Hosang Jeon

A scatter correction using thickness iteration in dual-energy radiography

With area detectors, scattered radiation causes the dominant error in separation of different materials properly. Several methods for the scatter correction in dual energy imaging had been suggested and improved results. Such methods, however, need additional lead blocks or detectors, and additional exposures to estimate the scatter fraction for every correction. We suggest the scatter correction by using a database of the fraction and distribution of scattered radiations. To verify this method we did MCNP simulation. The generation of the scatter information for each combination of thickness of an aluminum-water phantom had been done. Based on the uncorrected signals, thickness of each material can be calculated by a conventional dual-energy algorithm. And then the scatter information of corresponding thickness from the look-up table is used to correct the original signals. The iterative scatter correction reduced relative-thickness error in results. This scatter correction method can be applied to two-material dual-energy radiography like mammography, contrast imaging, or industrial inspection.

Fabrication and comparison Gd 2 O 2 S(Tb) and CsI(Tl) films for X-ray imaging detector application

During the last decade, digital X-ray imaging systems have been replacing analog X-ray imaging systems of conventional X-ray film-screen combination for radiography applications. Indirect detection methods consisted of an X-ray converter (or a scintillator film) and photodiode arrays are more widely used in medical diagnoses and industrial fields. Two major scintillation materials such as terbium doped gadolinium oxysulfide (Gd 2 O 2 S:Tb, GOS) and thallium doped cesium iodide (CsI:Tl) are commonly used. In this work, GOS scintillator films were manufactured by mixing and thermal hardening of Gd 2 O 2 S:Tb powder, dispersion agent, hardening agent, and other organic additives. And CsI:Tl scintillator films with columnar structure were also fabricated by the thermal evaporation method. The scintillation properties, such as emission spectrum and light yield etc., of the GOS and CsI:Tl films were measured by X-ray luminescence and photo-luminescence (PL) methods. The maximum luminescent intensity of both scintillators was observed at 540–560nm wavelength. In order to investigate the imaging performances of both GOS and CsI:Tl films as converters of X-ray imaging detectors, both scintillator films were coupled with an CCD sensor. The light response to X-ray dose, signal-to-noise ratio (SNR), spatial resolution were measured and analyzed under the same X-ray conditions. As X-ray dose increases, the SNR curves showed linear relationship. And the spatial resolution of two scintillator films was resolved at 7∼8lp/mm.

Simulation Study on DOI-PET Module Design Using LSO and New Silicon Photomultiplier

SiPM (silicon photomultiplier), which is 2D arrays of avalanche photodiodes operated in Geiger mode, is one of the candidates for photosensors of PET. In this paper, DOI-PET detector, which consists of 2 SiPM of 1.5 mm times 1.5 mm pixel size and 3 cm long LSO crystals, was designed using TCAD and DETECT2000. Because light photon distribution of scintillation crystal becomes different with respect to surface treatments, simulations to improve of DOI information were done with different surface conditions. Side surface treatments of LSO, which have best DOI information, were determined by DETECT2000 simulation. High quantum efficient SiPM was also designed using TCAD. In order to enhance quantum efficiency of SiPM, shallow junction is made by implantation and anti-reflection coating is also optimized.

Radiation damage monitoring using gamma-camera at cyclotron facility

Though there are number of measuring and monitoring devices, the gamma camera is very useful monitoring system in various industrial, environmental and medical diagnostic fields and it for industrial applications was developed so far.

A Simulation Study on Spatial Resolution and Noise Power Spectra of a URA-based Multi-hole Collimator in a Small Gamma Camera

Presently the gamma scintillation camera is widely used in various industrial, environmental and medical diagnostic fields. Two major performance parameters, spatial resolution and noise, are primarily determined by the collimator. The imaging performance of the simple collimator, single pin-hole, and a coded-aperture collimator, uniformly redundant array (URA), are analyzed in this study. Though parallel-hole collimators are used in some medical applications, in principle its performance is equivalent to that of a single-hole collimator and moreover its use is limited to the near-field application only. A coded aperture imaging (CAI) techniques have been proposed for gamma-ray imaging especially for far-field applications such as the astrophysics study or environmental monitoring in order to overcome these limitations of a pin-hole or a parallel-hole collimator. In this study, the simulated gamma images using a monte carlo code, MCNP, were acquired with two different energy windows and two-type and four-different collimators. For the evaluation of spatial resolution and noise power spectra, modulation transfer function (MTF) and normalized noise power spectra (NNPS) were computed for all images. In the result, we found that MTF and NNPS can reflect the different properties of different gamma camera systems.

Monte Carlo Study on Optimization of Reflectors in Pixelated CsI Film for Mammographic Application

Isotropic light spread in the scintillator film decreases the spatial resolution of scintillator-based digital X-ray imaging systems such as digital radiography and mammography. Pixelation of the scintillator film could be a good solution to overcome this limitation. This has been demonstrated with pixilated CsI:Tl layers which was made by thermal evaporation process on a pre-patterned substrate or which was post-patterned by laser after preparation. Additionally, in order to minimize cross-talk between pixels and to maximize light collection efficiency, a reflection material can be coated on the top and side surfaces of each pixilated scintillator block. In This paper, several materials such as Al, and quarter-wave multilayer reflector with SiO2 and TiO2 were considered as the reflector materials of pixilated CsI:Tl scintillator blocks. Through a serious of simulations given below on these simple 2-dimensional scintillator blocks coated by a reflector, reflectivity, cross-talk, and modulation transfer function were calculated to find the optimum reflector.

A small gamma camera combined with a uniformly redundant array (URA) collimator: the performance evaluation using detective quantum efficiency (DQE)

The gamma scintillation camera systems are recently used in various industrial, environmental and medical diagnostic fields. Moreover, the portable use in various fields is possible using miniaturized gamma camera systems. The necessity of objective and quantitative factors for the performance evaluation of gamma camera systems increase as the usefulness of these systems increase. The detective quantum efficiency(DQE), which is already applied to the performance evaluation of X-ray imaging systems, was partially revised and applied to our gamma camera system. In this study, our gamma camera system consisted of a scintillation crystal, a PSPMT with 5-inch diameter and read-out electronics. The four collimators, three pinhole collimators with different pinhole diameters and a coded-aperture collimator using uniformly redundant array (URA), were manufactured for imaging experiments. In the result, the spatial resolution slightly decrease as pinhole diameter increase because of thick scintillation material for high energy photons detection. The normalized noise power spectra obviously decrease as pinhole diameter increase because of signal to noise ratio. Finally, the DQE increase as pinhole diameter increase. The highest DQE was presented by the URA collimator. In spite of slightly low spatial resolution, URA present highest performance because of very low noise power. Our results present interrelation with conventional evaluating factors, for example, FWHM and uniformity. We found that DQE can be effectively used to evaluate imaging performance of gamma camera systems.

An experimental study on the variation of MTF and NPS caused by x-ray beam conditions for three indirect digital radiographic imagers

Digital radiographic X-ray imagers are increasingly used in many clinical areas. So it is important to evaluate the performance of imagers. There is growing consensus in the scientific world that the detective quantum efficiency (DQE) is the best-suited parameter for describing the imaging performance of an X-ray imaging device. DQE is calculated with modulation transfer function (MTF) and noise power spectrum (NPS). As a preliminary study, MTF and NPS of three indirect digital radiographic imagers were measured and analyzed for comparison of imager performance for three imagers. The imagers were evaluated using two standard X-ray beam qualities, RQA5 and RQA9 recommended by an International Electrotechnical Commission (IEC) standard. For MTF and NPS measurement, edge method and two-dimensional Fourier analysis method were adopted each other. In result, the highest MTF is observed in the imager with smallest pixel pitch and preprocessing of raw data among three imagers. There is an obvious MTF difference with preprocessing steps and a little difference between RQA5 and RQA9. NNPS is also affected by preprocessing steps. There are the significant increase of noise at low frequency because of the increase of the multiplicative noise component as exposure increases except CDXI-40G (Canon). The low NPS is relatively caused by high exposure level because of noise conversion process. In comparison, the lowest NPS imager was found. The evaluation of imaging performance using DQE concept will be done as our further study.