Jiwoong Jung

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.

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.

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

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 <;3% (n=256) for 60 minutes and simultaneous PET and MR images of brain phantom were successfully acquired. Experimental results indicate that the simultaneous PET and MR imaging is feasible using MR compatible PET insert proposed in this study without significant interference.

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.

A FPGA-based high speed multi-channel simultaneous signal acquisition method for PET

A PET data acquisition (DAQ) system based on multiple high-speed, 8-channel DAQ cards is being developed. Each of these DAQ cards has 8-channel 14-bit 100 MHz simultaneous ADCs, a six million-gate FPGA with 104 MHz onboard clock and a 16-bit 128 MB SDRAM. The SDRAM has a data transfer bandwidth of 450 MB/S, thus for raw data acquisition, only two channel ADCs can be simultaneously used at sampling rate of 100 MHz (for 8 channel simultaneous acquisition, the maximum sampling rate is about 28 MHz). In this study, data packages containing pulse arrival time, baseline, energy and position were saved instead of raw data of whole gamma signal pulse to reduce the amount of output data. Parallel signal processing containing arrival time detection, baseline calculation, pulse energy calculation and channel ID generation were implemented by model-based design method to achieve high performance PET signal processing. Finally, data packaging and down sampling approaches were employed to achieve 8-channel simultaneous signal acquisition at 100 MHz sampling rate without exceeding the SDRAM data transfer bandwidth. To examine the functionality of the signal acquisition method, PET images were obtained using a pair of detectors. The PET detector is composed of a Geiger mode avalanche photodiode (GAPD) array (3 mm × 3 mm, 4×4) coupled with LYSO array crystal (3 mm × 3 mm × 20 mm, 4 × 4). By rotating a pair of PET detectors for 180 degrees (6 degrees × 30 times) and simultaneously acquiring 8 channel (4 channels from each GAPD array) signals using one acquisition card, PET images of two F-18 line sources were successfully acquired. The energy resolution for 8-channel gamma signals was 19% and the timing resolution for the coincidence channels was 1.5 ns. The spatial resolution for the acquired PET image was 3.1 mm. Finally, a brain PET system using three DAQ cards has been developed and high quality PET images were acquired.

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.