Yohei Kikuchi

Polarization Phenomena in TlBr Detectors

A comparison of long-term stabilities of TlBr detectors with Au and Tl electrodes was performed in this study, in order to reveal polarization phenomena in TlBr detectors. Au/TlBr/Au detectors exhibited polarization phenomena at room temperature, observed as degradation of spectral performance and reduction of leakage current with elapsed time. Applying reverse bias voltage to the detectors recovered the detector performance temporarily. No polarization phenomena were observed in Tl/TlBr/Tl detectors at room temperature. The detector exhibited stable spectral performance for 24 hours.

Polarization phenomena in TlBr detectors

A comparison of long-term stabilities of TlBr detectors with Au and Tl electrodes was performed in this study in order to reveal polarization phenomena in TlBr detectors. Au/TlBr/Au detectors exhibited polarization phenomena at room temperature observed as degradation of spectral performance and reduction of leakage current with elapsed time. Applying reverse bias voltage to the detectors recovered the detector performance temporary. No polarization phenomena were observed in Tl/TlBr/Tl detectors at room temperature. The detector exhibited stable spectral performance for 24 hours.

Evaluation of TlBr detectors with Tl electrodes

Long-term stability of TlBr detectors with Tl electrodes was evaluated in this study. Tl/TlBr/Tl detectors were fabricated and tested with a charge sensitive preamplifier and a spectroscopy amplifier and a gated integrator. Significant improvement of energy resolutions was realized by applying the gated integrator to the TlBr detectors. The full-energy peak positions, counts and resolutions for 662 keV gamma-rays from 137Cs were monitored during the course of measurements (10 days). By changing the polarity of the applied bias voltages every 24 hours, the Tl/TlBr/Tl detectors exhibited stable spectroscopic performance for totally over 240 hours.

Basic evaluation of three-dimensional position sensitive CdTe detector unit for clinical use of ultra-high resolution human PET scanner

We develop ultra-high resolution human PET scanner with the worlds first resolution of less than 1 mm, which is used CdTe type semiconductor detectors. The aim of this study is that we develop three-dimensional position sensitive CdTe detector units which construct PET scanner, and evaluate basic performance of this scanner. Here we report about progress and outcome of its development. As the detector of our scanner, we developed position sensitive CdTe detectors (2D-PSD) two-dimensional . It can acquire interaction position inside the detector with positional resolution of 1.2 mm. By densely stacking 2D-PSD and subsequent circuits (Amplifiers, ADC, and other processing circuits), we developed three-dimensional position sensitive CdTe detector units. By placing these detector units with shape of decagon, we constructed PET gantry. In order to determine the operating conditions of the scanner, we performed evaluation and improvement of each detector unit. We confirmed high performance of each detector unit from their results, but we found out some of detectors indicate low performance and they need more improvement. Also we could determine the operating conditions (Setting values of bias voltage and each signal processing) to detectors from their results. For polarization phenomena of CdTe detector, we developed a bias reset system and decided its operating condition by experimental results. Next we evaluated the basic performance of the scanner.We moved a point source three directions in the field of view, and we evaluated basic spatial resolution. Finally we performed some phantom measurements, and we could obtain reconstruction images of their phantoms. By this research, we could forward development of the scanner to the stage of phantom study. However improvement of detector performance is a significant issue in the future. We will continue to evaluate performance of the scanner for clinical use.

Application of List Mode Image Reconstruction to Fine DOI-PET Scanner Using Position-Sensitive CdTe Semiconductor Detector Unit

We report the effects of implementing the list mode image reconstruction method (LMR) in a new brain scanner based on positron emission tomography. The benefits of LMR include reduced load in imaging processes and conservation of fine spatial data-sampling, the latter of which may be lost by data binning in histogram mode image reconstruction (HMR). In our project, we aim to build a high-spatialresolution scanner that employs three-dimensional positionsensitive CdTe semiconductor detector units. We experimentally confirmed that the unit could measure the detected positions of annihilation photons with a position resolution of 1 mm3 full width at half maximum (FWHM). Therefore, the scanner can potentially minimize the parallax-induced decrease in the spatial resolution of off-center positions in the field of view (FOV), thereby providing high resolution throughout the FOV because it obtains accurate depth of interaction (DOI) information at a sampling pitch ~1 mm finer than that of conventional DOI technologies (~4 mm). We simulated the proposed scanner. using the GEANT4 Application for Tomographic Emission. Reconstructed images were obtained by LMR or HMR based on the maximum likelihoodexpectation maximization (ML-EM) algorithm. The system matrix for ML-EM reconstruction was calculated by Siddon’s ray-driven method to fully exploit accurate detection positions. The spatial resolution at the center of the FOV acquired by LMR is 0.6 mm FWHM, which is superior to that acquired by HMR (1 mm FWHM). We confirmed that the resolutions of both methods are maintained at off-center positions, and the resolutions acquired by LMR at these positions are superior to those acquired by HMR. Furthermore, we simulated Hoffmanbrain- phantom imaging to evaluate the image quality of the scanner and LMR in human brain studies.

Evaluating radiocesium retention ability of root-mat horizon using micro-PIXE analysis

A lot of radiocesium had been deposited onto pastures and grasslands following Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. More radiocesium was accumulated in root-mat horizon than in both above ground plant bodies and mineral soils. In this study, factors causing higher radiocesium concentrations in root-mat horizon were evaluated by the addition of stable cesium solution and particle-induced X-ray emission (PIXE) analysis. Results suggest that adsorption onto root surfaces played a significant role in Cs accumulation in root-mat horizon. Furthermore, absorption of Cs was key to its long-term preservation. The adsorption of Cs by clay minerals also contributed to its retention. A slow water infiltration rate may also affect the enrichment of radiocesium in root-mat horizon. Based on these results, it is concluded that both biotic and abiotic factors contributed to the effective retention of radiocesium in root-mat horizons following the FDNPP accident.