The impact of soft-tissue phantoms on the in vivo quantification of lead in bone using portable X-ray fluorescence spectrometry

Abstract

Portable X-ray fluorescence spectrometry (pXRF) has emerged as a promising technique for the in vivo quantification and monitoring of bone lead (Pb) through the detection of characteristic Pb L Xrays. Accurate calibration requires bone and soft-tissue surrogates that replicate photon attenuation and scattering at the measurement site. While significant efforts have focused on bone-mimicking materials, less attention has been given to soft-tissue equivalents. In this study, two soft-tissue mimicking materials—Lucite (PMMA) and paper (98% cellulose)—with thicknesses ranging from 0 to 7 mm were investigated and compared with porcine tissue, which closely approximates human soft tissue. The experimentally determined mass attenuation coefficients for Pb L X-rays at 10.5 keV were 5.22, 4.17, and 4.94 cm2/g for Lucite, paper, and porcine tissue, respectively, indicating that paper provides attenuation properties comparable to those of soft tissue. Calibration curves were generated using six hydroxyapatite (HAp) bone phantoms doped with Pb at varying concentrations and measured for 180 s live time using a Niton XL5 spectrometer (Thermo Fisher Scientific, USA) operating in mining mode with the main filter. At 2.00 mm thickness, the minimum detection limits (MDLs) were 2.2, 1.7, and 2.2 μg Pb/g Ca for Lucite, paper, and porcine tissue, respectively. At 5.00 mm, the MDLs increased to 10.6, 4.3, and 6.2 μg Pb/g Ca. Pb concentrations in three cadaveric tibiae overlaid with porcine tissue were quantified using direct calibration, adjusted coherent normalization, and Compton interpolation. No significant differences were observed among the methods (p > 0.05) using Lucite. Despite paper showing attenuation coefficients closer to porcine tissue, Lucite more accurately reproduced the combined attenuation and scattering behaviour of human soft tissue, supporting its use in vivo pXRF calibration.