In Vivo Quantification of Bone Strontium Using X-Ray Fluorescence

Abstract

Strontium (Sr) is an element naturally present in the human skeleton and is acquired through dietary means. Exposure to strontium has been linked to both harmful and beneficial effects on skeletal health. Recently, the administration of strontium has been shown to induce a therapeutic effect of increasing bone strength and bone mineral density in women suffering from post-menopausal osteoporosis. The advent of this new therapy has warranted the continued development of an energy dispersive x-ray fluorescence (EDXRF) system that may be used as a diagnostic tool for non-invasive measuring and monitoring of in vivo bone strontium levels. This device is currently housed at McMaster University and has been previously optimized to measure bone strontium in vivo. One shortcoming with this system is the inability to quantify absolute amounts of bone strontium in vivo due to Sr x-ray absorption by soft-tissue overlying bone. This work describes an attempt to examine several imaging modalities to determine which modality may provide overlying tissue thickness readings with an acceptable range of accuracy to correct for Sr x-ray absorption. A performance comparison between magnetic resonance imaging, x-ray computed tomography, 8, 25 and 55 MHz ultrasound, in estimating the tissue thickness of seven cadaver fingers, illustrated that 55 MHz ultrasound provided a superior range of accuracy at 3.2%. It further indicated that the currently used 8 MHz ultrasound may be used to accurately estimate tissue thickness, though with a diminished accuracy of 6.6%. EDXRF measurements were performed on cadaver fingers ex vivo. Analysis of results indicated that quantification might be achieved if signals are normalizated to the 35 keV coherent scatter peak and correction of both soft-tissue absorption of Sr x-rays and differences in 125I excitation source activity are carried out. Four EDXRF measurements were performed on a strontium citrate supplemented individual starting six months after Sr medicating had begun. Analysis of strontium levels revealed that bone strontium was already at a plateau by the first measurement and that these levels did not change in the 6 months following.