Laser-induced hydrodynamics in water-saturated tissue: III. Optoacoustic effects

Abstract

Studied in this work are specific features of acoustic vibrations generated at the hot blackened tip of an optical fiber (the so-called hot tip) delivering moderate-power (1–10 W) CW laser radiation in contact with water or a water-saturated biotissue. Generated upon such contact is a wideband acoustic signal whose characteristics largely depend on the object exposed and treatment scheme. Placing the hot tip in an acoustic resonator is demonstrated to cause distinct amplitude modulation of the acoustic noise. The formation of laser channels in an intervertebral disc or the intramedullary cavity of a bovine thighbone gives rise to the emission of a quasiperiodic train of pulses associated with the explosive growth and collapse of steam-gas bubbles in the hot-tip-to-biotissue contact region. The resultant pressure pulses, 20 ± 15 MPa in amplitude, cause damage to the adjacent tissue and facilitate the production of a laser channel at a rate of some 0.4–5 mm s−1. During the course of laser treatment the biotissue gradually gets saturated with steam-gas bubbles, which results in the development of low-frequency pressure oscillations in the range 0.1–10 Hz and a gradual pressure rise to around 200 kPa, leading to reduction of the natural frequencies of the resonance modes of the biotissue. The possible effect of these acoustic vibrations on the biotissue is discussed.