Impact of low-amplitude pulse interspersed with histotripsy treatment of Staphylococcus aureus biofilms on Surgical Mesh

Ultrasound histotripsy has shown great potential for destroying bacteria biofilms on hernia mesh. The therapy could be further enhanced by reducing the necessary treatment times. In this study, we evaluated the inclusion of a low-amplitude pulse spaced between each of the high-amplitude histotripsy pulses on the effectiveness of the therapy for different treatment durations. The low-amplitude pulse consisted of between 20 and 500 cycles at 1.05 MHz with rarefactional pressure amplitudes varying from 0 (no low-amplitude pulse) to −1.9 MPa. The scan speed was varied as 0.2 $\mathrm{mm}/s$, 0.8 $\mathrm{mm}/s$, and 1.5 $\mathrm{mm}/s$ (0.18 BW/$s$, 0.72 BW/$s$, 1.35 BW/$s$ where BW is beamwidth). The step size between scan lines was 0.2 mm or 0.4 mm (0.18 BW and 0.36 BW). The pulse repetition frequency (PRF) was varied between 62.5 and 500 Hz with the low-amplitude pulses (if present) positioned $500\mu s$ after the histotripsy pulses. The number of colony-forming units on the mesh samples was significantly reduced when a low-amplitude pulse was included between the therapy pulses provided the mechanical index (MI) of the low-amplitude pulse exceeded 1.5. Slower scan speeds also increased biofilm destruction, but the effect was much less pronounced except for very slow scan speeds of 0.18 BW/$s$. Thus, the use of secondary pulses reduced treatment times from 20.9 $\min /{\mathrm{cm}}^2$ to 2.78 $\min /{\mathrm{cm}}^2$. Varying the PRF had no impact when the secondary pulse was not included, but biofilm destruction increased at higher PRFs when a secondary pulse was present. The number of cycles in the secondary pulse also had little impact over the range of values tested.