Kyu Bom Kim

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.

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.

Signal Transmission With Long Cable for Design of PET Detector for Hybrid PET-MRI

Recently, we reported a hybrid positron emission tomography-magnetic resonance imaging (PET-MRI) approach involving the transmission of a photo-sensor charge signal to a preamplifier using a long cable. However, in this application, pulse distortions may degrade the performance of the PET detector. The purpose of this study is to further extend the charge signal transmission (CST) approach and to design a PET detector module capable of transmitting the photo-sensor charge signal to PET electronics remotely located outside the MRI room for the hybrid PET-MRI. To achieve this, we developed a pulse restoration circuit (PRC) compensating the performance degradations caused by using the long cable from Geiger-mode avalanche photodiode (GAPD) to PET electronics. The effect of the cable length on the PET detector performance was examined using different lengths of flexible flat cable ranging from 0 to 21 m. Rise time, fall time and amplitude of the GAPD output were measured as a function of the cable length. Photopeak position, energy resolution and time resolution were also measured using a 100 MS/s data acquisition unit. PRC based on the filtering and shaping networks consisted of amplitude, and rise and fall restorations. The amplitude restoration was designed using op-amp and resistors. The rise time-fall time restoration was designed using RC shaping filter. The measured pulse shapes passing through the long cable with PRC were similar to the output of GAPD with the cable length of 0 m (amplitude 169 mV, rise time 35 ns and fall time 210 ns), and the degrading PET detector performance caused by long cable were restored (energy resolution 16%, photopeak position 400 ch and time resolution 1.3 ns). There were no significant changes in the PET detector module performance using the CST approach with PRC as a function of the cable length, both outside and inside the MRI room. There were no obvious artifacts or changes in the MR phantom images. These results show that the proposed approach using extended charge signal transmission method with long cable and simple pulse restoration circuit is reliable and useful for the development of a hybrid PET-MRI system.

A depth-encoding PET detector inserting glass plate between crystal layers

This study introduces a depth-encoding PET detector inserting a glass plate between the pixilated scintillation crystal layers. The principle of the proposed design was that the relative amount of light received by each photosensor will be altered by using the glass plate. This change in the light distribution can generate a pattern diagram of the 2D flood histogram that identifies depth position as well as X-Y position of γ-ray interaction. A Monte Carlo simulation was conducted for the assessment of the DOI-PET detector of 4 ×4 array photosensor coupled with 2-layers of LSO arrays which consist of 4 ×4 arrays of 3 ×3 ×10 mm3 discrete crystals. The traced light distribution for each event was converted by the modified resistive charge division networks into the 2D flood histogram. Optical glass plates with 11 different thicknesses that range from 0 to 10 mm with a 1 mm step were modeled. This was done, to estimate the thickness which allows the extraction of the depth information from the 2D flood histogram. An experimental study was performed to acquire the flood histograms of the DOI-PET detectors with 3 and 5 mm thick glass plate. The effect of glass plate on light loss and count rate loss were assessed for two detector configurations with and without glass plate. The simulation results showed that the flood histogram without overlapping of each crystal position can be generated for the detectors by inserting the glass plates with thickness of 3 ~ 10 mm. They were also demonstrated in the acquired representative flood histograms which were obtained by the experimental study. The light and count rate losses measured from DOI-PET detector with 3 mm thick glass plate was ~ 5% and ~ 2%, respectively. This study demonstrated that the proposed DOI-PET detector can extract the 3D γ-ray interaction position without considerable performance degradations of PET detector from the 2D flood histogram.

A pulse restoration circuit minimizing performance degradation of PET detector caused by using long cable

Recently, we have reported the approach of transmitting photo-sensor charge signal to preamplifier using long cable for development of hybrid PET-MRI. However, pulse distortions degrading the performance of PET detector might occur in this approach. The purpose of this study is to develop a pulse restoration circuit (PRC) compensating the performance degradations caused by using the long cable from Gaige-mode avalanche photodiode (GAPD) to PET electronics. The effect of the cable length on the PET detector performance was examined using different lengths of flexible flat cable (FFC) ranging from 0 to 21 m. Rise time, fall time and amplitude of the GAPD output were measured as a function of the cable length. Photopeak position, energy resolution and time resolution were also measured using a 100 MS/s data acquisition unit. The PRC based on the filtering and shaping networks was consisted of amplitude, rise time-fall time restoration parts. The amplitude restoration part was designed using op-amp and resistors and the rise time-fall time restoration parts were designed using RC shaping filter. There were changes in the PET detector performance as a function of the FFC lengths ranging from 0 to 21m. The amplitude and the photopeak position decreased from 169 mV to 100 mV and from 400 ch to 300 ch, respectively. The rise time, fall time and the time resolution increased from 35 ns to 61 ns, from 210 ns to 450 ns and from 1.1 ns to 2.2 ns, respectively. No considerable changes were observed in the energy resolution of 16%. These performance changes was improved by employing the PRC. The measured pulse shapes passing through the long cable with PRC, were identical to the GAPD output with the cable length of 0 m, and the photopeak position was restored. The time resolution was also improved from 2.2 ns to 1.3 ns. This study demonstrated that the PRC could improve the performance degradations caused by using long cable.

An improved time over threshold method using bipolar signals

The time over threshold (TOT) method has been recently proposed as a signal processing method used to calculate time and energy information by measuring the pulse arrival time and pulse duration over a preset threshold. Although TOT has been reported as an effective method for front end readout in PET applications, it has several limitations, including its non-linearity, lower dynamic range, and a trade-off between energy resolution and coincidence resolving time (CRT). In this study, we propose a novel design we developed to improve performance with regard to these problems occurring in the conventional TOT by employing a bipolar signal and two comparators. Using a high frequency CR shaping filter, a detected signal was converted into a bipolar signal, and the positive pulse of the converted bipolar signal had a fast rising time, while the negative pulse had a linear slope. The bipolar TOT circuit was composed of a preamplifier, a CR shaping filter, and two comparators. The PET detector was composed of a single LYSO coupled with 4  ×  4 SiPM arrays, a bipolar TOT circuit, and an FPGA based TDC. And this was constructed to evaluate the performance of the proposed bipolar TOT method. A 16-ch PET detector module consisting of a 4  ×  4 array LYSO coupled to a 4  ×  4 SiPM arrays, an Anger logic discretized positioning circuit, and a 4-ch bipolar TOT circuit was also constructed to evaluate the functionality of the bipolar TOT method for PET applications. The pulse height resolution and CRT were measured using both the bipolar TOT method and the conventional TOT method. While the bipolar TOT method provided a similar pulse height resolution (10.4%  ±  0.1%), the integral non-linearity (1.4%) and CRT (168  ±  4 ps) measured using the bipolar TOT method were greatly improved compared to those (17.2% and 258  ±  15 ps, respectively) measured with the conventional TOT method. The positions of the crystals were clearly identified, as seen in the flood histogram acquired using the 4-ch bipolar TOT circuit. The measured average pulse height resolution and average CRT for the 16-ch detector module were 11.5%  ±  0.2% and 516  ±  24 ps. The results obtained in this study indicate that the bipolar TOT method requiring a relatively small number of electronic components could effectively improve the CRT, linearity and dynamic range. Furthermore, they also demonstrated the extendibility allowing the development of a PET system that consists of a large number of detectors.

Analog and digital signal processing method using multi-time-over-threshold and FPGA for PET

PURPOSE The goal of this study was to develop an analog and digital signal processing method using multi-time-over-threshold (MTOT) and field programmable gate arrays (FPGAs) to extract PET event information by using the internal clock of FPGA (~350 MHz), without ADC and TDC. METHODS The PET detector modules were composed of a 4 × 4 matrix of 3 × 3 × 20 mm3 LYSO and 4 × 4 SiPM array. Output charge signals of PET detector modules were amplified and fed into four comparators to generate trigger signals. The energy of the detected gamma ray was calculated by integrating the digitized pulse and the arrival time was determined from the time stamp of the lowest trigger signal by FPGA. The data packet containing energy, time, and position information was stored in list mode on the host computer. RESULTS The performance of analog and digital signal processing circuits using MTOT method and FPGA was evaluated by measuring energy and time resolution of the proposed method and the values were 19% and 900 ps, respectively. CONCLUSION This study demonstrated that the proposed MTOT method consisting of only FPGA without ADC and TDC could provide a simple and cost-effective analog and digital signal processing system for PET.

Realtime intraoperative imaging system combining annihilation γ-ray detectors and video projectors

Realtime near-infrared (NIR) fluorescence imaging system could be exploited for intraoperative imaging guidance. However, the NIR fluorescence imaging technique attains only small penetration depths (a few centimeters) due to the absorption and scattering of light. The purpose of this study was to develop a realtime intraoperative imaging system using annihilation gamma-ray detectors for localization and laser projectors for visualization of surgical region embedded deep in tissue. The proof-of-principle gamma-ray detectors consisted of two front detector blocks and one backside detector block. The front detector block consisted of 8 detector modules arranged in a blank square shape and the backside detector block was composed of 4 × 8 detector modules. Each detector module was composed of a 4 × 4 array LYSO coupled to a 4 × 4 GAPD array. Front and backside detectors were located at opposite sides of each other. Commercial portable LED laser projector with a luminous flux of 500 ANSI lumens was used for the intra-operative system. Two laser projectors were positioned behind each front detector. The design parameters of the gamma-ray imaging system were adjusted and optimized using a Monte Carlo simulation tool, GATE. The performance of the system spatial resolution and count rate were estimated by simulation and by experiment using 450 kBq Na-22 point source. Experimentally measured spatial resolution and count rate were 4.1 mm and 300 counts per second which were similar to those estimated by the simulation. The beam projected by laser projectors was accurately localized the position of radiation source. Experimental results indicate that realtime intraoperative imaging system combining annihilation gamma-ray detectors and laser projectors proposed in this study would be useful to localize and to visualize surgical region embedded deep in tissue.

Multi-voltage threshold based front-end circuit and DAQ with FPGA for PET

RECENTLY, there has been great interest in the study of deriving PET event information from digitally sampled event waveforms. We have reported the feasibility of Multi-Voltage Threshold (MVT) based PET DAQ system using field - programmable-gate-array (FPGA) without ADC and TDC. The goal of this study was to extend MVT based multi-channel analog circuit and DAQ with FPGA which could be utilized for the development of a functional PET system.