2015 年 1 巻 p. 52-61
The optimum dose for a computed radiography system (CR) was estimated from the relationship of the radiation dose to the root mean square (RMS) and the Wiener spectrum (WS).
Samples for analysis of noise characteristics were prepared using a 50.0 mm tough water phantom, as a scattering body, fitted with a 20.0 mm Al filter (SID 200 cm, tube voltage 80 kV). The dose level at which the film density in the standard screen-film system was 1.0 ± 0.05 was adopted as the criterion dose (23 mGy) and five samples were prepared at dose levels of 1/4, 1/2, 1, 2 and 4 fold relative to the criterion dose.
RMS was calculated from five 256×256 pixel images. WS was determined from the 320 segments yielded by two-dimensional fast Fourier transformation (2DFFT) of the 256×256 pixel images.
On the basis of the data on RMS and WS, the optimum dose was determined, with the threshold level defined as the point of intersection in linear approximation between the low dose range affected by the quantum mottle and the high dose range depending on the system noise. As a result, the dose for CR, determined from RMS and WS, was about 30% lower than the screen-film system criterion dose.
