Although various models have been proposed in an attempt to predict the usefulness of a radiographic image in terms of its physical characteristics, no previous work has shown whether a single physical image quality index, such as a signal-to-noise ratio, can reliably predict the performance of a human observer over a broad range of image characteristics. We studied the relationship between physical and visual image quality for the task of detecting nylon beads in radiographs. Thirty-seven imaging cases with different combinations of physical image characteristics were considered; these included variations in object size and magnification, X-ray beam quality, screen-film system, screen-film contact, film density and illumination, and viewing distance. For each imaging case, visual image quality was quantified in terms of observer performance in a 2AFC visual detection experiment. Physical image quality indices were calculated according to eight different models of the detection process; these indices combined data regarding object size and attenuation, screen-film system MTF, film gradient, noise Wiener spectrum, and visual system response. The results of this work indicate that, for the conditions studied, human observer detection performance most closely resembles that of a sub-optimal statistical decision process.
One of the difficulties in the measurement of diagnostic x-ray spectra is the high fluence rate of the photons, which may exceed the capability of the spectrometer system. Pinhole collimators are often used which reduce the count rate to an acceptable level. The measured spectrum, however, may be distorted due to the penetration of photons through the collimator material. In this study, the effects of collimators on the measured spectra were evaluated. As an indicator of collimator performance, we determined the transmission equivalent aperture (TEA), through which a portion of the incident photons impinge on the detector. The dependence of the TEA on the geometry of collimation, pinhole area, collimator material and thickness, detector area and incident x-ray energy was investigated. The penetration correction factors (PCF) for the measured spectra were also derived. We obtained good correspondence between the theoretical spectra and the measured spectra which were corrected for collimator penetration by means of the calculated PCFS. Our results indicate that diagnostic x-ray spectra generated at high tube current can be measured if one uses a collimator with a small aperture together with its PCF.
We applied Monte Carlo calculations to determine the radiation dose absorbed in water phantoms. Monoenergetic incident x-ray beams with energies from 15 to 100 keV and phantom thicknesses from 5 to 20 cm were considered in this study. We calculated the spatial distributions of energy absorption in the phantom, the rad/R conversion factors, the average rad/R conversion factors, and the scatter-to-primary ratios of absorbed dose. We also compared the relative absorbed doses under various imaging conditions when the transmitted radiation produced a given optical density on radiographic film. The information provided will be useful for the estimation of radiation doses in various radiographic procedures.