モンテカルロ法による多重散乱線を考慮したX線スペクトルの計算(I) : 水ファントムへの単色X線入射

Abstract

The accurate imaging of the internal structure of the body and the accurate measurment of absorbed dose in the patient are limited by multiple interaction of X-ray photons with atoms in the body. For incident monoenergetic photon energies from 50 to 150keV and for uniform water phantoms(30×100×th cm^3 : th=2.5-30cm), we calculated the X-ray spectra including multiply-scattered photons using Monte Carlo simulation methods. The results indicate the following facts ; 1)A photon scatters about 3 times in average in the 10-cm water layer. About 2.5% of the incident photons are scattered more than 11 times for 100-keV incident beam. 2)Over 50% of the scattered photons are those multiply scattered. 3)As water layer thickness increases, the ratio of absorbed energy fluence to incident energy fluence gradually saturates, because of the saturation of the fraction of backscattering. 4)In high incident photon energy region, absorbed energy fluence calculated using the Monte Carlo method is greater than that using the linear absorption coefficient, whereas the former is less than the latter in low energy region. These facts are due to the existing of scattered photons. To evaluate the image quality and to obtain the accurate dose in the patient, it is necessary to consider the multiply-scattered photons.