Ex vivo evaluation of a coherent normalization procedure to quantify in vivo finger strontium XRS measurements

PURPOSE: Energy dispersive x-ray fluorescence spectroscopy (XRS) measurements were performed on human cadaver index fingers to measure bone strontium content in the presence of intact overlying soft-tissue. This work assesses the feasibility of applying a normalization procedure including soft-tissue correction of x-ray absorption as a means to quantify an ex vivo bone strontium XRS measurement. METHODS: Bone strontium measurements were made using an excitation-detection system incorporating an (125)I x-ray excitation source and an Ortec® Ametek-AMT Si(Li) detector in 180° backscatter geometry. Spectral processing was accomplished using an in-house nonlinear least-squares Marquardt fitting routine. Bone strontium was quantified using an egs5 Monte Carlo based x-ray soft-tissue correction algorithm in conjunction with the normalization of strontium x-rays to the coherent scatter peaks of 35.5 keV (125)I γ-rays. RESULTS: Comparison of tissue intact and bare bone finger XRS measurement quantification attempts revealed an overall discrepancy of 18.6% that is attributed primarily to the significant contribution of soft-tissue to coherent scatter of 35.5 keV source γ-rays and to a lesser degree, inconsistencies with the simulated tissue correction model. Work toward the beginnings of an experimentally derived tissue correction model, as a means to validate the simulated model, have been reported. Two observations hinted at a systematic inflation of the observed Kβ peak area. First, strontium concentrations estimated by Kα peak areas were less than the Kβ peak areas by 28.6% (p < 0.0001) and 10.5% (p < 0.001) for tissue intact and bare bone measurements, respectively. Second, the Kα:Kβ x-ray average ratios between tissue corrected (3.61 ± 0.55) and bare bone predicted (4.4 ± 0.4) did not agree (p < 0.0001) and pointed to shortcomings with the current processing treatment of strontium K x-ray peak area extraction. Through finger bone XRS measurements, bone strontium concentration in the Caucasian population was estimated at 95 ± 15 μg Sr/g dry bone. CONCLUSIONS: The discrepancies observed: between quantification attempts of tissue corrected and bare bone measurements, the inflated estimates of Kβ relative to Kα peak concentrations and between observed and expected Kα:Kβ ratios, have indicated that shortcomings with the bone strontium coherent normalization and tissue correction procedure exist. Coherent scatter contribution of soft-tissue overlying bone, tissue correction model limitations, and spectra processing issues are all mentioned as sources of observed discrepancies.