Hitoshi Kanamori

Direct measurement of mammographic x-ray spectra using a CdZnTe detector

Our purpose is to directly measure mammographic x-ray spectra with collimators and a low-efficiency CdZnTe detector developed recently and to find out the best fit response function of CdZnTe detector to correct the measured spectra. Photon spectra (target Mo or Rh) produced by a mammographic x-ray unit at 25-32 kV and 240 mAs (= 3 times of 80 mAs) and transmitted through 0.03 mm Mo or 0.025 mm Rh filter and object (0.1 mm Al to 0.8 mm Al phantoms) have been analyzed. Since detected spectra were distorted by the response of CdZnTe detector and did not present the true photon spectra, the correction was applied by the stripping procedure. The response function of detector used in this procedure has been determined by the evaluation of interactions (K-escape, coherent scattering, and Compton scattering processes) and incomplete charge collection calculated using the Monte Carlo method. We have used Kalpha1, Kalpha2, Kbeta1, Kbeta2 radiations of Cd, Zn, and Te, respectively and have used the weight function for the incomplete charge collection and have considered Compton scattering. The Monte Carlo simulations were continued by changing the important factors (mean path length of hole lambda(h), dead layer of the CZT crystal and weight factor Wq) of incomplete charge collection until the best fit response function was found out. Corrected photon spectra were compared with the mammographic x-ray spectral data of Bureau of Radiological Health (BRH) measured using a Ge detector. Attenuation curves of aluminum for 25-32 kV were calculated from the corrected photon spectra and compared with the attenuation curves measured using an ionization chamber. These results obtained using the CdZnTe detector agreed with the mammographic x-ray spectral data of BRH and attenuation curves obtained by the ionization chamber.

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.

Information spectra for radiographs modified by scatter

The information spectrum given by H. Kanamori and M. Matsumoto (1984) is modified by the effect of scatter which decreases the contrast of radiographs. The modification is simply that the signal Wiener spectrum is multiplied by (1-s)2, where s is the scatter rate. An example is demonstrated, where the detectability of low-contrast objects and resolution of high-contrast objects are successfully explained by information spectral values in low- and high-frequency ranges, respectively.

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.

Influence of the crossover effect on X-ray tube voltage dependence of quantum mottle

AbstractThe influence of the crossover effect on the tube voltage dependence of the quantum mottle for the front and back emulsion has been investigated. A new method is proposed for separating the Wiener spectra of the screen mottle for the front and back emulsions with the crossover effect into those of the quantum mottle and the structure mottle and a method for obtaining the detective quantum efficiency (DQE) and the noise-equivalent number of quanta (NEQ).The numbers of incident X-ray photons with the crossover effect were smaller than those without the effect for all tube voltages studied. However, at tube voltages below about 75 or 80 kV, the Wiener spectral values of the quantum mottle with the crossover effect at 0 mm 1 were smaller than those without the effect. The Wiener spectral value increased with the tube voltage and, at tube voltages above about 75 or 80 kV, exceeded that without the effect. Because the Wiener spectrum of the quantum mottle is proportional to the reciprocal of the NEQ, th...

Reduction of patient dose on medical radiographs using scattered x rays

We propose a new method for reduction of the patient dose by using scattered X-rays in order to achieve the same density without scattered rays. The minimum perceptible thickness difference (delta) Xmin of the object was calculated using psychophysical analysis for various radiographic densities, scatter fractions and luminous exitances of the viewer. The mAs values to obtain many densities were measured using four kinds of anti-scattered X-ray grid with their scatter fractions. These measured values were applied to above calculated psychophysical results. The smallest value of (delta) Xmin for acrylate of thickness 20cm was 0.18mm, if the scattered X-rays were negligible. The value of (delta) Xmin increases with increasing scatter fraction. The perceptibility of human eye is influenced by luminous exitance of the viewer. By increasing the luminous exitance from 1500 lm(DOT)m-2 to 8000 lm(DOT)m-2, the patient dose can be reduced 33 percent in maximum under the same perceptibility of (delta) Xmin. The method of changing grid will be considered.

A New Method for Evaluating the Radiographic Imaging Quality using the Beam Stopper

A new method was developed for evaluating radiographic imaging quality by introducing a beam stopper. The sharpness is estimated by this method for a radiographic screen-film system by taking a radiograph of the beam stopper instead of the conventional X-ray slit. Because the contrast difference relates directly to the visibility of the radiographs, the beam stopper estimation relates directly to the visual rating of blurring systems.

Influence of scattered x rays on object sharpness of radiographs

We investigated the influence of the scattered x-rays on the object sharpness of radiographs for tube voltages of 50 to 100 kV. For the purpose, using a phantom of the polymethyl methacrylate (PMMA) with an aluminum plate of low contrast object, the edge images including the blurs due to the x-ray focal spot, the screen-film system, and the scattered x-rays were produced. However, the shape of the overall modulation transfer function (MTF) derived from the edge image is considered not to be influenced by the scattered x-rays. As a result of comparing the overall MTFs with the product of the MTFs of the x-ray focal spot and the screen-film system, we found that the scattered x-rays gave different influence on the contrast at each spatial frequency for each tube voltage. To investigate the influence of the scattered x-ray on the overall MTF in detail, considering the MTF of the x-ray focal spot, we developed a method for determining the primary and scatter components of the overall MTF, whose components are proportional to the MTFs of the screen-film system and the scattered x-rays, respectively. Using the method, it was found that (1) the overall MTF depends on the MTF of scattered x- rays as well as the MTFs of the focal spot and screen-film system; and (2) at frequencies near 0 mm-1, the influence of the blur due to the scattered x-rays on the object sharpness was greater than that occurred in the intensifying screen and increased with the tube voltage.

Experimental Verification of Optimum Density Obtained from Psychophysical Gradients

AbstractThe optimum density is the density at which the best detectability is attained. The theoretical optimum density was obtained using psychophysical gradients (psychophysically modified gradients). The experimental optimum densities obtained by a visual experiment were compared with the theoretical optimum densities for radiographs of three types of object: two periodic fabrics having spatial frequencies oft and 3 mm-1 and a step-edge fabric using two kinds of film. The experimental optimum densities coincided in all cases with the theoretical optimum densities at which psychophysical gradient values reached maxima. The psychophysical gradient ivas confirmed to be useful for obtaining the optimum density.