The preparation of strontium-substituted hydroxyapatite bone phantoms with high strontium concentrations

Abstract

The current in vivo x-ray fluorescence spectrometry (IVXRF)-based method of bone strontium quantification is dependent on calibration against plaster of Paris (poP; ) calibrators/phantoms. Issues with the dissimilarity of the poP in comparison to bone mineral and the known issue with high levels of strontium contamination in commercial poP materials has led to the development of a strontium-free hydroxyapatite bone mineral phantom material. The analyte is introduced into this material by doping of a high concentration setting solution. This phantom material is not particularly well suited for the preparation of strontium containing bone phantoms which are high in strontium concentration (in the mol % concentration range) and thus when the (Ca + Sr)/P mole ratio is to be kept at 1.67: that of hydroxyapatite. This work thus evaluates the feasibility of preparing high concentration strontium-substituted hydroxyapatite phantoms using a CaHPO4 · 2H2O/Ca(OH)2/Sr(OH)2 · 8H2O cement mixture. It was found that a strontium-substituted HAp phantom material can be prepared by mixing suitable quantities of CaHPO4 · 2H2O, Ca(OH)2 and Sr(OH)2 · 8H2O to a (Ca + Sr)/P mole ratio of 1.67. Setting of the phantom using a 1 M setting solution (to increase phosphate concentration) produced a phantom composed of ()5(PO4)3OH of known strontium concentration. The final phantom material was found to be a strontium-substituted product (through x-ray diffraction and Raman spectroscopy measurements) to the degree of substitution expected. This work thus describes the preparation of a bone phantom material which can be used for the calibration of IVXRF systems of bone strontium quantification, as well as for other modalities (i.e. dual energy x-ray absorptiometry and quantitative ultrasound) in the case that bone strontium concentrations are expected to be high such as in the case of individuals taking strontium salts for the treatment/prevention of osteoporosis.