Photo-acoustic phase-delayed excitation of guided waves in coated bone phantoms

Photo-acoustic skeletal quantitative ultrasound enables assessment of the fundamental flexural guided wave (FFGW) propagating in bone. This mode, consistent with the F(1,1) tube mode can now be measured through a coating of soft tissue. Interference due to ultrasound propagation in the soft tissue surrounding the bone is reduced by using phase-delayed ultrasound excitation. Photo-acoustic phase-delayed excitation was done on five axisymmetric bone phantoms (1-5 mm wall thickness), coated by a 5 mm thick soft-tissue mimicking layer. A fiber head comprising a linear array of four optical fibers (400 μm diameter), illuminated by pulsed laser diodes (905 nm wavelength) generated ultrasound. This sound was received by a small (10 mm diameter) custom-made piezo transducer from the top surface of the coating. Tuning the phase delay allowed selection of the excited mode(s). FFGW was detected in the 20-40 kHz band when the power ratio between FFGW and interference was tuned to a local maximum. This tuning removed interference and improved SNR of the FFGW mode by >10 dB. Fitting the theoretical F_LC(1,1) mode of a liquid-coated (LC) tube to the measured FFGW phase-velocity provided accurate (11% ± 7% rms deviation) estimate for the cortical thickness. These results suggest that photo-acoustic phase-delayed excitation may enable in vivo assessment of cortical thickness based on FFGW.