Keiji Kobashi

A Pulsed Bias Voltage Shutdown Circuit for a Matrix Readout System of a CdTe Radiation Detector

A pulsed bias voltage shutdown circuit to suppress the polarization effect in a CdTe radiation detector equipped with a matrix readout system was investigated. The matrix readout system is suitable for radiation detection systems that have many pixels and that are used at a relatively low counting rate, such as single photon emission computed tomography (SPECT). CdTe detectors have a time-dependent polarization effect that must be suppressed. One method to suppress this effect is pulsed bias voltage shutdown. However, on the bias voltage supply side of the matrix readout system, the speed of the voltage change depends on a time constant consisting of a bias resistor and a coupling capacitor. Therefore, the voltage cannot be changed within the time constant for the matrix readout system. To overcome this limitation, three circuits have been added: a clamp circuit to the noise filter resistor of the bias voltage supply, a clamp circuit to the bias resistor, and protection circuits at the charge amplifier inputs. These circuits make it possible to change the bias voltage faster than the time constant of the bias resistor and the coupling capacitor. Although a noise signal is observed after the bias voltage has recovered, the noise decays below 20 keV about 30 ms after the bias voltage recovery. Our results demonstrated that the proposed pulsed bias voltage shutdown method is very efficient to suppress the polarization effect within 100 ms for the CdTe detector with the matrix readout system.

A sophisticated maximum capacity analysis for large turbine generators considering limitation of temperature

In this paper, the authors propose a sophisticated analysis method to determine possible maximum generator capacity and to evaluate the generators potential ability. The method is developed to survey generator design parameters on a large scale efficiently and objectively. The possible maximum generator capacity acquired by the proposed method satisfies the specifications of short circuit ratio, power factor and temperature limitations of armature and field windings. Armature voltage, armature current and field current are optimized simultaneously. An example adopting the proposed method is also shown in this paper. The results indicate the sensitivities of the calculated possible maximum generator capacity and the calculated loss for various development factors. The effectiveness of this method in the R&D phase is clarified by the example.

Dual Isotope SPECT Study With Epilepsy Patients Using Semiconductor SPECT System

Purpose We developed a prototype CdTe SPECT system with 4-pixel matched collimator for brain study. This system provides high-energy-resolution (6.6%), high-sensitivity (220 cps/MBq/head), and high-spatial-resolution images. The aim of this study was to evaluate dual-isotope study of CBF and central benzodiazepine receptor (BZR) images using 99mTc-ECD and 123I-IMZ with the new SPECT system in patients with epilepsy comparing with single-isotope study using the conventional scintillation gamma camera. Methods This study included 13 patients with partial epilepsy. The BZR images were acquired at 3 hours after 123I-IMZ injection for 20 minutes. The images of IMZ were acquired with a conventional 3-head scintillation gamma camera. After BZR image acquisition with the conventional camera, 99mTc-ECD was injected, and CBF and BZR images were acquired simultaneously 5 minutes after ECD injection with the new SPECT system. The CBF images were also acquired with the conventional camera on separate days. The findings were visually analyzed, and 3D-SSP maximum Z scores of lesions were compared between the 2 studies. Results There were 47 abnormal lesions on BZR images and 60 abnormal lesions on CBF images in the single-isotope study with the conventional camera. Dual-isotope study with the new system showed concordant abnormal findings of 46 of 47 lesions on BZR and 54 of 60 lesions on CBF images with the single-isotope study with the conventional camera. There was high agreement between the 2 studies in both BZR and CBF findings (Cohen &kgr; values = 0.96 for BZR and 0.78 for CBF). In semiquantitative analysis, maximum Z scores of dual-isotope study with the new system strongly correlated with those of single-isotope study with the conventional camera (BZR: r = 0.82, P < 0.05, CBF: r = 0.87, P < 0.05). Conclusions Our new SPECT system permits dual-isotope study for pixel-by-pixel analysis of CBF and BZR information with the same pathophysiological condition in patients with epilepsy.

Monte Carlo-based scatter correction considering the tailing effect of a CdTe detector for dual-isotope brain SPECT imaging

A Monte Carlo (MC)-based scatter correction method considering the tailing effect of a CdTe detector was developed for dual-isotope brain single-photon emission computed tomography (SPECT) imaging using technetium-99m (99mTc) and iodine-123 (123I), and its accuracy was validated by measuring phantoms. The tailing effect was modeled by convolutions of energy spectra obtained by geometry and tracking (GEANT) simulation with energy smoothing kernels. In our experimental phantom studies, quantitative accuracy and image contrast in the reconstructed image of dual-isotope-filled phantoms with our MC-based scatter correction method (Dual_SC) were compared with those of single-isotope-filled phantoms (Single_SC). The quantitative accuracy was evaluated by the percent error between the estimated activity concentration and true activity concentration. In our six-compartment phantom study with six different activity concentrations, the mean absolute percent errors of 99mTc for Single_SC and Dual_SC were 1.7% and 2.8%, respectively, while those of 123I for Single_SC and Dual_SC were 4.8% and 5.6%, respectively. In our striatal phantom study, the percent errors in the background regions for Single_SC and Dual_SC were less than 2% for both 99mTc and 123I. The image contrast was evaluated by the percent contrast of a cold or hot spot region to a background region. In our cold rod phantom study, the mean percent contrasts in the cold rod regions of 99mTc for Single_SC and Dual_SC were 66.2% and 65.0%, respectively, while those of 123I for Single_SC and Dual_SC were 59.3% and 61.6%, respectively. In our striatal phantom study, the percent contrasts in the striatal regions of 99mTc for Single_SC and Dual_SC were 56.1% and 56.1%, respectively, while those of 123I for Single_SC and Dual_SC were 39.8% and 40.0%, respectively. In conclusion, dual-isotope imaging with the CdTe-based brain SPECT system and our MC-based scatter correction method can provide comparable quantitative accuracy and image contrast to those of single-isotope imaging.

Performance assessment study of a 250 MVA air-cooled turbo-generator

Today, many customers require generators that combine high energy efficiency with user-friendly operation. The long term reliability of the unit is another of the most important customer considerations. To meet these expectations, Hitachi has designed a large air-cooled turbo-generator equipped with an Inter Cooler Ventilation System (ICVS), which achieves an efficiency of over 98.8% at 250 MVA. New findings based on studies of the 250 MVA generator are reported here. Due to the degree of circulating current losses, the temperature does not necessarily reach its highest point at the innermost position. After identifying the critical points, 1000 sensors were set in the test generator and its operational performance was evaluated. The temperature evaluations related to the stator windings and the measured flow in the air gap demonstrated a positive correlation with the measured values. The highest temperature in the generator had a wide margin relative to the class B temperature limitations. This performance-based assessment will assist in assuring the long-term reliability of the generator.

Semiconductor Detector-Based Scanners for Nuclear Medicine

Semiconductor detectors have the potential to improve the quantitative accuracy of nuclear medicine imaging with their better energy and intrinsic spatial resolutions than those of conventional scintillator-based detectors. The fine energy resolution leads to a better image contrast due to better scatter rejection. The fine intrinsic spatial resolution due to a pixelated structure leads to a better image contrast and lower partial volume effect. Their pixelated structures also improve the count-rate capability. The authors developed CdTe semiconductor detector-based positron emission tomography (CdTe-PET) and single-photon emission computed tomography (CdTe-SPECT) in order to test the potential of using semiconductor detectors in nuclear medicine. The physical performances of both systems were measured in several phantom experiments. The capability of using CdTe-PET to measure the metabolic distribution of tumors was evaluated through scans of cancer patients and rat tumor models. The feasibility of using CdTe-SPECT for simultaneous dual-radionuclide imaging was evaluated through scans of phantoms and healthy volunteers. The results suggest that the prototype CdTe-PET can identify intratumoral metabolic heterogeneous distribution and that CdTe-SPECT can accurately acquire dual-radionuclide images simultaneously. Although there are still problems to be solved, semiconductor detectors will play significant roles in the future of nuclear medicine.

Collimator for Variable Sensitivity and Spatial Resolution Without the Need for Exchange

A new design of collimator is proposed that has variable sensitivity and spatial resolution, eliminating the need for exchanging collimators in a gamma camera. Using Monte Carlo simulations, the present article evaluates the shielding of undesirable gamma rays in a parallel-hole collimator. It consists of a number of layers of rectangular holes. These layers consist of alternately stacked fixed and movable collimators. In high-resolution mode, the movable collimators are shifted by half the aperture pitch along the diagonal direction. The first collimator (type A) has 50 layers with fixed thicknesses of 1.2 mm. The second collimator (type B) has 25 layers with a thickness of 1.0 mm on the object side and 25 layers with a thickness of 1.4 mm on the opposite side. The third collimator (type C) has 20 layers with non-uniform thicknesses. The ratios of the maximum artificial peak to the main-peak are calculated for point-source responses. The ratios for types A, B, and C collimators are 0.78, 0.08, and 0.03, respectively. The same performance for shielding undesirable gamma rays is achieved in the type C collimator as for a conventional collimator.