The 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, these phenomena are explained using the tube voltage dependence of the NEQ. From these phenomena, it was found that if the crossover effect was appropriately included in the radiographs, the patient exposure and the quantum mottle could be reduced at low tube voltages.
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