Investigation of the potential effects of bone turnover on x-ray fluorescence measurements of bone lead in young women

Abstract

The potential effect of bone turnover on bone lead k X-Ray Fluorescence (XRF) measurements in young women were examined in this thesis. Published data has shown that men and women do not release lead back into blood from bone stores at the same rate until after menopause. Recently our laboratory re-investigated this phenomenon, and currently unpublished data showed that the release rate for young women may be better modelled through a non-linear function. One possible explanation is sex differences in cortical porosity. However, it was unknown whether cortical porosity could create a measurement artifact. Using 3D printing techniques, a phantom design was created that emulates cortical porosity at structural sizes of 300 microns. A calibration set of lead-doped cortical porosity phantoms were created and compared with past bone lead phantoms, there was no statistically significant differences observed. In phantoms with homogenous distributions of lead, cortical porosity does not appear to affect phantom measurements. Monte-Carlo simulations were performed using MCNP to verify and expand upon these results. Findings agreed with experimental results, and it was also shown that heterogenous distribution of lead did not affect K-XRF measurements until the distributions were extreme. Cortical porosity does not appear to result in XRF measurement artifacts, and the non-linearity in the relationship between blood and bone levels observed in young women appears to be real. Published analysis of in vivo data sets of young women found measurements in morbidly obese women were inaccurate. The effect of soft tissue overlay on bone lead measurements was therefore examined using MCNP. Results showed as soft tissue overlay thickness increases, the average measured bone lead value decreases. MCNP further showed that while the measured bone lead level in an individual is reduced, an increase in measurement uncertainty measurement masked the reduction. This may explain why the phenomenon has not been clearly observed and understood until now.