Sungho Chang

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 of maximum-likelihood position estimation in small gamma camera with position-sensitive photomultiplier tube (PSPMT)

In the miniature scintillation camera that is composed of a position-sensitive photomultiplier tube (PSPMT) and a scintillation crystal, a computer simulation of maximum-likelihood position estimation (MLPE) for better correction of image than that by a conventional algorithm has been studied. So far MLPE has been proposed in several papers, and mostly experiments finding look-up table (LUT) is necessary to apply this method. In this paper, the MLPE using a simulation method instead of experiments, which can consider all stochastic processes of converting the location of gamma-ray interaction with scintillator into position signals, has been proposed, implemented, and compared to conventional algorithm. From the analysis and comparison of images, the quality of image was better than that of resulting from the conventional algorithm. We verified this simulation method through comparison this simulation images with experimental images using small gamma camera of a NaI(Tl) and a PSPMT. From the result, it is proposed that this simulation method could be applied for all PSPMT with only one simulation instead of every tedious experiment to obtain the LUT for different PSPMT even with the same specification.

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.

A New algorithm for radioisotope concentration monitoring in cooling water outlet of nuclear power plant

In the nuclear power plant (NPP), cooling water is continuously sampled through a small pipe at the outlet before flowing back to the ocean for the purpose of radioactivity concentration monitoring in water. We propose a new algorithm to identify the artificial radioisotopes probably produced in the NPP and released into the cooling water which can be added to the existing water monitoring unit without any hardware cost but by inserting a simple algorithm. The key component of the algorithm is a conversion matrix which changes the measured pulse height spectrum into the energy spectrum of gamma sources in water tank at a given period of time. This is helpful to easily identify the type of artificial gamma emitters and its activity concentration in water, specially in the case of reactor incidents or accidents, by removing the background activities, scattering effect inside water, and most importantly complex spectral components such as Compton continuum and escape peaks. The developed algorithm is based on the Monte Carlo simulation of ideal gamma energy spectrum for a uniformly distributed source in a sample tank and its comparison with measurements.