With the advancement of the Silicon Photomultiplier (SiPM) technology, these devices are becoming the mainstream for use in high resolution PET detectors especially where the compatibility with magnetic field is critical. Large-area SiPM arrays are now widely used in high resolution PET detector modules. The challenges are emerged in the readout and timing circuitry by the increase of pixel density in the area. The application-specific integrated circuits (ASICs) are developed to readout arrays with standard anode/ cathode outputs. SiPM detection principle, which produces dark current noise in the inactive pixels, makes the multiplexing techniques more challenging rather than the PMT signals. In this work a new generic PET detector block using SiPM arrays with three outputs (anode, cathode, and fast) is presented which is suitable for applications in small animal, brain, and clinical PET imaging. The block detector accepts a 12×12 array with both standard and fast output signals with an overall detector dimension of 50×50 mm 2 . A new version of the scrambled crosswire readout (SCR) method was implemented to reduce 144 'fast outputs' to 9 tile signals and 144 standard outputs to 16 energy channels. The tile signals are also used to generate time pickoff information for timing resolution. We implemented time-to-digital converter (TDC) in Xilinx's SPARTAN6 field programmable gate array (FPGA) by using the 64-tap delay line. This low cost chip performs all required processes for position, energy, time, and interface architecture. The attached 24×24 LYSO:Ce array with 2×2×10 mm 3 pixels is imaged and pixels are resolved clearly in the room temperature. The measured energy resolution of the detector block after calibration for all crystal pixels is 12.4% at 511 keV. Also, the coincidence resolving time for two identical modules was measured at 1.85 ns.
Large area scintillation detectors applied in gamma cameras as well as Single Photon Computed Tomography (SPECT) systems, have a major role in in-vivo functional imaging. Most of the gamma detectors utilize hexagonal arrangement of Photomultiplier Tubes (PMTs). In this work we applied large square-shaped PMTs with row/column arrangement and positioning. The Use of large square PMTs reduces dead zones in the detector surface. However, the conventional center of gravity method for positioning may not introduce an acceptable result. Hence, the digital correlated signal enhancement (CSE) algorithm was optimized to obtain better linearity and spatial resolution in the developed detector. The performance of the developed detector was evaluated based on NEMA-NU1-2007 standard. The acquired images using this method showed acceptable uniformity and linearity comparing to three commercial gamma cameras. Also the intrinsic and extrinsic spatial resolutions with low-energy high-resolution (LEHR) collimator at 10 cm from surface of the detector were 3.7 mm and 7.5 mm, respectively. The energy resolution of the camera was measured 9.5%. The performance evaluation demonstrated that the developed detector maintains image quality with a reduced number of used PMTs relative to the detection area.
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