Hyun Keong Lim

A prototype MR insertable brain PET using tileable GAPD arrays

The aim of this study is to develop a MR compatible PET that is insertable to MRI and allows simultaneous PET and MR imaging of human brain. The brain PET having 72 detector modules arranged in a ring of 330 mm diameter was constructed and mounted in a 3-T MRI. Each PET module composed of 4 × 4 matrix of 3 mm × 3 mm × 20 mm LYSO crystals coupled to a tileable 4 × 4 array Geiger-mode avalanche photodiode (GAPD) and designed to locate between RF and gradient coils. GAPD output charge signals were transferred to preamplifiers using flat cable of 3 m long, and then sent to position decoder circuit (PDC) identifying digital address and generating an analog pulse of the one interacted channel from preamplifier signals. The PDC outputs were fed into FPGA-embedded DAQ boards. The analog signal was digitized, and arrival time and energy of the signal were calculated and stored. LYSO and GAPD were located inside MR bore and all electronics including preamplifiers were positioned outside MR bore to minimize signal interference between PET and MR. Simultaneous PET/MR images of a hot-rod and Hoffman brain phantom were acquired in a 3-T MRI using the MR compatible PET system. The rods down to a diameter of 3.5 mm were resolved in the hot-rod PET image. Activity distribution patterns between white and gray matter in Hoffman brain phantom were well imaged. No degradation of image quality of the hot-rod and Hoffman brain phantoms on the simultaneously acquired MR images obtained with standard sequences was observed. These results demonstrate that simultaneous acquisition of PET and MR images is feasible using the MR insertable PET developed in this study.

Development of brain PET using GAPD arrays.

PURPOSE In recent times, there has been great interest in the use of Geiger-mode avalanche photodiodes (GAPDs) as scintillator readout in positron emission tomography (PET) detectors because of their advantages, such as high gain, compact size, low power consumption, and magnetic field insensitivity. The purpose of this study was to develop a novel PET system based on GAPD arrays for brain imaging. METHODS The PET consisted of 72 detector modules arranged in a ring of 330 mm diameter. Each PET module was composed of a 4 × 4 matrix of 3 × 3 × 20 mm(3) cerium-doped lutetium yttrium orthosilicate (LYSO) crystals coupled with a 4 × 4 array three-side tileable GAPD. The signals from each PET module were fed into preamplifiers using a 3 m long flat cable and then sent to a position decoder circuit (PDC), which output a digital address and an analog pulse of the interacted channel among 64 preamplifier signals transmitted from four PET detector modules. The PDC outputs were fed into field programmable gate array (FPGA)-embedded data acquisition (DAQ) boards. The analog signal was then digitized, and arrival time and energy of the signal were calculated and stored. RESULTS The energy and coincidence timing resolutions measured for 511 keV gamma rays were 18.4 ± 3.1% and 2.6 ns, respectively. The transaxial spatial resolution and sensitivity in the center of field of view (FOV) were 3.1 mm and 0.32% cps/Bq, respectively. The rods down to a diameter of 2.5 mm were resolved in a hot-rod phantom image, and activity distribution patterns between the white and gray matters in the Hoffman brain phantom were well imaged. CONCLUSIONS Experimental results indicate that a PET system can be developed using GAPD arrays and the GAPD-based PET system can provide high-quality PET imaging.

Development of PET/MRI with insertable PET for simultaneous PET and MR imaging of human brain.

PURPOSE The purpose of this study was to develop a dual-modality positron emission tomography (PET)/magnetic resonance imaging (MRI) with insertable PET for simultaneous PET and MR imaging of the human brain. METHODS The PET detector block was composed of a 4 × 4 matrix of detector modules, each consisting of a 4 × 4 array LYSO coupled to a 4 × 4 Geiger-mode avalanche photodiode (GAPD) array. The PET insert consisted of 18 detector blocks, circularly mounted on a custom-made plastic base to form a ring with an inner diameter of 390 mm and axial length of 60 mm. The PET gantry was shielded with gold-plated conductive fabric tapes with a thickness of 0.1 mm. The charge signals of PET detector transferred via 4 m long flat cables were fed into the position decoder circuit. The flat cables were shielded with a mesh-type aluminum sheet with a thickness of 0.24 mm. The position decoder circuit and field programmable gate array-embedded DAQ modules were enclosed in an aluminum box with a thickness of 10 mm and located at the rear of the MR bore inside the MRI room. A 3-T human MRI system with a Larmor frequency of 123.7 MHz and inner bore diameter of 60 cm was used as the PET/MRI hybrid system. A custom-made radio frequency (RF) coil with an inner diameter of 25 cm was fabricated. The PET was positioned between gradient and the RF coils. PET performance was measured outside and inside the MRI scanner using echo planar imaging, spin echo, turbo spin echo, and gradient echo sequences. MRI performance was also evaluated with and without the PET insert. The stability of the newly developed PET insert was evaluated and simultaneous PET and MR images of a brain phantom were acquired. RESULTS No significant degradation of the PET performance caused by MR was observed when the PET was operated using various MR imaging sequences. The signal-to-noise ratio of MR images was slightly degraded due to the PET insert installed inside the MR bore while the homogeneity was maintained. The change of gain of the 256 GAPD/scintillator elements of a detector block was <3% for 60 min, and simultaneous PET and MR images of a brain phantom were successfully acquired. CONCLUSIONS Experimental results indicate that a compact and lightweight PET insert for hybrid PET/MRI can be developed using GAPD arrays and charge signal transmission method proposed in this study without significant interference.

Development of PET using 4 × 4 array of large size Geiger-mode avalanche photodiode

Geiger-mode avalanche photodiode (GAPD) has been demonstrated to be a high performance PET sensor because of high gain, fast response, low excess noise, low bias voltage operation and magnetic field insensitivity. The purpose of this study is to develop a PET for human brain imaging using 4 × 4 array of large size GAPD. PET detector modules were designed and built to develop a prototype PET. The PET consisted of 72 detector modules arranged in a ring with an inner diameter of 330 mm. The LYSO arrays consisted of 4 × 4 array of 3 × 3 × 20 mm3 pixels, which were 1-to-1 coupled to 4 × 4 arrays of 9 mm2 GAPD pixels (SensL, Ireland). The GAPDs were tiled together using flip chip technology on glass and operated at a bias voltage of 32 V for a gain of 3.5 × 106. The signals of the each module were amplified by a 16 channel preamplifier circuit with differential outputs and then sent to a position decoder circuit (PDC), which readout digital address and analog pulse of the one interacted channel from 64 signals of 4 preamplifier boards. The PDC output signals were fed into FPGA-embedded DAQ boards. The analog signal was sampled with 100 MHz, and arrival time and energy of the digitized signal were calculated and stored. The coincidence data were sorted and reconstructed by standard filtered back projection. The energy and time resolution of LYSO-GAPD block detector for 511-keV was 20.4% and 2.4 ns, respectively. The developed PDC could accurately provide the interacted PET signal and reduce the number of the readout channels of PET detector modules based on array type GAPD. The rods down to a diameter of 3.5 mm were resolved in hot-rod phantom image acquired with the brain PET which is similar to the image obtained by Monte Carlo simulation. Activity distribution pattern between white and gray matter in Hoffman brain phantom was well imaged. These results demonstrate that high performance PET could be developed using the GAPD-based PET detectors, analog and digital signal processing methods designed in this work. The prototype brain PET will be tested in a clinical 3T MRI to construct a combined PET-MRI.

A simple and improved digital timing method for positron emission tomography

A simple and improved digital timing method has been developed for positron emission tomography (PET). The so-called initial rise interpolation method is based on an important characteristic of gamma signal: a properly pre-amplified and sampled gamma signal pulse can be characterized to arrive with an initial rise from baseline and then to go up with a maximum rise. Pulse arrival time is obtained by calculating the intersection of the initial rise line with the baseline for each gamma signal pulse. In this study, a FPGA-based data acquisition (DAQ) card was used for data acquisition and processing. We measured coincidence timing resolution of two types (fast and slow) of recently developed 3 mm × 3 mm Geiger mode avalanche photodiodes (GAPDs) using 3 different digital timing methods: initial rise interpolation (IRI), digital CFD and maximum rise interpolation (MRI). Furthermore, simulation has been performed to evaluate effects of pulse rise time, pulse amplitude and front-end noise level on timing resolution estimated by the three digital timing methods. Measured results show that, IRI method provided the best timing resolution for both types of GAPDs: 0.7 ns FWHM for fast GAPD and 1.5 ns for slow GAPD (digital CFD: 1.5 ns and 2.2 ns; MRI: 1.8 ns and 2.7 ns). In accordance with measured results, simulation results also show that IRI method provided the best timing resolution. Based on these experimental results, we concluded that the developed simple and improved digital timing method is reliable and useful for the development of high performance PET.

MR compatible brain PET using tileable GAPD arrays

The aim of this study is to develop a MR compatible PET that is insertable to MRI and allows simultaneous PET and MR imaging of human brain. The brain PET having 72 detector modules arranged in a ring of 330 mm diameter was designed. Each PET module composed of 4 × 4 matrix of 3 mm × 3 mm × 20 mm LYSO crystals coupled to a tileable 4 × 4 array Geigermode avalanche photodiode (GAPD) and designed to locate between RF and gradient coils. Signals of the each module were transferred to preamplifiers using flexible flat cable of 3 m long, and then sent to a position decoder circuit (PDC), which outputs digital address and an analog pulse of the one interacted channel from preamplifier signals. The PDC outputs were fed into FPGA-embedded DAQ boards. The analog signal was digitized, and arrival time and energy of the signal were calculated and stored. All electronics were located outside MR bore to minimize signal interference between PET and MR. Basic performance of the PET components and cross-compatibilities of the PET module and MR were evaluated. Imaging performance of the designed brain PET was investigated using Monte Carlo simulation and experimental measurement. The degradation of PET performance caused by the 3 m long cable and the PDC was negligibly small. No obvious differences of the PET module performance measured inside/outside MR bore were observed. The SNR of various MR sequence phantom images acquired with/without the PET module were also similar. Activity distribution patterns of hot-rod phantoms were well imaged without distortion, and rods down to a diameter of 3.5 mm were resolved in both simulation and experiment. Gray and white matter of the Hoffman brain phantom was also well imaged. Preliminary experimental results demonstrate that MR compatible high quality PET imaging is feasible using the GAPD arrays, electronics, signal processing method and MR insertable PET design schemes developed in this study.

A dual-ended readout PET detector module based on GAPDs with large-area microcells

The use of a dual-ended readout PET detector module based on Geiger-mode avalanche photodiodes (GAPDs) with large-area microcells was proposed to obtain high photon detection efficiency (PDE) and overcome energy non-linearity problems. A simulation study was performed and experimental measurement were taken for the single- and dual-ended PET detector modules consisting of the two types of GAPDs with 50 × 50 μm2 and 100 × 100 μm2 microcells. A Monte Carlo simulation was conducted to predict the number of incident photons impinging on the GAPD entrance surface to estimate the light collection efficiency (LCE) and energy linearity performance. A depth of interaction (DOI) ratio histogram was also obtained. An experimental study was performed to acquire the spectra of different energy γ-rays, and the energy linearity was evaluated by analyzing the photo-peak channels. The simulation results showed that the LCE and energy linearity of the dual-ended PET detector modules were considerably improved compared to the single-ended one, with 100 × 100 μm2 microcell GAPDs. We also estimated that the proposed method can provide accurate (3–4 mm) and uniform DOI resolution. In the experimental measurement, the 511 keV photo-peak channels of the dual-ended PET detector modules were increased 26% and 71% compared to the single-ended one, with 50 × 50 μm2 and 100 × 100 μm2 microcell GAPDs, respectively. The coefficients of determination (R2) were increased from 0.97 to 0.99 and from 0.86 to 0.93 with 50 × 50 μm2 and 100 × 100 μm2 microcell GAPDs, respectively. The results of this study demonstrate that the dual-ended readout scheme using GAPDs with large-area microcells provides high LCE and DOI information with minimized energy non-linearity. This will enable investigators to configure PET detector modules with high sensitivity and resolution.

Development of filtering methods for PET signals contaminated by RF pulses for combined PET-MRI

This paper presents the development of filtering methods for positron emission tomography (PET) signals contaminated by radio frequency (RF) pulses for combined PET and clinical 3-T magnetic resonance imaging (MRI). The filtering methods include software, hardware, and hybrid correction methods. In the software correction method, PET signals are assessed, and valid signals are identified based on the characteristics of a typical PET signal using Field-Programmable Gate Array (FPGA)-based programming. The hardware correction method makes use of differential-to-single-ended and low-pass filter circuits for PET analog signals. The hybrid correction method involves the sequential application of both the hardware and software methods. Both valid and contaminated PET signals are measured with an oscilloscope. An evaluation is then made of the performance (energy resolution, photopeak channel, total counts, and coincidence timing resolution) of the PET detector modules with and without various MR sequences (gradient echo, spin echo T1 sequence). For all correction methods, the energy resolution, photopeak position, and coincidence timing resolution with MR sequences are similar (<; 3%) to those without MR sequences. However, the total count of each module depends greatly on the method applied. The hybrid correction method displays an ability to preserve (<; 1%) the total counts of the modules during various MR sequences. The results show that this filtering method, which can reject noise signals and reduce count loss while preserving the valid analog signals of MR sequences, is reliable and useful for the development of simultaneous PET-MRI.

MR insertable brain PET using tileable GAPD arrays

The aim of this study is to develop a MR compatible PET that is insertable to MRI and allows simultaneous PET and MR imaging of human brain. The brain PET having 72 detector modules arranged in a ring of 330 mm diameter was constructed and mounted in a 3-T MRI. Each PET module composed of 4 × 4 matrix of 3 mm × 3 mm × 20 mm LYSO crystals coupled to a tileable 4 × 4 array Geiger-mode avalanche photodiode (GAPD) and designed to locate between RF and gradient coils. GAPD output charge signals were transferred to preamplifiers using flat cable of 3 m long, and then sent to position decoder circuit (PDC) identifying digital address and generating an analog pulse of the one interacted channel from preamplifier signals. The PDC outputs were fed into FPGA-embedded DAQ boards. The analog signal was digitized, and arrival time and energy of the signal were calculated and stored. LYSO and GAPD were located inside MR bore and all electronics including preamplifiers were positioned outside MR bore to minimize signal interference between PET and MR. Simultaneous PET/MR images of a hot-rod and Hoffman brain phantom were acquired in a 3-T MRI using the MR compatible PET system. The rods down to a diameter of 3.5 mm were resolved in the hot-rod PET image. Activity distribution patterns between white and gray matter in Hoffman brain phantom were well imaged. No degradation of image quality of the hot-rod and Hoffman brain phantoms on the simultaneously acquired MR images obtained with standard sequences was observed. These results demonstrate that simultaneous acquisition of PET and MR images is feasible using the MR insertable PET developed in this study.

Characterization of cross-compatibility of small animal insertable PET and MRI

The purpose of this study is cross-compatibility of PET and MRI was characterized to explore the optimal method overcoming possible interferences between them. MR phantom images were acquired by placing the PET components inside and outside RF-coil in 7-T MRI to examine the effect of the relative position of PET detector and RF-coil on MRI. Theoretical evaluation of shielding effectiveness (SE) of plate and mesh Cu shielding were calculated to characterize criteria for reducing mutual interference between PET and MRI. Experimental studies were performed that MR image quality as a function of the area and thickness of Cu plate was examined to characterize the effect of plate shielding method. Also, MR images as a function of the open area of Cu mesh were acquired. Moreover, it was proposed and evaluated to minimize the cross-interference that only crystal and photo-sensor are placed inside MRI bore and the PET signals were transmitted to the signal amplifier circuits using long cable for developing hybrid PET-MR imaging system. Significant artifacts were generated on MRI by inserting the PET module inside RF coil, but obvious degradation of the MR image quality was not observed by placing the PET module outside RF-coil. In theoretical evaluation, Cu plate shielding need to be thicker than 15~30 μm and Cu mesh shielding need to be thicker than 0.7/1.5/ 5.5-mm for minimizing the mutual interference between PET and MRI when the hole sizes of mesh were 0.5/ 1/ 3-mm, respectively. Cu thickness did not affect the homogeneity but SNR of MR images changed from 150 to 110 when Cu thickness changed from 0 to 200 /an. The SNR and homogeneity were considerably changed from 270 to 29 and from 88 to 69 when 160 cnf and 640 cnf Cu area were employed. The temperature of Cu was risen by ~ 1TC when large area shielding (640 cnf) was used. MR image quality was not improved by increasing open area in GRE sequence. Moreover, this study verified charge signal transmission method using long cable, and it was feasible to acquire artifacts-free PET-MR images without any shielding material by placing the amplifier outside MR bore. In summary, cross-interferences between two imaging modalities would be minimized by placing the PET module outside RF-coil and inside gradient coil of MRI. In Cu plate shielding, area is a potentially bigger risk factor than the thickness in deteriorating MRI SNR, homogeneity and stable temperature operation. Cu mesh shielding with <4% open area will maximize the cross-compatibility of PET and MRI. Charge signal transmission using long cable between PET detector and preamplifier allows maximizing the cross-compatibility of PET and MRI and this approach is preferable because it allows locating only PET detector inside MRI without any shielding material.