Cavitation induced shear and circumferential stresses on blood vessel walls during photo-mediated ultrasound therapy

Photo-mediated ultrasound therapy (PUT) is a novel technique using combined laser and ultrasound to generate enhanced cavitation activity inside blood vessels. The stresses produced by oscillating bubbles during PUT are believed to be responsible for the induced bio-effects in blood vessels. However, the magnitudes of these stresses are unclear. In this study, a two-dimensional axisymmetric finite element method-based numerical model was developed to investigate the oscillating bubble-produced shear and circumferential stresses during PUT. The results showed that increased stresses on the vessel wall were produced during PUT as compared with ultrasound-alone. For a 50-nm radius bubble in a 50-mu m radius blood vessel, the produced circumferential and shear stresses were in the range of 100 kPa-400 kPa and 10 Pa-100 Pa, respectively, during PUT with the ultrasound frequency of 1 MHz, ultrasound amplitude of 1400 kPa-1550 kPa, and laser fluence of 20 mJ/cm(2), whereas the circumferential and shear stresses produced with ultrasound-alone were less than 2 kPa and 1 Pa, respectively, using the same ultrasound parameters. In addition, the produced stresses increased when the ultrasound pressure and laser fluence were increased but decreased when the ultrasound frequency and vessel size were increased. For bubbles with a radius larger than 100 nm, however, the stresses produced during PUT were similar to those produced during ultrasound-alone, indicating the effect of the laser was only significant for small bubbles.