Tetsuhiko Takahashi

Design of integrated radiation detectors with a-Si photodiodes on ceramic scintillators for use in X-ray computed tomography

An integrated radiation detector with a-Si photodiodes on ceramic scintillators is designed for use in X-ray computed tomography. The output signal is estimated using Monte Carlo simulation. Results show that this detector has higher sensitivity than a c-Si photodiode/scintillator detector when a transparent layer between the scintillator and the thin film semiconductor is optimized. The noise characteristic is also considered when an integrated detector is coupled with an operational amplifier. Good agreement is shown between predicted and measured parameters. >

Integrated Radiation Detectors with a-Si Photodiodes on Ceramic Scintillators

Solid-state monolithic-type X-ray detectors were fabricated by preparing amorphous silicon photodiodes monolithically on a Gd2O2S ceramic scintillator. The X-ray detection sensitivity of the solid-state detectors was comparable to that of a xenon ionization detector. It could be improved by optimizing the amorphous silicon photodiode and the intermediate materials between the photodiodes and the scintillator. The performance of the X-ray detector did not deteriorate even after 500,000-roentgen irradiation.

Laser‐stimulable transparent CsI:Na film for a high quality x‐ray imaging sensor

Transparent films, which can be stimulated by laser beams after x‐ray irradiation, have been searched to improve the spatial resolution of digital x‐ray imaging sensors. As a result, evaporated CsI:Na films are found to be efficiently laser stimulable around 77 K. The modulation transfer function (MTF) of the film is evaluated using a scanning Ga1−xAlxAs semiconductor laser. The high MTF value (57% at 2 1p/mm), strong x‐ray absorption, and high stimulation efficiency of the film ensure a high quality sensor for digital radiography.

Predicting Carotid Plaque Characteristics Using Quantitative Color-Coded T1-Weighted MR Plaque Imaging: Correlation with Carotid Endarterectomy Specimens

BACKGROUND AND PURPOSE: MR plaque imaging is used to evaluate the risk of embolic complications during carotid endarterectomy and carotid artery stent placement. However, its performance for characterizing intraplaque components has varied across studies and is generally suboptimal. Hence, we correlated MR imaging results with histologic findings to determine whether a combination of high-contrast T1-weighted imaging and quantitative image analysis could readily determine plaque characteristics. MATERIALS AND METHODS: We prospectively examined 40 consecutive patients before carotid endarterectomy by using a 1.5T scanner and axial T1-weighted spin-echo images under optimized scanning conditions. The percentage areas of intraplaque fibrous tissue, lipid/necrosis, and hemorrhage were calculated automatically by using the software with previously reported cutoff values and were compared with those of the specimens. The thickness of the fibrous cap was also measured manually. RESULTS: The percentage areas of fibrous, lipid/necrotic, and hemorrhagic components were 5.7%–98.7%, 1.3%–65.7%, and 0%–82.0%, respectively, as determined by the MR images, whereas the corresponding values were 4.8%–92.3%, 7.0%–93.8%, and 0%–70.4%, respectively, as determined by histologic examination. Significant positive correlation and agreement were observed between MR images and histologic specimens (r = 0.92, 0.79, and 0.92; intraclass correlation coefficients = 0.91, 0.67, and 0.89; respectively). Thickness of the fibrous caps on MR images (0.21–0.87 mm) and in the specimens (0.14–0.83 mm) also showed positive correlation and agreement (r = 0.61, intraclass correlation coefficient = 0.59). CONCLUSIONS: Quantitative analysis of high-contrast T1-weighted images can accurately evaluate the composition of carotid plaques in carotid endarterectomy candidates.

Cis-Trans Photoisomerization of Perinaphthothioindigo for Use as a Photo-Imaging Sensor Using Fluorescence under He-Ne Laser Excitation

An imaging sensor based on the cis/trans isomerization of thioindigoid dyes is proposed. A photo-image (~500 nm) is stored as a density distribution of the trans-form in the cis-form material, and read out as fluorescence (~700 nm) from the trans-form by scanning with He-Ne laser (633 nm) excitation. As the fluorescence decay time of perinaphthothioindigo is 1.8 ns, the sensor permits rapid read-out. For light exposure L from 10-4 to 10-1 J/cm2 at 500 nm, the fluorescence intensity under laser excitation varies as L0.8.

Noninvasive Evaluation of Collateral Blood Flow through Circle of Willis in Cervical Carotid Stenosis Using Selective Magnetic Resonance Angiography

BACKGROUND Preoperative assessment of intracranial collateral circulation is helpful in predicting cerebral ischemia during surgical procedures for cervical internal carotid artery (ICA) stenosis. However, magnetic resonance angiography (MRA) and other less-invasive techniques cannot evaluate collateral blood flow because these techniques are nonselective. Hence, by using a newly developed selective MRA technique, we attempted to visualize collaterals via the circle of Willis in patients with ICA stenosis. METHODS Twelve patients who underwent carotid endarterectomy were prospectively examined with a 1.5-T MR scanner. Both selective and nonselective MRA were obtained using a 3-dimensional time-of-flight technique, with or without a cylindrical saturation pulse that suppresses the flow signal from the region of the target ICA. Maximum intensity projection MRA images were generated and compared with digital subtraction angiography (DSA) images. RESULTS In all patients, the distal flow signal of the ipsilateral ICA was completely suppressed on selective MRA compared with nonselective MRA. In addition, collateral blood flow through the anterior and posterior communicating arteries was visualized in 5 and 2 patients, respectively. These findings corresponded well with the DSA imaging. CONCLUSIONS Selective MRA techniques can readily suppress signals from the distal blood flow of the target artery and visualize the presence of collateral flows through the circle of Willis in patients with cervical ICA stenosis.

A Robust Ultrashort TE (UTE) Imaging Method With Corrected k-Space Trajectory by Using Parametric Multiple Function Model of Gradient Waveform

Ultra-short TE (UTE) sequences with radial sampling make it possible to visualize tissues with very short T2 decay times. The UTE sequence acquires an echo signal from the central to the outer parts of k-space and is very sensitive to small trajectory errors. Therefore, k-space errors caused by imperfections in the gradient system performance, such as gradient delay and waveform distortion, must be corrected. During normal clinical use, these errors must be corrected to account for any gradient strength, or image obliquity. Because of time limitation on clinical examination, a simple, robust, and time-efficient correction method for use with UTE is needed. We demonstrated image degradation due to k-space errors by simulation and found that uncontrolled gradient time delays were the dominant cause of image degradation. They could be corrected by using a pre-scan calibration that works by comparison of half and full echo signals. Further improvements in image quality were achieved by using a one-time calibration of gradient waveform approximations that were built from multiple exponential functions and were used during image reconstruction. We have developed a robust UTE correction method that consists of a gradient waveform approximation that follows a short pre-scan for estimating gradient time delay errors.

Magnetic Resonance Imaging (MRI) Compatible Ultrasound Therapeutic System

Since an ultrasound (US) imaging system can image in real-time and interactively, it can be used as an image guidance assisting magnetic resonance imaging (MRI) for minimally invasive therapy. The effects of MRI-compatible US probes on MRI monitoring were evaluated, and it was found that the MRI-compatible US probes, whose backing material contained 100 ppm ferrite, did not disturb MR monitoring except at a few mm radius from the US probes position. MRI temperature monitoring of a swine liver irradiated with a high-intensity focused US beam from an MRI-compatible therapeutic transducer with US image guidance was then performed, and the potential usefulness of such a therapeutic system in minimally invasive therapy was demonstrated.

Highly stable solid‐state x‐ray detector array

A detailed analysis of the effects of temperature changes, over time and between array elements, on the generation of circular artifacts in images produced by x-ray computed tomography was reported. We give formulas for calculating--according to the x-ray energy, detector sensitivity, and observed image quality (contrast and spatial resolution)--the maximum offset temperature coefficient and maximum gain temperature coefficient that will allow circular-artifact-free imaging. A temperature-controlled and insulated solid-state x-ray detector array, consisting of Gd2O2S:Pr,Ce,F ceramic scintillators coupled to crystal Si pin-photodiodes and designed to meet the requirements for these coefficients, produced high-resolution artifacts-free CT images of a phantom head.

Positional lumbar imaging using a positional device in a horizontally open-configuration MR unit—Initial experience

To evaluate whether positional MR images of the lumbar spine, obtained with a horizontally open‐configuration MR unit, demonstrate positional changes of the dural sac, and to assess whether there are significant differences in positional changes between healthy volunteers and patients with chronic low back pain.