Stress wave emission and cavitation bubble dynamics by nanosecond optical breakdown in a tissue phantom

Abstract

Stress wave emission and cavitation bubble dynamics after optical breakdown in water and a tissue phantom with Nd: YAG laser pulses of 6 ns duration were investigated both experimentally and numerically to obtain a better understanding of the physical mechanisms involved in plasma-mediated laser surgery. Experimental tools were high-speed photography with 50000 frames s$^{-1}$, and acoustic measurements. The tissue phantom consisted of a transparent polyacrylamide (PAA) gel, the elastic properties of which can be controlled by modifying the water content. Breakdown in water produced a purely compressive stress wave. By contrast, in stiff PAA samples and for sufficiently large pulse energies, the compression wave was followed by an intense tensile wave, similar to the behaviour previously observed in cornea. The elastic/plastic response of the medium led to a significant decrease of the maximum size of the cavitation bubble and to a shortening of its oscillation period which was found to be related to the generation of the tensile stress wave upon breakdown. For increasing elastic modulus of the PAA, both the amplitudes of the bubble oscillation and of the stress wave emitted during bubble collapse decreased until the bubble oscillation was so strongly damped that no collapse stress wave was emitted. Numerical simulations were performed using a spherical model of bubble dynamics which includes the compressibility and elastic/plastic behaviour of the medium, viscosity, density and surface tension. The calculations revealed that consideration of the elastic/plastic behaviour of the medium surrounding the bubble is essential to describe the experimentally observed bipolar shape of the stress wave emitted upon optical breakdown. Water is a poor tissue model because the shape of the emitted stress waves and the bubble dynamics differ strongly for both materials. The mechanical properties of PAA were also found to be quite different from those of tissues. Experimental and numerical results provided evidence that the dynamic mechanical properties relevant for optical breakdown in PAA and tissue differ by as much as two orders of magnitude from the static values. The discovery of a tensile stress wave after optical breakdown in tissue-like media is of great importance for the assessment of collateral damage in laser surgery because biological tissues are much more susceptible to tensile stress than to compressive stress.

Laser-induced cavitation phenomenon studied using three different optically-based approaches – An initial overview of results

“…Assuming P 1 to be the pressure on the inside of the shocked region and P 2 to be the pressure on the outer shockwave front, Evans et al [4] obtained high-pressure difference values for velocities in a range from 1.7 up to 2 km/s and pressures up to 500 MPa. These experiments were carried out in agar gel where the bubble oscillation is damped and the amplitude of the pressure pulse during collapse is smaller than is the case in water [23] . In the present study, higher velocities (3 to 3.5 km/s) were obtained because distilled water was used.…”

“…Figure 6 shows a typical STM signal. For constant pressure and density conditions, the Rayleigh-Plesset equation [8,22,23] relates the collapse time to the maximum bubble radius reached with each laser pulse.…”

Time-resolved imaging of the laser forward transfer of liquids

“…In fact, the expansion of a plasma inside a liquid, as it occurs in this experiment, is a well-known process which leads to the generation of a cavitation bubble. [24][25][26] Therefore, the formation of the cavitation bubble in this LIFT experiment is induced by the plasma resulting from the ablation of the Ti film.…”

Effects of cavitation bubble interaction with temporally separated fs-laser pulses

“…The energy and threshold measurements were performed in water because for this sample medium an influence due to a local damage by a previous pulse can be excluded. Furthermore, the LIOB threshold in gelatin like in polyacrylamide (PAA) as well as agar gel [36][37][38][39] is nearly the same as in water. Here again, the energy was varied to scale the bubble lifetime and hence the temporal overlap with the subsequent laser pulse.…”