Elastic photon scattering and normalization ofIn Vivo XRF Analyses of Lead in Bone

One method of monitoring long-term exposure to Pb concerns the siting of an appropriate source-detector arrangement proximal to the tibial bone, detecting the x-ray fluorescence (XRF) in a backscattering geometry. Some systems make use of a relatively active source of 109 Cd mounted on the front of a thin-window high-resolution detector. The elastic peak at 88.036 keV, which is assumed to be due to the higher Z components of natural bone, is well separated from the Compton peak. Evaluation of the elastic peak allows normalization of the XRF signal, with results being stated in terms of micrograms of Pb per gram of bone mineral. In the present study, simulation has been made of elastic scattering from compact bone (ICRU-37 eight-element composition) and soft tissue (ICRU-44 nine-element composition) to which has been added Pb in the range 10-1000 ppm by mass. Interest focuses on the extent to which these Pb concentrations can affect the accuracy of the normalization process. Results are obtained for photon energies in the range 1-1000 keV and scattering angles of 10° and 165°. The contribution from Pb to total elastic scattering increases sharply for photon energies above the K-photoeffect threshold of Pb (i.e. above approximately 100 keV) and is more important to the total elastic cross-section at 165° than at 10°, For radionuclide source energies which are practical for the in vivo detection of Pb, the normalization procedure is affected at levels of much less than 1%.