An improved radioisotope-based X-ray fluorescence system for the in vivo measurement of bone strontium†

High levels of strontium can cause impaired bone growth in children, and excess mortality in animals, but at low doses there are no known toxic effects in humans. Strontium is purported to be beneficial for post-menopausal bone density loss and strontium citrate is used as a nutritional supplement by some women. This article describes development of a system to monitor bone strontium levels quickly and accurately for use in the health risk assessment of self-administered strontium supplements. Previous radioisotope-based strontium measurements take up to 30 minutes for measurements and are not portable. The new system is comprised of a silicon drift detector (SDD) with a ¹⁰⁹Cd source in a 180° backscatter geometry. Novel anthropomorphic phantoms were developed for system calibration from 3D printed PLA shells with strontium-doped hydroxyapatite cores. The system performance was investigated using two sources of different reported activity. During development, it was noted that while one ¹⁰⁹Cd source did emit an order of magnitude higher 88 keV γ-rays than the other, it did not emit an order of magnitude greater fluence of silver X-rays. This is attributed to differences in source encapsulation. This lower-than-expected X-ray fluence meant that the best minimum detectable limit (MDL) was determined to be 22 μg Sr per g Ca for a 30 minutes measurement. However, low system dead times indicated that the system was not used at maximum throughput, and it is predicted that with higher fluence silver X-ray sources, the system could achieve a minimum detectable limit of 7 μg Sr per g Ca.