Ji Yeon Hwang

Portable nuclear safeguard equipment using pinhole gamma camera

The collimator design for nuclear survey system should be differently taken into consideration from the collimator for medical environments because it has always been used in the high-energy radiation environments. The purpose of this study is to research the optimum pinhole design for accomplishing the improved image at high-energy radiation field under condition. The relative resolution and sensitivity advantages of pinhole collimators were investigated using analytic formulations and Monte Carlo simulations. Simulations using GATE (Geant4 Application for Tomographic Emission) were performed to model the pinhole gamma camera system. A gamma camera consists of a cone-shaped pinhole collimator with a tungsten aperture and a CsI(Tl) scintillation crystal 6.0 mm thick and 50.0 mm ´ 50.0 mm in area. The focal length and the acceptance angle of the pinhole collimator were set to 60.35 mm and 45°, respectively. The intrinsic spatial resolution and sensitivity were simulated by changing the pinhole diameter and channel height. The point source was located 60.35 mm above the center of the pinhole, and the projection data was estimated for pinhole diameter values from 2.0 mm to 4.0 mm while the channel heights were fixed between 2.0 mm and 6.0 mm. The optimal ranges of channel height and pinhole diameter were determined through evaluation of the intrinsic resolution and sensitivity tradeoff curves. The channel type is selected by these analyses and we also verify these simulation results through experimental test of three types of collimators. We allowed to determine the optimal values of pinhole diameter and channel height to be 2, 3, 4 mm and 6, 4, 6 mm, respectively. The simulated and experimental results agreed with 6% and 14% discrepancies in sensitivities and spatial resolution, respectively. The results demonstrated that the pinhole collimator designed in this study could be utilized to perform radiation monitoring system.

PET detector configuration with thick light guide and GAPD array having large-area microcells

Light sharing PET detector configuration coupled with thick light guide and Geiger-mode avalanche photodiode (GAPD) with large-area microcells was proposed to overcome the energy non-linearity problem and to obtain high light collection efficiency (LCE). Theoretical evaluations were performed for 90 types of PET detector modules. A Monte Carlo simulation was conducted for the three types of LSO block, 4×4 array of 3×3×20 mm3, 6×6 array of 2×2×20 mm3, and 12×12 array of 1×1×20 mm3 discrete crystals, to investigate the scintillation light distribution after conversion of the γ-rays in LSO. The incident photons were read out by three types of 4×4 array photo-sensors, which were PSPMT of 25% quantum efficiency (QE), GAPD1 with 50×50 μm2 microcells of 30% photon detection efficiency (PDE) and GAPD2 with 100×100 μm2 of 45% PDE. The number of counted photons in each photo-sensor was analytically calculated. The LCE, linearity and flood histogram were examined for each PET detector module as a function of light guide thickness ranging from 1 to 10 mm. The performance of PET detector modules based on GAPDs was significantly improved by using the thick light guide. The LCE was increased from 24 to 30% and from 14 to 41%, and the linearity was also improved from 0.96 to 0.99, from 0.78 to 0.99, for GAPD1 and GAPD2, respectively. In the contrary, the performance was not changed for PSPMT based detector. The flood histogram of 12×12 array PET detector modules using 3 mm light guide coupled with GAPDs was obtained by simulation, and all crystals of 1×1×20 mm3 size was clearly identified. PET detector module coupled with thick light guide and GAPD array with large-area microcells was proposed to obtain high QE and high spatial resolution, and its feasibility was verified. It demonstrates GAPDs could be a competitive and cost-effective photo-sensor respect to the high QE (∼40%) PMT.

Monte carlo simulation of four-layer DOI detector with relative offset in animal PET

We have built a four-layer detector to obtain the depth of interaction (DOI) information in which all four layers have a relative offset of a half crystal pitch with each other. The main characteristics of the detector, especially the energy and spatial resolutions, strongly depend on the crystal surface treatments. As a part of the work for the development of an animal PET, we have investigated the effect of crystal surface treatment on the detector performances by Monte Carlo simulation, in order to optimize the surface condition of crystals composing a four-layer detector. The proposed detector consists of four LYSO layers with a crystal dimension of 1.5 mm × 1.5 mm × 7.0 mm. A simulation tool, DETECT2000, was used and validated against the experimental results, flood images acquired by prototype module. Spatial resolution and sensitivity were simulated by varying the surface treatment in crystals. In this study, the optimal surface condition of the four-layer crystals was derived for small animal PET with a view towards achieving high sensitivity as well as high and uniform radial resolution.

Development of a large-angle pinhole gamma camera using depth-of-interaction detector for nuclear survey

Pinhole imaging is receiving notice as a method to enable gamma camera for nuclear survey. In our previous work, the pinhole gamma camera with depth of interaction (DOI) based on three layers of monolithic crystals and maximum-likelihood position-estimation (MLPE) algorithm was designed for the environmental monitoring and the methodology for 3D event positioning in the detector was established by using Monte Carlo simulation. The aim of this study was to evaluate the performance of our detector module experimentally. The proposed detector module was developed by stacking three layers of monolithic CsI(Tl) crystals, each of which has a dimension of 50.0 × 50.0 × 2.0 mm3. The bottom surface of the third layer was directly coupled to an 8 × 8 channel position-sensitive photomultiplier tube (PSPMT, Hamamatsu H8500C). The PSPMT was read out using a resistive charge divider, which multiplexes 64 anodes into 8(X) + 8(Y) channels. Gaussian-based MLPE methods have been implemented by using experimentally measured detector response functions (DRFs). To measure the resolution in each DOI layer, the gamma events were generated at 25 different points over one fourth of the detector area with a spacing of 5.0 mm in the X and Y dimensions. The FWHMs obtained from the first, second, and third layers had mean values of 2.86, 2.67 and 2.87 mm, respectively. The layer misclassification rate was measured over all pixels as 21.4% for the DOI direction. In this paper, our new detector proved to be a reliable design to characterize the event position in all three dimensions with high and uniform spatial resolution and the large-angle pinhole camera could be a useful tool in the environmental monitoring.

Preliminary experimental results of a quasi-monolithic detector with DOI capability for a small animal PET

In our previous work, the new detector module with depth of interaction (DOI) based on a quasi-monolithic crystal array and maximum-likelihood position-estimation (MLPE) algorithm was designed and the methodology for 3D event positioning in the detector was established by using Monte Carlo simulation. The aim of this study was to evaluate the performance of our detector module experimentally. The detector module consisted of a 1D array of eight LYSO crystals of 2.0 (axial) × 20.0 (trans-axial) × 10.0 (depth) mm3 optically coupled to a Hamamatsu H7546B flat panel position sensitive photomultiplier tube (PSPMT). The PSPMT was read out using a resistive charge divider, which multiplexes 64 anodes into 8(X) + 8(Y) channels. Gaussian-based MLPE methods have been implemented using experimentally measured detector response functions (DRFs). The results demonstrated that the detector module could identify the position of the gamma ray interaction inside the crystal with a resolution of 2.0 mm in all three directions. The new DOI detector for a small animal PET was developed and verified experimentally with a view towards achieving high sensitivity as well as high and uniform radial resolution.