Coregistered photoacoustic and ultrasonic signatures of early bone density variations.

This study examines the application of backscattered ultrasound (US) and photoacoustic (PA) signals for assessment of bone structure and density variations. Both methods were applied in the frequency-domain, employing linear frequency modulation chirps. A near-IR laser (800 nm) was used for inducing the PA signal. The backscattered pressure waves were detected with a 2.2-MHz US transducer. Experiments were focused on detection and evaluation of PA and US signals from in-vitro animal and human bones with cortical and trabecular sublayers. It was shown that PA signals can be detected as deep as a few millimeters below trabecular and cortical layers. The occurrence of multiple scattering was demonstrated in PA detected signals from cancellous bone. Osteoporotic changes in the bone were simulated by using a very mild demineralization ethylenediaminetetraacetic acid solution. Changes in the time-domain signals as well as integrated backscattering spectra were compared for the samples before and after demineralization. The results demonstrated the sensitivity of PA to variations in bone minerals. In comparison to PA, US was capable of generating detectable signals from deeper bone sublayers (few centimeters). However, while US signal variations with changes in the cortical layer were insignificant, PA proved to be sensitive even to minor variations of the cortical bone density.