Hideaki Kubota

Optimum tube voltage for chest radiographs obtained by psychophysical analysis

Many kinds of x-ray films having various characteristic curves have been developed for chest radiographs. In general, a phototiming device for determination of a mAs value which gives a proper exposure has been used for a chest radiography. For each film, however, the x-ray tube voltage has been determined by the subjective evaluation of radiologists or radiological technologists. In this paper, we propose a new method for determining the optimum tube voltage for chest radiographs using psychophysical analysis. The optimum density and the optimum density range of a screen/film system are obtained from the gradient curve of film and the minimum perceptible contrast delta Dmin [Acta Radiol. Diagnos. 4, 463-476 (1966)]. The optimum tube voltage, by which the lowest density of a mediastinum and the highest density of a lung field just cover the optimum density range, is obtained using the x-ray photon spectrum and sensitivity spectrum of the screen. This objective method does not depend on personal subjective evaluation, therefore it is available for the determination of optimum tube voltage for chest radiographs to be observed by many doctors of various departments.

X-ray tube voltage dependence of Wiener spectra of quantum mottle obtained by determining detective quantum efficiencies of a screen

By using the detective quantum efficiency (DQE), which rep- resents the transfer efficiencies of the signal and noise, we expressed the Wiener spectrum of the quantum mottle. Using the new equation of the quantum mottle and the equation of the structure mottle proposed in 1992, we modified our method for separating the screen mottle, and proposed a new method for obtaining the DQE. Using the new method, we experimentally obtained the tube voltage dependence of the DQE, and the density and tube voltage dependence of the Wiener spectra of the quantum mottle. To explain the tube voltage dependence of the DQEs at a spatial frequency of 0 mm 21 , we theoretically obtained the tube voltage dependence of the first and second moments of x-ray pho- ton spectra absorbed by the screen. We found that the tube voltage dependence of the DQEs was caused by the fact that the output signal from the screen varied only slightly with tube voltage, while the output noise increased. Using the tube voltage dependence of the DQEs and the number of x-ray photons incident on the screens, we explain the dependence of the Wiener spectral values of the quantum mottle.

Determination of x-ray tube focal spot position.

In x-ray spectral measurement using a semiconductor detector system, accurate positioning of the detector in front of the anode is very important. This needs accurate determination of the focal spot position on the target in an x-ray tube. We present a simple technique to determine the focal spot position accurately. Using a pinhole technique, the focal image made by x rays is overlapped on the target image made by visible rays emitted when filament heating takes places. Both images are made in the same geometry with the tube diaphragm and filter removed.

Equivalent spatial frequency and optimum film densities for the perceptibility of radiographic contrast of step-edge images

AbstractPrevious experimental results for minimum perceptible contrasts of step-edge images and sinusoidal periodic patterns were transformed to minimum perceptible luminous-exitance differences. By comparing the differences between the two kinds of patterns, the equivalent spatial frequency for the perceptibility of step edges was determined as 1.6-1.8 mm 1. From the minimum perceptible contrasts of step-edge images, the optimum film densities for step edges were obtained as 1.1 - 1.3 for Kodak XRP and OC medical films and 1.7 -2.0 for Fuji Industrial 100 and FR industrial films.

Fundamental considerations of density-thickness curves for soft X-ray radiographs

AbstractWe consider the fundamental issues of densitythickness curves, which express the contrast and the latitude of soft X-ray radiographs. Firstly, soft X-ray spectra for various thicknesses of polyester objects were obtained experimentally and attenuation curves of X-ray exposure, defined by the ionization of air, were derived. As a soft X-ray radiograph is usually exposed directly, without a screen, the energy absorbed in the emulsion layers was supposed to be more suitable to describe the input to an X-ray film than the total X-ray exposure used hitherto. Secondly, X-ray absorption efficiency spectra of emulsion layers of X-ray films were measured and attenuation curves of energy absorbed in the emulsion layerswere obtained. Thirdly, density-thickness curves were derived, by combining the characteristic curve of an X-ray film with the two kinds of attenuation curves, and compared with those of radiographs of a polyester step wedge. As a result the energy absorbed in the emulsion layers was shown to ...

Effects of x-ray tube-voltage ripple and current mode on patient dose

This paper proposes a radiographic sensitivity attenuation curve for screen-films systems. This curve expresses the relative inverse value of mAs value needed to give the same photographic density. This curve decreases more slowly with increasing object thickness than the exposure attenuation curve generally used hitherto. By using the radiographic sensitivity attenuation curve and the exposure attenuation curve, the voltage-ripple dependence of mAs values and patient dose for the same photographic density was obtained. As predicted theoretically in a previous paper [Med. Phys. 18, 921 (1991)], it was verified quantitatively the reverse order that larger ripple values resulted in lower patient dose and lower mAs value for the unsaturating current mode at large ripple. This reverse order clearly appeared in two-pulse units. These phenomena were experimentally verified. As a result it was found that in radiographing with a two-pulse unit, larger mA value results in lower patient dose.

Psychophysical RMS Granularity for the Evaluation of Radiographic Mottle

AbstractThe psychophysical RMS (root mean square) granularity ap is proposed for the evaluation of radiographic mottle, i.e., the granularity appearing in a radiograph made with an X-ray screen-film system. The value of is calculated from the Wiener spectrum of the radiographic mottle and the frequency dependence of the minimum perceptible contrast, which expresses a psychophysical property of human vision. Examples are presen ted for three typical X-ray films. The density dependence of a,, is in close agreement with the results of subjective rating that (V the most perceptible density is 1.0 and (2) the perceptibility at density 2.5 is much worse than that at 0.6. Whereas the Selwyn granularity coefficient, which relates to the physical RMS granularity, does not correlate with the results of subjective rating.

X-ray tube voltage dependence of Rossmann's three factors in Wiener spectra of radiographic mottles of a low-speed screen/film system

We obtained the tube voltage dependence of the Wiener spectra of the radiographic mottles of the front and back emulsions made using a low-speed screen and an anticrossover film. The Wiener spectral values of the front emulsion are almost constant, while those of the back emulsion increase with the tube voltage. The spectral values of the front emulsion are greater than those of the back emulsion at most of densities. In order to investigate the reason of these phenomena, we separated the radiographic mottle into the three factors. As a result, we have found that the reason of the dependence of the radiographic mottle is that the contribution of the quantum mottle is less than that of the structure mottle. The Wiener spectral values of the structure mottle of the front screen are greater than those of the back screen for most of the tube voltages. The Wiener spectral values of the front screen decrease and those of the back screen increase with the tube voltage, and the two curves cross at high voltage. These reasons were explained from the spatial fluctuation of the thickness of the screen due to the nonuniform screen structure and the attenuation curve of the x-ray intensity. The tube voltage dependence of the quantum mottle was explained from that of the number of photons absorbed in the screen.

Relation between radiographic mottle for double and single emulsions

The radiographic density fluctuations produced by using dual screen-film systems are designated as the radiographic mottle. The density fluctuation of the radiographic mottle for the double emulsions at a density of the double emulsions consists of those for the front and back emulsions on a radiograph. However, the relation between the Wiener spectra of the radiographic mottle for the double and single emulsions had not been studied. Hence we compared the Wiener spectra of the radiographic mottle for the double emulsions with the sum of those for the front and back emulsions on the same radiographs and with the sum of those for the emulsions at the same densities. At all densities of more than 0.62 for lower spatial frequencies (< or = 1 mm-1), the Wiener spectral values of the radiographic mottle for the double emulsions were greater than the sum of those for the front and back emulsions for both comparisons on the same radiographs and at the same densities. In order to investigate the reason of the above phenomena, we separated the Wiener spectral values of the radiographic mottle for various densities into those of the three factors, i.e., quantum mottle, structure mottle, and film granularity, and performed the same comparisons as the radiographic mottle. Also, to explain the results for the three factors, we obtained the Wiener spectral values of the spatial fluctuations of the light exposure or the fluorescence intensity and the gradients of the characteristic curves of the film for the double and single emulsions of the x-ray film. As a result of the investigation, we found that the phenomena on the radiographic mottle were caused by that (1) on the same radiographs the squares of the gradients of the characteristic curves for the double emulsions were about 5.3 times as great as those for the single emulsion at densities of more than 0.62 of the double emulsions, and (2) at the same density of more than 0.62 those were more than about 2.2 times as great as those for the single emulsion.

Edge enhancement effect of vision on X-ray radiographs made using screen—film systems

Abstract Theoretical and experimental approaches for the edge enhancement effect of vision on X-ray radiographic images were carried out. Psychophysically enhanced portions in the density unit at both sides of a step edge image were derived from modulation transfer functions (MTFs) of vision and radiographic systems. Experimental values were obtained using a new method, in which a step edge image was compared with a standard film having continuous density change. Experimental values distributed around theoretical values and their qualitative behaviour were similar to those of theoretical values. Discrepancy between theoretical and experimental results was due to the non-linearity of the human visual system.