Evaluating calcium phosphate glass phantoms for the calibration of in vivo X-ray fluorescence spectrometry-based methods of bone strontium quantification

Abstract

In vivo X-ray fluorescence spectrometry is a technique used to quantify trace elements in bone tissue by using an external γ-ray source to irradiate a section of the body. The current system for bone metal quantification is capable of detecting metals such as strontium in human bone by using an ¹²⁵I radionuclide excitation source with photon detection using a Si(Li) detection system. The historical choice of phantom material, plaster of Paris, has introduced numerous complications in terms of its suitability for the calibration of systems of bone metal quantification. A chemically pure hydroxyapatite phantom material of known composition and stoichiometry has been developed for the purpose of calibrating systems of bone metal quantification as a replacement for plaster of Paris while also removing the required coherent conversion (correction) factor from the protocol. Hydroxyapatite, however, is not necessarily stable over time and thus this study assessed common calcium phosphate glasses prepared with metal oxides as potential candidate calibrators. Monte Carlo simulations were carried out using the bench-marked EGS5 code to evaluate if calcium phosphate glasses can be used in the same fashion as hydroxyapatite. Selected glasses were assessed for their differential cross-sections for coherent scattering as well as the implications on the correction factor. The calcium phosphate glasses did not show much agreement with hydroxyapatite when comparing the slopes of the normalized calibration curves nor when directly calculating the correction factor from the ratios of the differential coherent scattering cross-sections for bone mineral to the glass phantoms. This simulation study thus concludes that the removal of the correction factor from the calibration protocol requires the use of a hydroxyapatite phantom and that conventional calcium phosphate glasses, although desirable, cannot be used reliably as calibrators.