flight (TOF) PET and MRI imaging system in recent years due to its fast timing response and immunity to magnetic field. The aim of this work is to develop multi-channel fast SiPM readout electronics for clinical TOF PET detector. The detector module is assembled by directly coupling a 12×12 LYSO crystal block to an 8×8 Sensl FB30035 SiPM array. We are developing two approaches of compact readout electronics to handle the large number of SiPM output channels. One is a low noise and high bandwidth, 128-channel ASIC to readout the 64 fast outputs and 64 standard outputs of the 8×8 SiPM array. The other approach is a conventional electronics based multiplexing readout scheme using discrete components. A charge division resistor network reduced 64 channels of standard output to four energy signals, and multiplexing and summing circuits reduced 64 channels of fast output to one timing signal, making the detector block highly compact and scalable. The ASIC design has been finished and its ready for tape-out. Preliminary performance of the detector module readout by conventional electronics was evaluated. 12×12 LYSO crystals were well resolved and identified in the flood image and the average energy resolution was 15.6% ±1.3%. For single crystal pair, 240ps coincidence resolving timing (CRT) was achieved. It was dropped down to 500ps for detector block to single crystal configuration, due to the combination effects of: 1) degradation of fast signal rising edge by output capacitance addition of SiPMs and 2)large white electronic noise and ultrahigh dark count rate. The readout design will be further optimized by increasing the signal bandwidth and reducing noise to get better timing performance at a cost of increasing circuit complexity to a reasonable level. The performance of ASIC readout will be evaluated and good timing performance is expected due to the individual pixel timing pick-off.

Geometrical calibration of a PET-scanner-based multi-pinhole SPECT using LYSO background radiation

In previous work, we have developed a cost-effective small-animal SPECT imaging system by adding a multi-pinhole cylinder collimator into an animal PET scanner. To achieve high-quality reconstructed SPECT image, accurate geometrical calibration of the collimator is critical. The pinhole position in PET-scanners coordinate system (PSC) could be factorized into two parts: pinhole position in collimators coordinate system (fixed, only need one measurement) and collimator position in PSC (would vary after times of use after times of use). In this paper, we propose a novel geometrical calibration method for the second part based on the coincidental beta-gamma events of LYSO background radiation of the PET scanner. To obtain sufficient conditions for describing the collimator in PSC, a mark ball was added to one end of the collimator and imaged with collimator by LYSO background radiation simultaneously. With the reconstructed image of the collimator and the add-on mark ball, the axis of the collimator and the center of the mark ball were derived using least-squares fitting and centroid calculation, individually. Monte-Carlo simulation studies with five groups of different positions of collimator were implemented and geometrical parameters were calibrated. The proposed method was implied on a real system. Both simulation and experiment studies show that the proposed method could achieve high accuracy for parameter calibration. In conclusion, it is a practical and efficient way to perform SPECT collimator calibration by using PET scanners LYSO background radiation in the studied PET/SPECT system.

Accurate image reconstruction based on standard-Gaussian-model fitted system matrix in multipinhole small animal SPECT imaging

For multi-pinhole small animal SPECT imaging, system matrix (SM) is critical for accurate reconstruction. It is typically achieved by analytical calculation, experimental measurement, or Monte-Carlo (MC) simulation. MC simulation method is relatively accurate and avoids long-time measurements. However, to achieve high resolution for small animal reconstruction, SM should be extremely huge, and would have severe noise produced when simulation time is limited.

Development and assessment of statistical iterative image reconstruction for CT on a small animal SPECT/CT dual-modality system

We developed a statistical iterative CT image reconstruction software for a newly constructed high-resolution small animal SPECT/CT dual-modality system, and assessed its performance at different radiation exposure levels. The objective of this work was to preserve or improve reconstructed image quality at either the same or reduced animal x-ray radiation exposure. The SPECT/CT system used a single detector for both the CT and SPECT modalities that consists of a micro-columnar CsI(TI) phosphor, a light image intensifier (LII) and a CCD sensor. The CT reconstruction software was based on the ordered-subset-convex (OSC) algorithm, and the system matrix was calculated through a ray-driven approach. A self-calibration method was implemented to calculate the offset of the axis of rotation (AOR), an important geometry parameter of the system. An endovascular stent was imaged to evaluate the high resolution performance of the statistical reconstructed image. A sacrificed mouse was scanned at different exposure levels to assess the effect of statistical noise on the image. The mouse studies were reconstructed with both the statistical reconstruction software and a filtered back-projection (FBP) program. The images were assessed and compared by contrast to noise ratio (CNR) in the region of interest. The images yielded by the statistical reconstruction software were artifact free and show superior noise performance to those from FBP reconstruction at different radiation exposure levels. The statistical reconstructed images with reduced exposure showed obviously higher image quality than those from FBP reconstruction at full exposure.

Optical simulation of a 9×9 LYSO block detector with PQS technology using GATE

Design of the light sharing mechanisms has a significant influence on the performance of PET block detector. Monte Carlo simulation plays an important role in PET detector design and optimization. In this work, GATE is used to simulate the transportation processes of gamma rays and scintillated optical photons in a PMT-based 9×9 L YSO block detector of a self-developed animal PET system. The aim of this work is to optimize the optical parameters of the block detector design. Scintillation photons are generated and transported in the L YSO crystal array and finally detected by the 4 PMTs. Based on 2D Anger logic, the position of the crystal from which optical photons originate is calculated from the number of optical photons detected by the 4 PMTs. Different simulation parameters such as crystal side and bottom roughness and optical coupling pattern between crystals and reflecting films are tested in the simulation and 2D-MAPs of the detector are obtained. By comparing the 2D-MAPs acquired, we conclude that crystal side surface roughness has a significant influence on PET detector performance and a relatively rougher crystal side surface is recommended for applications while the detector performance is not sensitive to crystal bottom surface roughness and optical coupling pattern between crystals and reflecting films is key parameter for PET detector design as it determines the amounts of light sharing. The developed method is feasible for facilitating PET detector design and performance prediction.

Dedicated brain PET system of PET/MR for brain research

This work is to replace PET ring in human brain PET/MR system with a dedicated wearable PET insert, aimed at improving both patient feasibility and system performance for brain imaging. The designed PET/MR system includes two parts: the inside parts, including a radio frequency (RF) coil and PET ring, are mounted on patient’s head, and the outside part, a MR imager, is dependent of patient. The RF coil is the innermost layer, surrounded by an outer PET-ring layer. They are supported by a MRcompatible structure. And both RF coil and PET detectors are placed inside a standard clinical 3-T MR imager. From the design of the system we can infer that some advantages can be achieved. First, high sensitivity will be achieved with the same amount crystals as the PET ring is more close to region-of-interest area, at a reduced cost. Second, by using a 2-layer depth of interaction (DOI) detector, the parallax effect can be minimized. The resolution will benefit from short positron range caused by magnetic field and smaller ring diameter will also reduce the effect of non-collinearity. Thirdly, as the PET ring is mounted on head, impact of patient motion will be reduced.