The measurement of trabecular bone mineral density using coherent and Compton scattered photons in vitro

A photon scattering method for measuring the trabecular bone mineral density (TBMD) in vitro is described. This method involves the measurement of the ratio of coherent to Compton 90 degrees scattered photons from Am-241 by using a narrow beam geometry with an intrinsic germanium detector. The feasibility of using smaller scattering angles for better counting efficiency and the associated problems in their application for in-vivo measurements were investigated. Calibration of the system with fresh trabecular bone samples showed a linear relationship between the coherent to Compton ratio R of the detected counts and the TBMD (r = 0.94). The effect of the overlying soft tissue on the R ratio was significant while the effect of self-attenuation by the trabecular bone itself and the cortical layer was negligible. It was found that the marrow fat content could alter the value of the R ratio. Our results show that for a 10% increase in the fat content in the interstices of the trabecular bone there is a 2.5% decrease in the R ratio. This technique together with soft tissue corrections will enable us to measure the TBMD of the calcaneum in vivo, assuming a small variation in the trabecular fat content. The estimated absorbed dose to the bone marrow is about 139 mrad.