Abstract
A hydroxyaptite [HAp; Ca5(PO4)3OH] phantom material was developed with the goal of improving the calibration protocol of the 125I-induced in vivo X-ray fluorescence (IVXRF) system of bone strontium quantification with further application to other IVXRF bone metal quantification systems, particulary those associated with bone lead quantification. It was found that calcium can be prepared pure of inherent contamination from strontium (and other elements) through a hydroxide precipitation producing pure Ca(OH)2, thereby, allowing for the production of a blank phantom which has not been available previously. The pure Ca(OH)2 can then be used for the preparation of pure CaHPO4 ⋅ 2H2O. A solid state pure HAp phantom can then be prepared by reaction of Ca(OH)2 and CaHPO4 ⋅ 2H2O mixed as to produce a Ca/P mole ratio of 1.67, that in HAp and the mineral phase of bone, in the presence of a setting solution prepared as to raise the total phosphate concentration of the solution by increasing the solubility CaHPO4 ⋅ 2H2O and thereby precipitating HAp. The procedure can only be used to prepare phantoms in which doping with the analyte does not disturb the Ca/P ratio substantially. In cases in which phantoms are to be prepared with high concentrations of strontium, the cement mixture can be modified as to introduce strontium in the form of Sr(OH)2 ⋅ 8H2O as to maintain a (Ca + Sr)/P ratio of 1.67. It was found by both X-ray diffraction spectrometry and Raman spectroscopy studies that strontium substitutes for calcium as in bone when preparing phantoms by this route. The necessity for the blank bone phantoms was assessed through the first blank bone phantom measurement and Monte Carlo simulations. It was found that for the 125I-induced IVXRF system of bone strontium quantification, the source, 125I brachytherapy seeds may be contributing coherently and incoherently scattered zirconium X-rays to the measured spectra, thereby requiring the use of the blank bone phantom as a means of improving the overall quantification methodology. Monte Carlo simulations were employed to evaluate any improvement by the introduction of HAp phantoms into the coherent normalization-based calibration procedure. It was found that HAp phantoms remove the need for a coherent conversion factor (CCF) thereby potentially increasing accuracy of the quantification. Further, it was found that in order for soft tissue attenuation corrections to be possible using spectroscopic information alone, HAp along with a suitable soft tissue surrogate material need to be employed. The HAp phantom material was used for the evaluations of portable X-ray analyzer systems for their potential for IVXRF quantification of lead and strontium with a focus on a comparison between tungsten, silver and rhodium target systems. Silver and rhodium target X-ray tube systems were found to be comparable for this quantification.
Highlights
1.1 Strontium and boneStrontium is a group II alkaline Earth metal (Z = 38) and shares similar chemistry, and by extension biochemistry, to the most abundant of the alkaline Earth metals in the human body, calcium (Z = 20)
This work describes the preparation of a pure HAp phantom material proposed here as a material for the purpose of calibrating in vivo X-ray fluorescence (IVXRF) systems of bone strontium and lead quantification as a replacement for plaster of Paris (poP)
The need for the analytical blank bone phantom developed by Da Silva et al.[1] is discussed in the context of assessing spectral interferences, which has an influence on the determination of analytical figures of merit for the IVXRF system of bone strontium quantification
Summary
X-ray fluorescence spectrometry (XRF) has been successfully applied to the measurement of various trace and minor elements of toxicological and epidemiological interest, namely lead, uranium and strontium, in the bone tissue of human subjects in vivo.[19–23]. The coherent normalization procedure being developed by Somervaille et al.[1] in the context of a bone lead analysis using a 109Cd source and later extrapolated to other IVXRF systems of bone metal quantification.[2–8] The development of this method of calibration was in response to the complexity of the calibration protocol proposed by Ahlgren et al.[10] and Ahlgren and Mattsson[11] based on excitation using a 57Co source, which required the use of orthoplanar X-rays, estimations of bone mineral concentration/density through the scattered source radiation and the construction of custom phantoms on a per-subject basis composed of bone ash and wax. This becomes of particular importance when large scale epidemiological studies are desired (i.e. for bone lead as a biomarker for cumulative exposure),[7,8] or, in the case of a bone strontium measurement, if clinical use is desired and portability becomes a convenient feature (i.e. to monitor patients undergoing strontium therapy for osteoporosis).[5,6]
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