Single photon emission computed tomography(SPECT) is a useful medical imaging modality using single photon detection from radioactive tracers, such as 99 Tc and 111 In, however further development of increasing the contrast in the image is still under investigation. A novel method (Double Photon Emission CT / DPECT) using a coincidence detection of two cascade gamma-rays from 111 In is proposed and characterized in this study. 111 In, which is well-known and commonly used as a SPECT tracer, emits two cascade photons of 171 keV and 245 keV with a short delay of approximately 85 ns. The coincidence detection of two gamma-rays theoretically determines the position in a single point compared with a line in single photon detection and increases the signal to noise ratio drastically. A fabricated pixel detector for this purpose consists of 8 × 8 array of high-resolution type 1.5 mm thickness Ce:GAGG (3.9% @ 662 keV, 6.63g/cm 3 , C&A Co. Ce:Gd 3 Ga 2.7 Al 2.3 O 12 2.5 × 2.5 × 1.5 mm 3 ) crystals coupled a 3 mm pixel SiPM array (Hamamatsu MPPC S13361-2050NS-08). The signal from each pixel is processed and readout using time over threshold (TOT) based parallel processing circuit to extract the energy and timing information. The coincidence was detected by FPGA with the frequency of 400 MHz. Two pixel detectors coupled to parallel-hole collimators are located at the degree of 90 to determine the position and coincidence events (time window = 1 µs) are detected and used for making backprojection image. The basic principle of DPECT is characterized including the detection efficiency and timing resolution.
K: Gamma camera, SPECT, PET PET/CT, coronary CT angiography (CTA); Data acquisition circuits; Front-end electronics for detector readout 1Corresponding author.
A: Compton imaging is a useful method to localize gamma sources without using mechanical collimators. In conventional Compton imaging, the incident directions of gamma rays are estimated in a cone for each event by analyzing the sequence of interactions of each gamma ray followed by Compton kinematics. Since the information of the ejection directions of the recoil electrons is lost, many artifacts in the shape of cone traces are generated, which reduces signal-to-noise ratio (SNR) and angular resolution. We have developed an advanced Compton imaging system with the capability of tracking recoil electrons by using a combination of a trigger-mode silicon-oninsulator (SOI) pixel detector and a GAGG detector. This system covers the 660-1330 keV energy range for localization of contamination nuclides such as 137 Cs and 134 Cs inside the Fukushima Daiichi Nuclear Power Plant in Japan. The ejection directions of recoil electrons caused by Compton scattering are detected on the micro-pixelated SOI detector, which can theoretically be used to determine the incident directions of the gamma rays in a line for each event and can reduce the appearance of artifacts. We obtained 2-D reconstructed images from the first iteration of the proposed system for 137 Cs, and the SNR and angular resolution were enhanced compared with those of conventional Compton imaging systems.
K: Compton imaging; Image reconstruction in medical imaging 1Corresponding author.
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