  Photoacoustic (PA) technology can provide information on both the physical
structure and chemical composition of bone, showing great potential in bone
assessment. However, due to the complex composition and porous structure of
cancellous bone, the PA signals generated and propagated in cancellous bone are
complex and difficult to be directly used in cancellous bone analysis. In this
paper, a photoacoustic differential attenuation spectrum (PA-DAS) method is
proposed. By eliminating the PA spectrum of the optical absorption sources, the
propagation attenuation characteristics of cancellous bone are studied
theoretically and experimentally. An analytical solution for the propagation
attenuation of broadband ultrasound waves in cancellous bone is given by
applying high-frequency and viscous corrections to Biot's theory. An
experimental system of PA-DAS with an eccentric excitation differential
detection system is established to obtain the PA-DAS of cancellous bone and its
acoustic propagation characteristic on the rabbit osteoporosis model. The
PA-DAS quantization parameter slope is further extracted to quantify the
attenuation of high and low frequency components. The results show that the
PA-DAS can distinguish osteoporotic bone from normal bone, enabling
quantitative assessment of bone mineral density and the diagnosis of
osteoporosis.
