Measurement of electron density and effective atomic number using dual-energy x-ray CT

The information on the electron densities of bodies is important for the treatment planning of radiotherapy. In order to obtain the electron densities directly, we have developed dual-energy x-ray computed tomography(CT) using synchrotron radiation. It was experimentally proved that the electron density was deduced with about 1 % accuracy from two linear attenuation coefficient. However, the linear attenuation coefficient were measured by a few percent lower than the theoretical ones. We assumed that the less accurate linear attenuation coefficients were caused by the influence of scattered radiation and the non-linearity in the response of the detector. In comparison of the scattered radiation to the simulation results, the scattered radiation contributed at the most 0.5 % to the linear attenuation coefficients. Correcting the non-linearity in the detector response functions, the values of linear attenuation coefficient were improved drastically.ROME 2004 NSS - MIC - SNPS and RTS

AAPM Anual meeting 2004

AAPM　Anual meeting 200

Performance evaluation of the first model of 4D CT

World Congress on Medical Physics and Biomedical Engineering 200

Estimation of scattered radiation amount and its effects on image quality in 4D CT

We have developed a prototype of 4-dimensional (4D) CT-scanner that employs continuous rotation of cone-beam, the number of elements are 912 channels x 256 segments; element size is approximately 1mm x 1mm. Scanning time is 1.0sec/rotation. Data sampling rate is 900 views (frames)/sec. The potential drawback of cone beam CT is a larger amount of scattered x-rays than conventional CT-scanner. Our purpose in this work was to estimate scattered-to-primary ratio (S/P) and observe its effects on the reconstructed images as variation of a beam width (5-128mm). S/P was measured by scanning cylindrical water phantom and interrupting the primary with Pb disk. The effects of scattered radiation on the images was evaluated with noise and uniformity of CT number, the low contrast detectability and the differences of CT number between air cavity in the water phantom and air region outside it.\nThough S/P increased as the beam width became wider, noise and uniformity of CT numbers depended little on the beam width. Low contrast detectability did not depend on it. However CT numbers differed 38 at 5mm beam width and 67 at 128mm beam width between the air cavity and the air region outside. S/P of 200mm-diam-water phantom for 120kVp was approximately 8% (128mm beam width) and approximately 1% (10mm beam width). The results suggested that scattered radiation seemed little effect on image quality except quantitativeness of CT-number in 4D CT.RSNA(Radiological Society of North America

Magnitude and effects of x-ray scatter in a 256-slice CT scanner

We developed a prototype 256-slice CT scanner that employs continuous rotation of a cone-beam with a larger cone angle than conventional multidetector CTs (MDCT) to ensure a wide field of view. However, a larger cone angle may result in image deterioration due to increased x-ray scatter. Scattered radiation causes the detected signals to deviate from the true measurement of primary x-ray intensity and may result in artifacts (e.g., cupping and streak artifacts), quantitative inaccuracy in reconstructed CT number, and degradation of contrast-to-noise ratio (CNR). To reduce the effects of scatter, the 256-slice scanner incorporates an antiscatter collimator. Here, we estimated the magnitude of x-ray scatter in the prototype 256-slice CT scanner under clinical scan conditions and quantified the effects of this scatter on CT number accuracy, image noise, uniformity, and low contrast detectability. Although most experiments were performed with the antiscatter collimator, we also estimated the magnitude of x-ray scatter without the collimator to evaluate the scatter rejection efficiency of the collimator. The scatter-to-primary energy fluence ratio (SPR) without the collimator increased as cone angle increased, with estimated values of 49.7% for a 138 mm beam width with a phantom of 200 mm diameter, and 78.5% for a 320 mm diameter phantom. Estimated SPR was drastically decreased with the collimator, with an SPR reduction rate (ratio of SPR with and without the collimator) of 12.7% and 16.8% for the 200 and 320 mm diameter phantoms, respectively. The reduction in x-ray scatter by the collimator resulted in a considerable reduction in scatter effects. The measured uniformity was good and was independent of scatter amount. Although scatter still affected CT number accuracy, this could be corrected by rescaling. Further, although the CNR was decreased, in theory at least, the change was so subtle that it had no substantial effect on low-contrast detectability

Evaluation of weighted FDK algorithms applied to four-dimensional CT (4D-CT)

Nuclear Science Symposium/ Medical Imaging Conference (Rome 2004