THE MONTE-CARLO MODELING OF IN-VIVO X-RAY-FLUORESCENCE MEASUREMENT OFLEAD IN TISSUE

A Monte Carlo model has been developed, using the EGS4 code, to model the in vivo x-ray fluorescence (XRF) measurement of Pb in non-superfic ial bone/tissue. Unlike previous work in this field the current model incorporates a correction for Doppler broadening of the Compton scatte r peak due to the electron momentum distribution of the medium (tissue /water) in which the photons are Compton scattered by convolving the C ompton peak of the Monte Carlo generated spectrum with a modified Comp ton profile for water. This correction improves the agreement between the measured spectral shape obtained using an experimental in vive x-r ay fluorescence Pb analyser with a Cd-109/180 degrees source/geometry combination, measuring a bone phantom at depth in water and the genera ted spectral shape obtained from the equivalent Monte Carlo model. The model enables improved estimates to be made of the spectral backgroun d beneath the Pb K alpha(1) and K alpha(2) x-ray peaks compared with e stimates based on simpler models that assume that Compton interactions are with 'free' electrons and hence permits better optimization of in vive analyser system design.