Shaohui An

Engineering and performance (NEMA and animal) of a lower-cost higher-resolution animal PET/CT scanner using photomultiplier-quadrant-sharing detectors.

The dedicated murine PET (MuPET) scanner is a high-resolution, high-sensitivity, and low-cost preclinical PET camera designed and manufactured at our laboratory. In this article, we report its performance according to the NU 4-2008 standards of the National Electrical Manufacturers Association (NEMA). We also report the results of additional phantom and mouse studies. Methods: The MuPET scanner, which is integrated with a CT camera, is based on the photomultiplier-quadrant-sharing concept and comprises 180 blocks of 13 × 13 lutetium yttrium oxyorthosilicate crystals (1.24 × 1.4 × 9.5 mm3) and 210 low-cost 19-mm photomultipliers. The camera has 78 detector rings, with an 11.6-cm axial field of view and a ring diameter of 16.6 cm. We measured the energy resolution, scatter fraction, sensitivity, spatial resolution, and counting rate performance of the scanner. In addition, we scanned the NEMA image-quality phantom, Micro Deluxe and Ultra-Micro Hot Spot phantoms, and 2 healthy mice. Results: The system average energy resolution was 14% at 511 keV. The average spatial resolution at the center of the field of view was about 1.2 mm, improving to 0.8 mm and remaining below 1.2 mm in the central 6-cm field of view when a resolution-recovery method was used. The absolute sensitivity of the camera was 6.38% for an energy window of 350–650 keV and a coincidence timing window of 3.4 ns. The system scatter fraction was 11.9% for the NEMA mouselike phantom and 28% for the ratlike phantom. The maximum noise-equivalent counting rate was 1,100 at 57 MBq for the mouselike phantom and 352 kcps at 65 MBq for the ratlike phantom. The 1-mm fillable rod was clearly observable using the NEMA image-quality phantom. The images of the Ultra-Micro Hot Spot phantom also showed the 1-mm hot rods. In the mouse studies, both the left and right ventricle walls were clearly observable, as were the Harderian glands. Conclusion: The MuPET camera has excellent resolution, sensitivity, counting rate, and imaging performance. The data show it is a powerful scanner for preclinical animal study and pharmaceutical development.

A lower-cost high-resolution LYSO detector development for positron emission mammography (PEM)

We have developed several positron emission tomography (PET) cameras using photomultiplier-quadrant-sharing (PQS) geometry; in which each detector block is optically coupled to four round PMTs, and each PMT is shared by four blocks. Although PQS design reduces the cost of high-resolution PET systems, when the camera consists of detector panels that are made up of square blocks, half of the PMT’s sensitive window remains unused at the detector panel edge. Our goal was to develop a LYSO detector panel, which minimize the unused portion of the PMTs while maintaining the low cost, high resolution, and high sensitivity of positron emission mammography (PEM) camera. Our plan was to modify PQS design by using elongated blocks at panel edges and square blocks in the inner area. For elongated blocks, symmetric and asymmetrical reflector patterns were developed, and PQS and PMT-half-sharing (PHS) arrangements were implemented in order to obtain a suitable decoding. The performance of our blocks was good, producing good crystal-decoding and average energy resolution. Using a modified PQS geometry and asymmetric block design, we reclaimed the unused PMT region at detector panel edges, thereby increasing field-of-view and overall detection sensitivity and minimizing undetected breast region near the chest wall. This lower cost design using regular round PMT allowed us to use larger detector panels and hence to build a lower-cost, high-resolution, high-sensitivity PEM camera.

Monte Carlo simulation study on the time resolution of a PMT-quadrant-sharing LSO detector block for time-of-flight PET

We developed a detailed Monte Carlo simulation method to study the time resolution of detector for time-of-flight positron emission tomography (TOF PET). The process of gamma ray reaction in detector, scintillation light emission and transport inside the detector, the photoelectron generation and anode signal generation in the photomultiplier tube (PMT), and the electronics process of discriminator are simulated. We tested this simulation method using published experimental data, and found that it can generate reliable results. Using this method, we simulated the time resolution for a 13×13 detector block of 4×4×20 mm3 lutetium orthosilicate (LSO) crystals coupled to four 2-inch PMTs using PMT-quadrant-sharing (PQS) technology. We analyzed the effects of several factors, including the number of photoelectrons, light transport, transit time spread (TTS), and the depth of interaction (DOI). The simulation results indicated that system time resolution of 300–350ps should be possible with currently available fast PMTs. This simulation method can also be used to simulate the time resolution of other detector design method.

A Lower-Cost High-Resolution LYSO Detector Development for Positron Emission Mammography (PEM)

In photomultiplier-quadrant-sharing (PQS) geometry for positron emission tomography applications, each PMT is shared by four blocks and each detector block is optically coupled to four round PMTs. Although this design reduces the cost of high-resolution PET systems, when the camera consists of detector panels that are made up of square blocks, half of the PMTs sensitive window remains unused at the detector panel edge. Our goal was to develop a LYSO detector panel which minimizes the unused portion of the PMTs for a low-cost, high-resolution, and high-sensitivity positron emission mammography (PEM) camera. We modified the PQS design by using elongated blocks at panel edges and square blocks in the inner area. For elongated blocks, symmetric and asymmetrical reflector patterns were developed and PQS and PMT-half-sharing (PHS) arrangements were implemented in order to obtain a suitable decoding. The packing fraction was 96.3% for asymmetric block and 95.5% for symmetric block. Both of the blocks have excellent decoding capability with all crystals clearly identified, 156 for asymmetric and 144 for symmetric and peak-to-valley ratio of 3.0 and 2.3 respectively. The average energy resolution was 14.2% for the asymmetric block and 13.1% for the symmetric block. Using a modified PQS geometry and asymmetric block design, we reduced the unused PMT region at detector panel edges, thereby increased the field-of-view and the overall detection sensitivity and minimized the undetected breast region near the chest wall. This detector design and using regular round PMT allowed building a lower-cost, high-resolution and high-sensitivity PEM camera.

The system design, engineering architecture and preliminary results of a lower-cost high-sensitivity high-resolution Positron Emission Mammography camera

A lower-cost high-sensitivity high-resolution Positron Emission Mammography (PEM) camera is developed. It consists of two detector modules with the planar detector bank of 12×20 cm2. Each bank has 60 low-cost PMT-Quadrant-Sharing (PQS) blocks arranged in a 10×6 array with two types of geometries. One is the symmetric 19.36×19.36 mm2 block made of 1.5×1.5×10 mm3 LYSO crystals in 12×12 array. The other is the 12×13 asymmetric elongated block made of 1.5×1.9×10 mm3 crystals. One row (10) of the elongated blocks are used along one side of the bank to reclaim the half empty PMT photocathode in the regular PQS design to reduce the dead area at the edge of the module. The bank has a high overall crystal packing fraction of 88%, which results in a very high sensitivity. Mechanical design and electronics have been developed for low-cost, compactness and stability. Each module has four Anger-HYPER decoding electronics that can handle a count-rate of 3 Mcps for singles. A simple two-module coincidence board with a hardware delay window for accidental events has been developed with an adjustable window of 6–15 ns. Some of the performance parameters have been studied by preliminary tests and simulations, including the decoding quality and energy resolution of the detectors, and the point source sensitivities, spatial resolutions and phantom images of the whole system.

The systematic errors in the random coincidence estimation using a delayed window

With the advance of new technology, PET systems nowadays have much higher sensitivities than before with the adoption of faster scintillators, faster electronics, larger solid angle detectors and full 3D data collection mode. All of these lead to the increasing of the random coincidences that will affect the image quality. Accurate random correction is important for the quantitative imaging in PET applications. Delayed window method is widely used for random coincidences estimation. However, this method is not mathematically accurate. The difference between the delayed window randoms and the prompt window randoms can be significant under certain conditions. We studied the details of these differences using Monte Carlo simulations since this is the only way to separate the real random events from the true and scattered events. Three phantoms are used in the study, including the NEMA1994 20-cm long phantom, NEMA2001 70-cm long phantom and a 10-cm long, 1-cm diameter small cylinder phantom. The PET system used in this study is a 12-module LYSO camera with 20-cm axial FOV and 54-cm ring diameter to simulate a brain PET system. Two policies for the multiple events handling are used: reject-all-multiples, and take-all-goods. Random rates and NECR as the function of activities from prompt window and delayed window are obtained. The simulation results show that 1) The take-all-goods policy will underestimate the randoms rate and the reject-all-multiples will overestimate the randoms, which results overestimation of NECR for take-all-goods policy and underestimation of NECR for reject-all-multiples policy; 2) The discrepancy is more significant for smaller phantom than bigger phantom. This study demonstrates the intrinsic discrepancy for random coincidence estimation by the delayed window method. When the phantom is relatively small compare to the FOV dimension of the PET system, the discrepancy is big enough to produce non-negligible errors.

High-definition positron emission tomography using restored sinograms

Resolution recovery becomes an important technology for high-resolution and high-sensitivity Positron Emission Tomography (PET) cameras. High-definition PET images can be created after correcting detector parallax errors using point-spread functions. Many studies have demonstrated that better resolution and quantitative accuracy can be achieved by iterative reconstruction method with system respond function (SRF). In this paper, we use the SRF to restore sinograms first and then reconstruct images using the restored sinograms with conventional reconstruction methods. SRF is derived from Monte Carlo simulations with GEANT software. We modeled the SRF for the detector geometry of our prototype animal PET camera (RRPET) and human PET camera (HOTPET) for the evaluation. Na-22 point source experiment results demonstrated that resolution degradations in both cameras were eliminated. As expected, with sinogram restoration method the image quality was dramatically improved in the small animal PET system evaluated with micro-deluxe phantom and rat data. However, there was no improvement of Hoffman brain-phantom image in the human PET with rotatable gantry, which may relate to the detector gaps and scatter background existing in the original sinograms.

An Accurate Timing Alignment Method With Time-to-Digital Converter Linearity Calibration for High-Resolution TOF PET

Accurate PET system timing alignment minimizes the coincidence time window and therefore reduces random events and improves image quality. It is also critical for time-of-flight (TOF) image reconstruction. Here, we use a thin annular cylinder (shell) phantom filled with a radioactive source and located axially and centrally in a PET camera for the timing alignment of a TOF PET system. This timing alignment method involves measuring the time differences between the selected coincidence detector pairs, calibrating the differential and integral nonlinearity of the time-to-digital converter (TDC) with the same raw data and deriving the intrinsic time biases for each detector using an iterative algorithm. The raw time bias for each detector is downloaded to the front-end electronics and the residual fine time bias can be applied during the TOF list-mode reconstruction. Our results showed that a timing alignment accuracy of better than ±25 ps can be achieved, and a preliminary timing resolution of 473 ps (full width at half maximum) was measured in our prototype TOF PET/CT system.

Design study of a lower-cost ultrahigh-resolution high-sensitivity PET for neuroimaging

Current clinical PET with 4-6 mm intrinsic resolution (6-9 mm practical) limits many important brain studies. The objective of this study is to use our existing technology for realizing an ultrahigh resolution high-sensitivity PET with a lower cost for neuroimaging. This proposed neuro-PET has a 54-cm detector ring diameter, a large 21-cm axial field of view (AFOV) for capturing the whole brain and carotid arteries for acquiring arterial input function for quantitating imaging. The system has 131,856 lutetium yttrium orthosilicate (LYSO) small detectors (1.4 × 1.4 mm2) coupled to 924 photomultiplier tubes (19-mm) with PMT-quadrant-sharing (PQS) design. We propose to use 11-mm shallow detectors to reduce the depth-of-interaction (DOI) image blurring and to reduce the costly LYSO material by half, and use the effective sensitivity gained by the time-of-flight (TOF) to compensate for the sensitivity loss by the shallow detectors. Despite the tiny 1.4 × 1.4 mm2 detector cross-section restricting light output, our preliminary study shows that a 550-ps (FWHM) TOF time resolution was achieved (enabled by the excellent timing characteristic of our PQS detectors). Hence, there would a gain of 2x effective sensitivity for the 18-cm brain. Monte Carlo simulations show transaxial image resolutions of 1.58 and 2.06 mm at 1, and 9 cm respectively, without resolution-recovery algorithm, demonstrating slow DOI degradation. The large AFOV and small detector ring diameter give this system another 2x higher sensitivity than the typical clinical PET. This camera will use our existing detector technology, transformable gantry, and production-engineering tooling developed in the last few years. The production cost of this ultrahigh resolution neuro-PET would be less than

A High-Resolution Time-of-Flight Clinical PET Detection System Using a Gapless PMT-Quadrant-Sharing Method

400K. This ultrahigh-resolution PET with resolution approaching MRI and CT, also allows more meaningful image correlation, and provides ultrahigh resolution functional imaging for small brain nuclei structures, which would open new doors for functional neuroimaging and neuroscience. The lower cost, larger AFOV and higher sensitivity would facilitate the use of this dedicated brain PET.