Hakjae Lee

Feasibility study of a plasma display-like radiation detector for X-ray imaging.

In this study we have investigated a 2-dimensional gas type detector based on plasma display technology as a candidate for the flat-panel radiation detector. By using the Garfield code, the dependence of X-ray absorption and multiplication on gas composition, cell gap and electric field were examined. Considering the simulation results, three prototype detectors were designed and fabricated. The performance of these detectors was evaluated by measuring the collected charge density, dark current density and sensitivity. The collected charge had the highest value at a condition when Xe 100% and 2.8 mm gap was 108.8 nC/cm² at 1000 V. The dark current of the same detector was varied from 0.0095 to 0.10 nA/cm² and about a fourth of the dark current density of a-Se based detector was at the bias range of 100-1000 V. The sensitivity of Xe 100% and 2.8 mm detector was 0.20 nC/mR·cm² at 0.36 V/um. It is about a tenth lower than that of a-Se based detector at 10 V/um.

Simulation Study of Plasma Display Panel-Based Flat Panel X-Ray Detector

Screen-film-based radiography is being rapidly replaced by digital radiography (DR). Thin-film-transistors (TFT) with amorphous silicon (a-Si) or amorphous selenium (a-Se) are usually used in DR X-ray imaging systems. Another flat panel display, plasma display panel (PDP), has a structure that is similar to that of the conventional gas type radiation detectors, and can be manufactured with lower costs than the TFT-based detector panels. The motivation of this study was to develop a cost-effective DR detector using the PDP. In order to apply the PDP technologies in gaseous detectors for X-ray imaging, we modified the pixels structure and optimized the materials inside the PDP panel. To maximize the signals intensity, we re-designed the panels structure based on the gas microstrip detector (GMD), and estimated the performance of the proposed detector using the Monte Carlo simulation method. Signal intensity of gaseous detector is determined by the amount of ionization as well as by the avalanche effect. The ionization and avalanche processes were simulated using the Geant4 and Garfield, respectively. Four types of gas mixtures and various values of electric fields have been explored. The results show that a higher proportion of Xe helps to generate more ionization electrons. The results also suggest that the electric field, which is applied between anode and cathode strips, is a dominant factor for the avalanche effect to occur. In this study, the GMD structure was adopted for the plasma-display-panel-based X-ray detector. A quantitative verification of the effectiveness of the proposed structure was performed as well.

FPGA-based multichannel data acquisition system for prototype in-beam PET

In-beam positron emission tomography (PET) is a clinically proven imaging technique for the online investigation of position emitters induced by hadron irradiation. Because the PET involves the use of many photodetectors, the data acquisition (DAQ) system of the PET system requires many elements and a high processing speed to handle many input signals and complex data sets simultaneously. To create a fast and compact DAQ system for a prototype in-beam PET system, we used an FPGA-based multichannel analysis board and developed a firmware program that was geared to acquire the prototype in-beam PET data. As a result, a flood map of the detector block with 22Na 10μCi sources was acquired, and all pixels were well separated. 18F-FDG (4-mm diameter, 3 mCi), which was located in the center of a PET ring, was explored to acquire preliminary results from the prototype in-beam PET system. We produced an energy histogram with an energy resolution of 26% at 511 keV and a reconstructed point image. The measured maximum count rate on the host PC using the developed DAQ system was 120,000 cps. Although there were many improvements in terms of the count rate, the calculation of the pulse timing and correction methods can still be improved, and we were able to assess the feasibility of the prototype in-beam PET system from the achieved preliminary results.

Development of plasma-display-panel-based x-ray detector (PXD)

The plasma display panel (PDP) is popular in the large area flat panel display market due to its relatively simple cell structure, low cost materials, and uncomplicated manufacturing process. The cell structure of PDP, which consists of electrodes and gas mixture, could be utilized in the manufacture of radiation detectors. In this study, we developed a plasma display panel based x-ray detector (PXD) based on Monte-Carlo simulation. This prototype detector panel has row and column strips, and it can thus be utilized as an imaging detector. To achieve the 2D x-ray image from the developed panel, a PXD dedicated driving and data acquisition circuit has been developed. Now we integrate the individual modules into a system. We hope to further study signal processing to achieve the first x-ray image of PXD.

3D breast registration for PET-CT and MR based on surface matching

The objective of this study is to develop a 3D breast image registration algorithm for PET-CT and MR system. Proposed algorithm consists of three stages: breast segmentation, surface matching, and image transformation. Contrast-enhanced MR image volume was used as a reference and CT volume was transformed with varying parameters to calculate similarity between two image modalities. At first, the breast regions were cropped separately by the pre-determined regional masks. For the CT image case, region growing based breast segmentation was explored to eliminate the breast dedicated jig which has been developed to hold the shape of breast during PET-CT scan. After the segmentation, each of the points set of breast surface has been extracted. The extracted point sets were used as a feature vector for the surface matching. For the surface matching, we developed modified version of elastic-convolved iterative closest point (ECICP) algorithm to obtain the optimal transformation parameters. Then PET image was transformed with those parameters and overlaid it onto MR image. The results of this study show that the difference between MR and CT image has been considerably reduced and the suspicious lesions on MR and PET images were matched as well.

Registered collimation for pixellated SPECT detectors

We investigated a high sensitive collimator (registered collimator) design and associated image reconstruction methods. Its opening was designed to align with the CZT pixel by having same pitch and shape. By the combination of wide opening holes and low septa height, the sensitivity of the collimator can increase dramatically. However, it may also lead to loss of system resolution. The goal of this study is 1) to determine the optimal parameters of the collimator which can increase sensitivity and decrease the charge sharing effect, and 2) to develop system matrix for iterative image reconstruction that can adequately model the proposed collimation scheme to achieve high resolution recovery in spite of using wider collimation holes. Thickness and height of septa in the registered collimator were analytically determined to 1 mm which blocks 97% of 140keV gamma rays and 5.48 mm which indicates 0.2 % efficiency that equivalently means about 10 times higher counts than low energy high resolution(LEHR) collimator. Then 2D fan beam-based geometrical response function(GRF) was developed to model the wide opening holes in the registered collimator. The GRF was included in the system matrix of ordered subset expectation maximization(OSEM) method to compensate the loss of resolution. For performance comparison, GATE simulation was performed using a 5 mm diameter hot sphere phantom, which is located at 1 cm off-centered radial position, and a 40.36 by 40.36 mm CZT detector module. The preliminary results showed that the proposed collimator achieved about 13 times more counts and improved image resolution. In the future, we will keep our experiments with3D cone beam model as well as 2D fan beam model using more realistic phantoms.

Development of a Multipurpose Gamma-Ray Imaging Detector Module With Enhanced Expandability

A silicon photomultiplier (SiPM) is considered a next generation photo-sensor for low-light applications. In this study, we propose and develop the general concept of a multipurpose submillimeter gamma-ray (MSG) imaging module that is applicable to most gamma-ray imaging devices. The main features of the MSG module are submillimeter intrinsic resolution, geometrically expandable structure, expandable data acquisition system, multi-energy capability and usability in intense magnetic fields. The elements of the gamma-ray detector are pixelated Ce:GAGG (

Development of a DAQ circuit for a plasma-display-panel-based X-ray detector

31\times 31

Deformable registration for breast PET-CT and MR images based on perturbation weighted feature information

pixels, 0.8 mm pitch), a 1.5-mm acrylic light guide, and one SiPM array (

Image registration for PET/CT and CT images with particle swarm optimization

8 \times 8