Hidetaka ARIMURA,¹ Takaharu IKEDA,² Takeshi IKARI,² Hideaki KUBOTA,³ Masao MATSUMOTO,⁴ Atsushi TAKIGAWA,⁴ Nobuyuki NAKAMORI² and Hitoshi KANAMORI²

¹Shimadzu Corporation, Nakagyo-ku, Kyoto, 604 Japan, ²Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto, 606 Japan, ³Information Processing Center, Shiga University, Hikone, 522 Japan, ⁴Faculty of Medicine, Osaka University, Toyonaka, 560 Japan

(Accepted January 24, 1995)

A low-speed screen/film system was used to investigate structure mottle, i.e., the density fluctuation of an x-ray film due to inhomogeneous screen structure. The tube voltage dependence of the Wiener spectra of the structure mottle was separately determined for front and back film emulsions. The Wiener spectral values of the structure mottle of the front emulsion were greater than those of the back emulsion at lower tube voltages. The spectral values of the structure mottle of the front emulsion decreased with tube voltage, while those of the back emulsion increased. We explain these phenomena by the behavior of the Wiener spectra of the relative spatial fluctuation of fluorescence intensity due to following reasons: (1) spatial thickness fluctuation of the screen produces a relative spatial fluctuation of x-ray energy absorbed in the screen, and (2) as the distance between the emulsion of the film and the average position of the x-ray absorption in the screen lengthens, the number of random scattering and absorption of light photons increase.

Key words : physical image quality, low-speed screen/film system, relative fluorescence fluctuation, screen structure mottle, tube voltage dependence.