Key Jo Hong

Tomographic image of prompt gamma ray from boron neutron capture therapy: A Monte Carlo simulation study

Purpose of paper is to confirm the feasibility of acquisition of three dimensional single photon emission computed tomography image from boron neutron capture therapy using Monte Carlo simulation. Prompt gamma ray (478 keV) was used to reconstruct image with ordered subsets expectation maximization method. From analysis of receiver operating characteristic curve, area under curve values of three boron regions were 0.738, 0.623, and 0.817. The differences between length of centers of two boron regions and distance of maximum count points were 0.3 cm, 1.6 cm, and 1.4 cm.

Readout Electronics and Data Acquisition of a Positron Emission Tomography Time-of-Flight Detector Module With Waveform Digitizer

A multi-element silicon photomultiplier (SiPM) based time-of-flight (ToF) detector module for positron emission tomography (PET) has been developed. The detector module is based on a 4 × 4 array of LYSO-SiPM elements (Hamamatsu MPPC S10931-050P), with individual bias supply for each element. Each element is read out by a wideband, low-noise RF amplifier to maximize timing performance. All 16 outputs are digitized with a high-speed CAEN V1742 digitizer module (32 + 2 channels, 5 GS/s sampling, 12-bit amplitude resolution, 500 MHz input bandwidth) to acquire raw pulse waveforms for offline timing and energy extraction. As the digitizer has no internal trigger for individual channels, a trigger board has been developed which produces a fast pulse that triggers the digitizer whenever any pixel of the detector detects a signal in coincidence with a reference detector. To assess the performance of the prototype module, a 4 × 4 LYSO scintillator array ( 3×3×5 mm3 elements) was coupled to the SiPM photodetectors and energy/timing resolution measurements were performed using a Ge-68 source. At 1.4 V overvoltage, the energy resolution, not corrected for saturation effects of the SiPM, varied from a minimum of 10.1% to a maximum of 13.3% with an average energy resolution of 11.4 ± 0.8% across the 16 channels. With a reference detector (single 3×3×5 mm3 LYSO crystal coupled to a Hamamatsu MPPC S10362-33), the average coincidence resolving time (CRT) across the detector module was 206 ± 7 ps FWHM at 2.4 V overvoltage-the best reported for a PET block (array) detector based on conventional photodetectors to date.

FPGA-based time-to-digital converter for time-of-flight PET detector

We have developed a FPGA-based time-to-digital converter (TDC) that can be used for a TOF-PET block detector based on silicon photomultiplier (SiPM) photodetectors. The tapped delay line (TDL) method implemented with a dedicated carry chain structure was used to measure short time intervals. The proposed TDC, implemented in a Spartan-6 FPGA, consists of a fine time measurement block, a coarse counter, a ring oscillator and a multiplexer. The ring oscillator generates a delay chain related frequency which is used to compensate process, voltage and temperature (PVT) effects in real-time without causing dead-time in the TDC. The multiplexer allows multiple channels to share the same delay chain which effectively reduces the amount of FPGA resources. As the TDC is implemented in an shared FPGA device, which already exists in a data acquisition system (DAQ), TOF capability can be implemented easily without requiring more resources. The performance of our proposed TDC was first measured with two input pulses which were generated from a pulse generator but with different delay lengths. Timing resolution of a TDC channel is 41.6 ± 1.1 ps FWHM (17.7 ± 0.5 ps RMS). The proposed TDC was also used to measure the timing resolution of a pair of TOF-PET detector with a Hamamatsu MPPC coupled to a 3 mm × 3 mm face of a 2 mm × 2 mm × 3 mm LYSO crystal. The measured coincidence time resolution was 197 ± 4 ps FWHM which agreed with the value measured by a high speed oscilloscope (195 ± 7 ps FWHM). These results verify the feasibility of our TDC for TOF-PET applications.

Therapy region monitoring based on PET using 478 keV single prompt gamma ray during BNCT: A Monte Carlo simulation study

We confirmed the feasibility of using our proposed system to extract two different kinds of functional images from a positron emission tomography (PET) module by using an insertable collimator during boron neutron capture therapy (BNCT). Coincidence events from a tumor region that included boron particles were identified by a PET scanner before BNCT; subsequently, the prompt gamma ray events from the same tumor region were collected after exposure to an external neutron beam through an insertable collimator on the PET detector. Five tumor regions that contained boron particles and were located in the water phantom and in the BNCT system with the PET module were simulated with Monte Carlo simulation code. The acquired images were quantitatively analyzed. Based on the receiver operating characteristic (ROC) curves in the five boron regions, A, B, C, D, and E, the PET and single-photon images were 10.2%, 11.7%, 8.2% (center region), 12.6%, and 10.5%, respectively. We were able to acquire simultaneously PET and single prompt photon images for tumor regions monitoring by using an insertable collimator without any additional isotopes.

Trends of Data Path Topologies for Data Acquisition Systems in Positron Emission Tomography

In this paper, we describe current trends in data path topologies for Positron Emission Tomography (PET) data acquisition (DAQ) systems which is used to capture signals from the detector modules and generate coincidence data for a image reconstruction computer. First, the requirements of DAQ systems in PET, such as event throughput, timing resolution, and energy resolution, are analyzed. Then, design issues related to DAQ designs are discussed including multiplexing schemes, DAQ boards interconnection topologies, and transmission protocols to transfer data to the image reconstruction computer. We also discuss how new trends in PET systems, such as time-of-flight (TOF) PET, depth-of-interaction (DOI) information, and PET/MRI, can affect the design of PET DAQ systems.

GPU-based prompt gamma ray imaging from boron neutron capture therapy

PURPOSE The purpose of this research is to perform the fast reconstruction of a prompt gamma ray image using a graphics processing unit (GPU) computation from boron neutron capture therapy (BNCT) simulations. METHODS To evaluate the accuracy of the reconstructed image, a phantom including four boron uptake regions (BURs) was used in the simulation. After the Monte Carlo simulation of the BNCT, the modified ordered subset expectation maximization reconstruction algorithm using the GPU computation was used to reconstruct the images with fewer projections. The computation times for image reconstruction were compared between the GPU and the central processing unit (CPU). Also, the accuracy of the reconstructed image was evaluated by a receiver operating characteristic (ROC) curve analysis. RESULTS The image reconstruction time using the GPU was 196 times faster than the conventional reconstruction time using the CPU. For the four BURs, the area under curve values from the ROC curve were 0.6726 (A-region), 0.6890 (B-region), 0.7384 (C-region), and 0.8009 (D-region). CONCLUSIONS The tomographic image using the prompt gamma ray event from the BNCT simulation was acquired using the GPU computation in order to perform a fast reconstruction during treatment. The authors verified the feasibility of the prompt gamma ray image reconstruction using the GPU computation for BNCT simulations.

Studies of electromagnetic interference of PET detector insert for simultaneous PET/MRI

We are developing a brain positron emission tomography (PET) system prototype with long optical cables to minimize mutual interference between our PET components and magnetic resonance imaging (MRI) system. Our PET system consists of 16 PET detector modules which are placed in Faraday cages spaced equally in a 32 cm diameter ring. By using 20 m length optical cables rather than electrical connections, the Faraday cage ground can float relative to the MRI RF ground which permits the RF field to transmit through PET ring. This could eliminate the need for custom RF coils in whole body inserts, or the need for a custom transmit coil in brain insert PET/MRI designs. The aim of this study is to investigate the feasibility of PET detectors with a floating ground from measurements of electromagnetic interference (EMI) shielding and numerical analyses of RF field attenuation under different conditions. The shielding effectiveness equation shows that a copper plate of 30 μm (~4×Skin Depth) thickness shields approximately 120 dB (99.9999 %) of both the 66.7 MHz analog-to-digital converter sampling frequency of the interior PET electronics and the 127.7 MHz Larmor frequency of the exterior 3-T MRI RF coil. Simulation results using ANSOFT Maxwell showed that a larger gap between PET detectors or a shorter height of PET Faraday cage results in less RF field attenuation. The two side plates of adjacent PET Faraday cage act as a capacitor. When the gap increases or height shrinks, capacitive impedance increases which then results in less RF power dissipation and thus more RF field transmission inside field of view (FOV). Simulation results showed 25 dB increase of the transmission level when the gap was increased by 2 mm and height was decreased by 20 mm. Further MR-compatibility analysis will be performed by acquiring MR images with the shielded PET detector ring inserted.

A pulse width modulation readout method for densely packed solid state photodetectors

The field of PET is moving toward systems comprising thousands of crystal elements coupled to thousands of semiconductor photodetector channels. Because of the large number of channels, developing methods for compact signal readout, while avoiding the need for new integrated circuit development is important. In this work a compact readout system for arrays of 0.5 mm × 0.5 mm × 1.0 mm LYSO crystal elements is presented. An 18×18 element crystal array was coupled to a 2×2 array of 5 mm × 5 mm position sensitive solid-state photomultipliers (PS-SSPM). Each PS-SSPM has 4 position channels. The 16 position channels of the array were multiplexed into a total of 6 readout channels. A novel version of pulse width modulation capable of high multiplexing ratios is used to readout the electronics instead of traditional analog-to-digital conversion. Each detector pulse was transformed into a digital pulse that was then readout by an FPGA-based TDC. The system successfully identified the 0.5 mm × 0.5 mm crystals at room temperature with a ratio of distance between crystal peaks to standard deviation of the peaks of greater than 3.2. The success of this simple compact readout strategy points to a practical readout strategy for PET systems comprising thousands of semiconductor photodetectors.

Characterization of PET data acquisition system with compressed sensing detectors

We evaluated the timing performance of our data acquisition system (DAQ) with compressed sensing detectors we used in a PET insert for simultaneous PET/MR brain imaging. Compressed sensing is a promising multiplexing method that can achieve high multiplexing ratio while resolving simultaneous hits on multiple pixels without serious degradation of timing and energy resolution for positron emission tomography (PET) scanners. In our compressed sensing detector, each of the 128 silicon photomultiplier (SiPM) pixels produces a unique pattern on the 16 read out channels (the multiplexing ratio is 8:1), which are digitized and recorded at the DAQ system. The unique decoding pattern for each SiPM is used to recover the crystal indexes. In this study, we used a 10 μCi 22Na radioactive point source to evaluate the coincidence timing resolution across the two lutetium-yttrium orthosilicate (LYSO) based block detectors. We also measured the timing integral non-linearity of the DAQ system by using a pulser to trigger both detectors with the triggering pulses for one detector offset by sequential ~ 2 ns steps. The integral non-linearity of the DAQ system is ~0.3 ns across the entire ADC sampling period, which is below the timing resolution for the measurements using a pulser as input (~ 1.0 ns FWHM). The coincidence timing resolution measured with a 22Na source over the entire 128 crystal block is 5.98 ± 0.09 ns. Possible factors that are limiting the current timing performance are also presented.

SU-E-J-104: Single Photon Image From PET with Insertable SPECT Collimator for Boron Neutron Capture Therapy: A Feasibility Study

PURPOSE The aim of our proposed system is to confirm the feasibility of extraction of two types of images from one positron emission tomography (PET) module with an insertable collimator for brain tumor treatment during the BNCT. METHODS Data from the PET module, neutron source, and collimator was entered in the Monte Carlo n-particle extended (MCNPX) source code. The coincidence events were first compiled on the PET detector, and then, the events of the prompt gamma ray were collected after neutron emission by using a single photon emission computed tomography (SPECT) collimator on the PET. The obtaining of full width at half maximum (FWHM) values from the energy spectrum was performed to collect effective events for reconstructed image. In order to evaluate the images easily, five boron regions in a brain phantom were used. The image profiles were extracted from the region of interest (ROI) of a phantom. The image was reconstructed using the ordered subsets expectation maximization (OSEM) reconstruction algorithm. The image profiles and the receiver operating characteristic (ROC) curve were compiled for quantitative analysis from the two kinds of reconstructed image. RESULTS The prompt gamma ray energy peak of 478 keV appeared in the energy spectrum with a FWHM of 41 keV (6.4%). On the basis of the ROC curve in Region A to Region E, the differences in the area under the curve (AUC) of the PET and SPECT images were found to be 10.2%, 11.7%, 8.2% (center, Region C), 12.6%, and 10.5%, respectively. CONCLUSION We attempted to acquire the PET and SPECT images simultaneously using only PET without an additional isotope. Single photon images were acquired using an insertable collimator on a PET detector. This research was supported by the Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, Information and Communication Technologies (ICT) & Future Planning (MSIP)(Grant No.2009 00420) and the Radiation Technology R&D program (Grant No.2013M2A2A7043498), Republic of Korea.