ultrasound measurements of overlying soft tissue thickness at four skeletal sites suitable for in vivo xray fluorescence due to signal attenuation in overlying soft tissue development of xray fluorescence systems to measure low atomic number elements such as strontium in human bone required a search for a skeletal site with thin overlying tissue this paper reports ultrasound measurements of overlying tissue on 10 subjects at four anatomical sites the average tissue thickness at the finger was 2907 mm the average tissue thicknesses were 3607 mm 4820 mm and 8417 mm at forehead tibia and heel respectively additionally both parametric and nonparametric approaches to the relationship between body mass index bmi and tissue thickness suggest that there is a significant linear correlation between the subjects bmi and overlying tissue at the finger and heel bone these correlations might be used as a criterion to perform xrf measurements however a larger data set is required to address these correlations more clearly