THE SKELETAL CALCIUM PHOSPHORUS RATIO - A NEW IN-VIVO METHOD OF DETERMINATION/

We describe a new method for assessing the Ca/P ratio of bone in vivo using gamma-ray photon absorptiometry. The theoretical approach of the method and the estimation of the variance are presented. Two radiatio n sources, Gd-153 (100 keV), and I-125 (27.5 keV), and a germanium det ector were used to determine this ratio. Measurements were made on bon e phantoms with different Ca/P ratios; also, the ratio was measured on lamb and sheep tibias, rabbit tibias, and human fingers. Since the ac curacy of the method is affected by the amount of fat and collagen in the measurement field, the effect of collagen and fat on the measureme nts also was investigated. In all cases, the precision of the method, expressed as the coefficient of variation (CV): 100 x standard deviati on/mean, was near to the theoretical one, ranging from 1.8% to 3.2%. F or human fingers, the CV was 3.2%, a value near to the theoretical 2.9 % with a dose to the skin ranging from 0.044 to 0.066 mGy, depending o n the width of the finger. (C) 1997 American Association of Physicists in Medicine.