Gyuseong Cho

Use of a flat-panel detector for microtomography: a feasibility study for small-animal imaging

We have applied a flat-panel detector to an X-ray cone-beam micro computed tomography (micro-CT) for small-animal imaging. The flat-panel detector consists of an active matrix of transistors and photodiodes with a pixel pitch of 50 /spl mu/m and a thallium-doped cesium iodide (CsI:Tl) scintillator as an X-ray-to-light conversion layer. The detector was fabricated with a complementary metal-oxide-semiconductor (CMOS) technology capable of a submicrometer design line width, hence, it has a pixel fill-factor as high as /spl sim/80%. In addition, the detector has a very fast response characteristic with an image lag less than 0.3% in the frame integration time of 5 s. The CMOS flat-panel detector has been tested in terms of modulation transfer function, noise power spectrum, and detective quantum efficiency. Tomographic imaging performances of the micro-CT system, such as voxel noise, contrast-to-noise ratio, and spatial resolution, have also been evaluated by using various quantitative phantoms. Experimental results of euthanized laboratory rat imaging suggest that the micro-CT system employing a CMOS flat-panel detector can be greatly used in small-animal imaging.

Characterization of dual layer phoswich detector performance for small animal PET using Monte Carlo simulation

A positron emission tomograph dedicated to small animal imaging should have high spatial resolution and sensitivity, and dual layer scintillators have been developed for this purpose. In this study, simulations were performed to optimize the order and the length of each crystal of a dual layer phoswich detector, and to evaluate the possibility of measuring signals from each layer of the phoswich detector. A simulation tool GATE was used to estimate the sensitivity and resolution of a small PET scanner. The proposed scanner is based on dual layer phoswich detector modules arranged in a ring of 10 cm diameter. Each module is composed of 8 x 8 arrays of phoswich detectors consisting of LSO and LuYAP with a 2 mm x 2 mm sensitive area coupled to a Hamamatsu R7600-00-M64 PSPMT. The length of the front layer of the phoswich detector varied from 0 to 10 mm at 1 mm intervals, and the total length (LSO + LuYAP) was fixed at 20 mm. The order of the crystal layers of the phoswich detector was also changed. Radial resolutions were kept below 3.4 mm and 3.7 mm over 8 cm FOV, and sensitivities were 7.4% and 8.0% for LSO 5 mm-LuYAP 15 mm, and LuYAP 6 mm-LSO 14 mm phoswich detectors, respectively. Whereas, high and uniform resolutions were achieved by using the LSO front layer, higher sensitivities were obtained by changing the crystal order. The feasibilities for applying crystal identification methods to phoswich detectors consisting of LSO and LuYAP were investigated using simulation and experimentally derived measurements of the light outputs from each layer of the phoswich detector. In this study, the optimal order and lengths of the dual layer phoswich detector were derived in order to achieve high sensitivity and high and uniform radial resolution.

Optimization of dual layer phoswich detector consisting of LSO and LuYAP for small animal PET

Dual layer scintillators for small animal PET have been developed to measure the depth of interaction and to improve resolution performances. The aim of this study was to perform simulations to optimize the order and length of each crystal composing the dual layer phoswich detector. A simulation tool GATE was used. The bases of our small PET were dual layer phoswich detector modules arranged in a ring with a 10 cm diameter. Each module consisted of 8/spl times/8 arrays of LSO and LuYAP crystals with 2 mm/spl times/2 mm sensitive area coupled to a Hamamatsu R7600-00-M64 PSPMT. The length of the front layer varied from 0 to 10 mm with 1 mm intervals while the total length (LSO+LuYAP) was fixed to 20 mm. The order of the crystal layers of phoswich detector was also changed. The radial resolutions were kept below 3.4 mm and 3.7 mm over 8 cm FOV and sensitivities were 7.4% and 8.0% for LSO 5 mm-LuYAP 15 mm and LuYAP 6 mm-LSO 14 mm phoswich detector, respectively. While the high and uniform resolutions were achieved by using the LSO front layer, the higher sensitivity was obtained by changing the order of crystals. Feasibilities for applying crystal identification methods to phoswich detector consisting of LSO and LuYAP were approved by simulation and experimental measurement of light outputs of each layer of phoswich detector. In this study, the optimal order and lengths of dual layer phoswich detector were derived to achieve high sensitivity and high and uniform radial resolution.

Cascade Modeling of Pixelated Scintillator Detectors for X-Ray Imaging

We have modeled the signal and noise propagation in a pixelated scintillator detector by using the cascaded linear-systems transfer theory. The main difference from the conventional homogeneous scintillator detector is the additional X-ray quantum sampling process at the beginning stage of the cascaded model. The additional sampling stage is expressed as a multiplication of the fill factor of the pixelated scintillator both in signal and noise. The numerical simulation shows that, while the detective quantum efficiency (DQE) degrades in the low spatial-frequency band due to the X-ray quantum sampling, the DQE maintains high values in the high-frequency band, which is due to the band-limited modulation-transfer function (MTF) property of the pixelated scintillator. The pixelated scintillator design is relatively insensitive to additive electronic noise in the DQE performance compared to the conventional design. As a potential solution to overcome the reduction of DQE in the low-frequency band with the pixelated scintillator design, we propose a partially pixelated scintillator design, which utilizes the concept of pre-filtering before the sampling process of the incident X-ray quanta.

Construction and characterization of an amorphous silicon flat-panel detector based on ion-shower doping process

Abstract In this paper, we introduce a new 36×43 cm 2 amorphous silicon flat-panel detector for digital radiography. A prototype flat-panel detector was fabricated using a p–i–n photodiode/thin-film transistor (TFT) array. The main difference of this flat panel detector to the similar general flat-panel detectors is p–i–n photodiode fabrication method. The p-layer of diode is formed using an ion shower doping method instead of the conventional PECVD method to increase the quality of array. The diode shows a leakage current of 2 pA/mm 2 at −5 V and dark current uniformity of the detector is 2.5%. The modulation transfer function (MTF) of the detector is 0.41 at 2 lp/mm .

Evaluation of maximum-likelihood position estimation with Poisson and Gaussian noise models in a small gamma camera

It has been reported that maximum-likelihood position-estimation (MLPE) algorithms offer advantages of improved spatial resolution and linearity over conventional Anger algorithm in gamma cameras. While the fluctuation of photon measurements is more accurately described by Poisson than Gaussian distribution model, the likelihood function of a scintillation event assumed to be Gaussian could be more easily implemented and might provide more consistent outcomes than Poisson based MLPE. The purpose of this study is to evaluate the performances of the noise models, Poisson and Gaussian, in MLPE for the localization of photons in a small gamma camera (SGC) using NaI(Tl) plate and PSPMT. The SGC consisted of a single NaI(Tl) crystal, 10 cm diameter and 6 mm thick, optically coupled to a PSPMT (Hamamatsu R3292-07). The PSPMT was read out using a resistive charge divider, which multiplexes 28(X) by 28(Y) cross wire anodes into four channels. Poisson and Gaussian based MLPE methods have been implemented using experimentally measured detector response functions (DRF). The intrinsic resolutions estimated by Anger logic, Poisson and Gaussian based MLPE were all 3.1 mm. Integral uniformities were 19.9%, 12.0% and 9.8%, and linearities were 1.0 mm, 0.5 mm and 0.05 mm, for Anger logic, Poisson and Gaussian based MLPE, respectively. MLPEs considerably improved linearity and uniformity compared to Anger logic. Gaussian based MLPE, which is easy to implement, allowed to obtain better linearity and uniformity performances than the Poisson based MLPE.

Electronic dose conversion technique using a NaI(Tl) detector for assessment of exposure dose rate from environmental radiation

An electronic dose conversion technique to assess the exposure dose rate due to environmental radiation especially from terrestrial sources was developed. For a 2/spl times/2 inch cylindrical NaI(Tl) scintillation detector, pulse-height spectra were obtained for gamma-rays of energy up to 3 MeV by Monte Carlo simulation. Based on the simulation results and the experimentally fitted energy resolution, dose conversion factors were calculated by a numerical decomposition method. These calculated dose conversion factors were, then, electronically implemented to a developed dose conversion unit (DCU) which is a microprocessor-controlled single channel analyzer (SCA) with variable discrimination levels. The simulated spectra were confirmed by measurement of several monoenergetic gamma spectra with a multichannel analyzer (MCA). The converted exposure dose rates from the implemented dose conversion algorithm in the DCU were also evaluated for a field test in the vicinity of the nuclear power plant at Kori as well as for several standard sources, and the results were in good agreement with separate measurement by a high pressure ionization chamber (HPIC) within a 6.4% deviation.

Artifacts associated with implementation of the Grangeat formula

To compensate for image artifacts introduced in approximate cone-beam reconstruction, exact cone-beam reconstruction algorithms are being developed for medical x-ray CT. Although the exact cone-beam approach is theoretically error-free, it is subject to image artifacts due to the discrete nature of numerical implementation. We report a study on image artifacts associated with the Grangeat algorithm as applied to a circular scanning locus. Three types of artifacts are found, which are thorn, wrinkle, and V-shaped artifacts. The thorn pattern is created by inappropriate extrapolation into the shadow zone in the radon domain. If the shadow zone is filled in with continuous data, the thorn artifacts along the boundary of the shadow zone can be removed. The wrinkle appearance arises if interpolated first derivatives of the radon data are not smooth between adjacent detector planes. In particular, the nearest-neighbor interpolation method should not be used. If the number of projections is not small, the bilinear interpolation method is effective to suppress the wrinkle artifacts. The V-shaped artifacts on the meridian plane come from the line integrations through the transition zones where derivative data change abruptly. Two remedies are to increase the sampling rate and suppress data noise.

Heterodyne wave number measurement using a double B-dot probe

An in situ method of wave number measurement inside a helicon plasma has been developed using a double B-dot probe with a heterodyne detection scheme. Each probe in the double B-dot probe measures the wave magnetic field. The signals from the two separately located probes inside the plasma are mixed with a local oscillator signal transforming the signals into transistor–transistor logic signals with intermediate frequency. The phase difference is obtained by a phase comparator yielding wave number information of a plasma wave.

Guard-Ring Structures for Silicon Photomultipliers

Si photomultipliers with three different guard-ring structures are fabricated, and a detailed comparative study on their device performances is performed. The virtual guard-ring structure shows a high-resolution full width at half maximum in the gamma spectrum and a high breakdown voltage of ~ 66 V but the lowest fill factor of 46.6%-59.8% among the examined structures. The best charge conversion performance, gain, and fill factor (67.1%) are achieved with the trench guard-ring structure. However, this structure shows a low energy resolution, which is supposed to be due to the trench-associated defects. The performance of the N-implantation guard-ring structure is intermediate in most aspects of the device performance compared to the other structures.