Metallic Component Preserving Algorithm Based on the Cerebral Computed Tomography Angiography in Aneurysm Surgery.

The purpose of this study was to investigate the viability of the proposed method in preventing the loss of metallic components including the clip and coil in cerebral computed tomography angiography (CTA). Forty patients undergoing surgery for aneurysms carried metallic materials. The proposed method is based on conventional bone subtraction CTA (BS-CTA) system. Briefly, the position of metal components was determined using the threshold value and a region of interest (ROI). An appropriate threshold was used to separate the background from the target materials based on the Otsu method. A three-dimensional (3D) rendering was performed from the proposed BS-CTA data carrying the extracted target information. The accuracy of clip and coil region measured using the dice similarity coefficient (DSC) and bidirectional Hausdorff distance (HD) is reported. The metallic components of the proposed BS-CTA were significantly visualized in various patient cases. Quantitative evaluation using the proposed method is based on the mean DSC of 0.93 with a standard deviation (SD) of ±0.05 (e.g., maximum value = 0.99, minimum value = 0.75, 95% confidence interval (CI) = 0.91 to 0.95, and all p < 0.05). The mean HD was 1.50 voxels with an SD of ± 0.58 (e.g., maximum value = 5.95, minimum value = 0.12, 95% CI = 1.10 to 1.90, and all p < 0.05). The proposed method demonstrates effective segmentation of the metallic component and application to the existing conventional BS-CTA system.

Development of a direction-sensitive gamma-ray monitoring system using a gamma camera with a dual-sided collimator: A Monte Carlo study.

Nuclear explosions, sabotage, and dirty bomb materials are considered a security threat. This paper discusses the development of a gamma-ray monitoring system that enables the screening of nuclear materials moving simultaneously on both sides of the system at ports. This direction-sensitive gamma-ray monitoring (DSGM) system consists of a monolithic plastic scintillator surrounded by 28 photomultiplier tubes and dual-sided parallel-hole lead collimators. With Monte Carlo simulation, the monitoring performance of the DSGM system was assessed for static and moving sources. A multilayer perceptron model was employed to estimate the energy-deposited position of the gamma-rays emitted by nuclear materials in the scintillator.

Performance evaluation of a small animal PET scanner a high level of multiplexing and charge-signal transmission.

Small animal positron emission tomography (PET) is a noninvasive imaging modality that enables in vivo imaging and quantification of the biological processes of small experimental animals. We have developed a small animal PET that utilizes a high-resolution multiplexed readout and charge signal transmission (CST) method. The small animal PET was composed of six detector blocks consisting of SiPMs and LYSO arrays. Six detector blocks were mounted on a PET gantry having an inner diameter of 76 mm, outer diameter of 112 mm, and axial length of 40.8 mm. The charge signals of SiPM output were transmitted to the input of multiplexed readout using 4 m flexible flat cables. The multiplexed readout was composed of six main boards, each of which included 36 detector boards, to reduce the number of readout channels by a factor of 36, with a multiplexing ratio of 144:4. The performance of the small animal PET was evaluated using NEMA NU 4-2008 standards, and its imaging capability was demonstrated by in vivo mouse imaging studies. The average energy and time resolutions were 13.2% ± 0.3% and 3.8 ns, respectively. The spatial resolution at the center of the transaxial FOV was 1.1 mm, and the peak sensitivity at the center of the axial FOV was 1.5%. The peak noise equivalent count (NEC) rate and scatter fraction were 21.1 kcps at 18.2 MBq and 21%, respectively. The acquired images demonstrated high quality tracer uptake patterns of small experimental animals. The results of performance evaluation and animal imaging indicate that the small animal PET developed in this study can provide high-quality small animal imaging with cost-effectiveness and compactness.

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.

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.

Quantitative analysis of prompt gamma ray imaging during proton boron fusion therapy according to boron concentration.

The purpose of this study is to evaluate the prompt gamma ray imaging technique according to the clinical boron concentration range during proton boron fusion therapy (PBFT). To acquire a prompt gamma ray image from 32 projections, we simulated four head single photon emission computed tomography and a proton beam nozzle using a Monte Carlo simulation. We used modified ordered subset expectation maximization reconstruction algorithm with a graphic processing unit for fast image acquisition. Boron concentration was set as 20 to 100 µg at intervals of 20 µg. For quantitative analysis of the prompt gamma ray image, we acquired an image profile drawn through two boron uptake regions (BURs) and calculated the contrast value, signal-to-noise ratio (SNR), and difference between the physical target volume and volume of the prompt gamma ray image. The relative counts of prompt gamma rays were noticeably increased with increasing boron concentration. Although the intensities on the image profiles showed a similar tendency according to the boron concentration, the SNR and contrast value were improved with increasing boron concentration. This study suggests that a tumor monitoring technique using prompt gamma ray detection can be clinically applicable even if the boron concentration is relatively low.

A prototype MR insertable brain PET using tileable GAPD arrays.

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.

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.