Med Phys
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Official radiogenic cancer risk estimates for low-dose, protracted exposure conditions have been based on linear, no-threshold downward extrapolation from medium and high-dose effects among a population of A-bomb survivors, with the application of a downward correction for an assumed reduced biological effectiveness at low doses and low dose rates (DDREF correction). Neither in the follow-up of populations exposed to the high-dose A-bomb flash, nor from epidemiological data after low-dose occupational or medical irradiation is there any convincing evidence for this DDREF hypothesis--even less for a zero-effect threshold dose. ⋯ Excess cancer mortality following occupational exposures to ingested fission products and radiation-associated teratogenic, genetic, and cancer detriment among diverse populations who had ingested small amounts of radioactivity after the precipitation of fallout at great distances from the Chernobyl nuclear explosion, suggest discrepancies of as much as 2 orders of magnitude with official risk estimates. Contrary to widely publicized statements, claiming that current regulations of population exposures are far too restrictive, thus unnecessarily costly for the radiation industries, the aggregate of radiation epidemiological evidence suggests that current standards are inadequate to protect public health.
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In electron beams, the dose in phantom under the central shielding depends on electron-beam energy, depth in phantom, and shield area and thickness. In our experiments, all shield thicknesses were larger than the range of electrons in the shield material. ⋯ The effects of shield area on the dose under the central shields were studied in detail and dose distributions are given as a function of shield lateral dimensions and electron-beam energy. It is shown that in clinical use of central shielding, the best approach to dose estimation under the shield is direct measurement in phantom under conditions of the actual clinical setup.
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In order to aid radiologists in the diagnosis of pneumothorax from chest radiographs, an automated method for detection of subtle pneumothorax is being developed. The computerized method is based on the detection of a fine curved-line pattern, which is a unique feature of radiographic findings of pneumothorax. Initially, regions of interest (ROIs) are determined in each upper lung area, where subtle pneumothoraces commonly appear. ⋯ A subtle curved line due to pneumothorax is then detected by means of the Hough transform. The detected pneumothorax pattern is marked on the chest image displayed on a CRT monitor. With the present computer method applied to 50 chest images (28 normals and 22 abnormals with pneumothorax), we were able to detect 77% of pneumothoraces, with 0.44 false-positives per image.
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Comparative Study
The effects of different correction techniques on absolute volume determination with SPECT using a threshold edge detection method.
Quantitation of planar radionuclide images is hampered by structures containing radioactivity which overlie or underlie the organ of interest. The introduction of single photon emission computerized tomography (SPECT) overcame this problem to a large extent and enhanced the contrast of the images. Attenuation of photons, however, degrades the resultant SPECT images and correction methods for photon absorption and scatter were subsequently proposed. ⋯ A negative correlation was found between threshold and volume. This reduction in threshold was most prominent when scatter and attenuation correction were combined. This study shows that correction methods for attenuation of photons influence the threshold value for volume quantitation and the use of a constant threshold value could lead to underestimation of larger volumes.
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A method is presented for integrating stereotactic projection and tomographic image data to give composite 3-D images (stereo pairs) of cerebral anatomy and vasculature. The technique serves to combine complementary information from each modality and allows the imaged volume to be viewed directly. The procedure is largely automated and requires no additional apparatus or information beyond that which is ordinarily employed during stereotactic surgical planning. ⋯ Also, calculations were performed to estimate the resolution of measurements made from digitized stereoscopic images. The resulting sub-pixel accuracy of the matched images suggests that the technique has potential for stereotactic applications. Preliminary results are presented illustrating combined CT-DSA and MR-DSA data sets.