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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

A MR compatible PET insert has been developed in our group employing SiPM and a charge signal transmission method that transmits the charge signal of SiPM to subsequent electronics through 4 m long cables. The previously developed PET/MRI system, however, had several limitations, such as the instability of PET performance due to temperature variations inside PET gantry and the degradation of SNR in the MR image due to the inserted PET. The purpose of this study was to optimize the design of PET/MRI system to maintain a stable temperature inside the PET gantry and to minimize mutual interference between PET and MRI. The PET insert was composed of 9 PET detector blocks that were arranged in a circle around the subject with 390 mm diameter and 60 mm axial FOV. Each PET detector block was composed of a 4 × 8 array of detector modules consisted of 4 × 4 array of SiPM and 4 × 4 array of LYSO. The PET detectors were cooled using a water chiller system to compensate temperature fluctuations caused by eddy currents. PET analog electronics were enclosed in an aluminum box and located 50 cm apart from the MR bore. The FPGA-embedded DAQ and power supply were installed at outside MR room. PET performance was measured inside and outside MRI using spin echo and gradient echo sequences. MRI performance was evaluated with and without the PET insert. Human brain imaging was also acquired to evaluate the performance of simultaneous PET and MR imaging. No significant degradation of the PET performance caused by MR was observed when the PET was operated under various MR imaging sequences. The homogeneities and SNRs of MR images obtained with and without the PET insert installed inside the MR bore were similar and changed by only 1% and 3%, respectively. High quality simultaneous PET and MR images of human brain were successfully acquired. Experimental results indicate that simultaneous PET and MR imaging is feasible without considerable mutual interference between PET and MRI by employing the PET/MRI proposed in this study.

SIMULTANEOUS functional and anatomical imaging using PET-MR system has been reported to be useful in preclinical research to investigate various in vivo biological processes [1-3]. The purpose of this study was to develop a MR compatible small animal PET insert and to evaluate the performance of the developed PET and the influence of the PET on MRI performance.

The purpose of this study was to develop a dual-modality PET/MRI with insertable PET for simultaneous imaging of human brain. The PET insert consisted of 18 detector blocks arranged in a ring of 390 mm diameter with 60 mm axial FOV. Each detector block was composed of 4 × 4 matrix of detector modules, each of which consisted of a 4 × 4 array LYSO coupled to a 4 × 4 GAPD array. The PET gantry was shielded with gold-plated conductive fabric tapes with 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 0.24 mm thickness. The position decoder circuit and FPGA-embedded DAQ modules were enclosed in an aluminum box with 10 mm thickness and located at the rear of the MR bore inside MRI room. A 3-T human MRI system with a Larmor frequency of 123.7 MHz and bore inner diameter of 60 cm was used for PET/MRI hybrid system. A custom-made radio frequency (RF) coil with inner diameter of 25 cm was fabricated. The PET was positioned between gradient and the RF coils. PET performance was measured outside and inside MRI scanner with spin echo, turbo spin echo and gradient echo sequences. SNR and of MR phantom image were also measured with and without PET insert. The stability of developed PET insert was evaluated and simultaneous PET and MR images of brain phantom were acquired. No significant degradation of the PET performance caused by MR was observed when the PET was operated with various MR imaging sequences. Uniformity and SNR of MR phantom image were degraded about 1% and 7% by the PET insert, respectively. The change of gain of PET detector was

A Geiger-mode avalanche photodiode (GAPD) is actively studied silicon-based photo-detector that is a promising photo-sensor for PET because of various advantages, such as gain, compact size, fast response and magnetic field insensitivity. Previously, we have developed a brain PET employing GAPD-based PET detector block, charge signal transmission method via 4 m long cable, high density position decoder circuit (PDC) and digital signal processing with FPGA-embedded ADC boards. The purpose of this study was to evaluate the performance of the developed brain PET system. The PET consists of 18 detector blocks, each of which comprises 4 × 4 GAPDs and 4 × 4 LYSO arrays. The output signals of PET detector block were amplified by preamplifier circuits and then transferred to the PDC. The PDC was used to identify the one valid channel among 256 channels of PET detector blocks and to output digital address analog pulse corresponding to the valid channel. Three FPGA-embedded DAQ boards were used to digitize analog output signals of the PDCs. The gain uniformity, spatial resolution, sensitivity, stability and count rate performance were measured. Tomographic image of 3D Hoffman brain phantom was also acquired to evaluate imaging capability of the PET. Normalized average photo-peak positions of PET detector block were 1.0 ± 0.14 without the gain correction and 1.0 ± 0.03 with the gain correction. Spatial resolution and sensitivity in the center of field of view were 3.1 mm and 0.8%, respectively. The average scatter fraction was 0.4 with lower energy threshold of 350 keV. The maximum true count rate and maximum NECR were measured 43.3 kcps and 6.5 kcps at an activity concentration of 16.7 kBq/ml and 5.5 kBq/ml, respectively. Activity distribution patterns between white and gray matter in the Hoffman brain phantom were well imaged. These results demonstrate that the developed GAPD based brain PET system is useful for high quality brain PET imaging.

There has been great interest in the development of combined PET/MR, a useful tool for both functional and anatomic imaging. We have developed a proof-of-principle MR compatible PET, employing a design concept that uses GAPD arrays as a PET photo-sensor and charge signal transmission method for human brain imaging. The purpose of this study was to design the 2nd version of brain PET with an extended axial field-of-view (FOV) and to evaluate its initial performance. The PET consisted of 18 detector blocks arranged in a ring of 390 mm diameter with 60 mm axial FOV. Each detector block was composed of a 4 × 4 matrix of detector module, each of which consisted of a 4 × 4 array L YSO coupled to a 4-side tileable 4 × 4 GAPD array. The PET gantry was shielded with gold-plated conductive fabric tapes with a thickness of 0.1 mm. PET signals were fed into the position decoder circuit (PDC) generating the digital address and analog pulse of the one interacted channel among the 256 output channels of the detector block, using a 4 m long flat cable. Commercial DAQ modules were used to digitize analog output signals of the PDCs and to store the data in list mode format. The flat cable was shielded with a mesh-type aluminum sheet, which had a thickness of 0.24 mm. All electronics were enclosed in an aluminum box, which had a thickness of 10 mm, located outside the MR bore. Average energy and timing resolutions of the developed PET measured outside the MR room were 18.1±3.2% (n=4,608) and 3.6 ns, respectively. The sensitivity and spatial resolution were 1.2% and 3.1 mm at the center of the field of view, respectively. No significant degradations of PET performance and the uniformity of MR image were observed. Simultaneous PET and MR images of hot-rod phantom and cat brain were successfully acquired. Experimental results indicate that the high performance compact and lightweight PET insert for hybrid PET-MRI can be developed using GAPD arrays and charge signal transmission method.

A PET employing Geiger-mode avalanche photodiode (GAPD) has been designed and developed in our laboratory for simultaneous human brain PET-MR imaging. The purpose of this study is to evaluate the effect of temperature on the performance of the brain PET. The PET gantry including LYSO and GAPD array were placed in a temperature controlled chamber. All electronics including preamplifiers were located outside the chamber to minimize the temperature effect. Energy resolution and photo-peak position of all channels of the brain PET were measured. Count rates of the PET were also measured using 2 different Schemes: fixed energy window and adjusted energy window. All measurement was performed at 10°C ∼ 40°C with 5°C step. Hot-rod phantom was acquired at 10°C, 25°C and 40°C. The energy resolution was degraded from 14% to 30% as the temperature raised. The photo-peak position the coincidence count rates were respectively decreased from 1314 ch to 560 ch, from 2.5 kcps to 0.9 kcps as the temperature raised. When the energy window was adjusted to the photo-peak position, the rates were stable from 10°C to 35°C. In hot-rod phantom images acquired at 10°Cand 25°C, the rods down to a diameter of 3.5 mm were clearly resolved. However the phantom image acquired at 40 °C demonstrated noticeable degradation in image quality. Future study is being conducted to optimize the adjustable window to obtain stable performance of PET system using GAPD as a function of temperature.

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.

Purpose: The aim of this study was to develop a prototype magnetic resonance (MR)-compatible positron emission tomography (PET) that can be inserted into a MR imager and that allows simultaneous PET and MR imaging of the human brain. This paper reports the initial results of the authors’ prototype brain PET system operating within a 3-T magnetic resonance imaging (MRI) system using newly developed Geiger-mode avalanche photodiode (GAPD)-based PET detectors, long flexible flat cables, position decoder circuit with high multiplexing ratio, and digital signal processing with field programmable gate array-based analog to digital converter boards. Methods: A brain PET with 72 detector modules arranged in a ring was constructed and mounted in a 3-T MRI. Each PET module was composed of cerium-doped lutetium yttrium orthosilicate (LYSO) crystals coupled to a tileable GAPD. The GAPD output charge signals were transferred to preamplifiers using 3 m long flat cables. The LYSO and GAPD were located inside the MR bore and all electronics were positioned outside the MR bore. The PET detector performance was investigated both outside and inside the MRI, and MR image quality was evaluated with and without the PET system. Results: The performance of the PET detector when operated inside the MRI during MR image acquisition showed no significant change in energy resolution and count rates, except for a slight degradation in timing resolution with an increase from 4.2 to 4.6 ns. Simultaneous PET/MR images of a hot-rod and Hoffman brain phantom were acquired in a 3-T MRI. Rods down to a diameter of 3.5 mm were resolved in the hot-rod PET image. The activity distribution patterns between the white and gray matter in the Hoffman brain phantom were well imaged. The hot-rod and Hoffman brain phantoms on the simultaneously acquired MR images obtained with standard sequences were observed without any noticeable artifacts, although MR image quality requires some improvement. Conclusions: These results demonstrate that the simultaneous acquisition of PET and MR images is feasible using the MR insertable PET developed in this study.