Standing wave suppression in transcranial ultrasound therapy using random-signal-modulation excitation

Recent papers have demonstrated that acoustic standing waves can be inhibited by frequency-modulated spread-spectrum excitation. An alternative method is studied here, which is expected to be more practical for implementation in phase-steered arrays. The method operates by means of phase-shift-keying (PSK) excitation. It is hypothesized that standing waves can be suppressed by breaking the symmetry of the incident and reflected waves via random phase changes at regular intervals. Sequential and random binary-PSK (BPSK) and quadrature-PSK (QPSK) with a carrier frequency of 250 kHz and a time segment of 10 signal cycles were generated by a circuit built in-house. Acoustic standing waves were induced in a plastic-walled chamber and raster acoustic pressure measurements were performed. Standing wave suppression using different PSK schemes were compared in terms of the square of the normalized acoustic pressure. Results showed that both of the random BPSK and QPSK excitations have superior performance over their sequential counterparts.