Pulsed holmium laser ablation of tissue phantoms: correlation between bubble formation and acoustic transients

Vapor bubble formation and related pressure transients during pulsed holmium laser ablation of tissues are suspected to induce damage. Poly(acrylamide) gels of 70-95% water content with a Young's modulus of 0.14-4.6 x 10(5) Pa served as tissue phantoms to evaluate such effects. Holmium laser pulses (wavelength: 2.12 mu m, duration: 180 mu s FWHM), were delivered through 400 and 600 mu m diameter optical fibers inserted into cubes of clear gel. Bubble effects were investigated using simultaneous flash micro-videography and pressure recording for radiant exposures of 20-382 J/cm(2). Bubble formation and bubble collapse induced pressure transients were observed regardless of phantom stiffness. Bubbles of up to 4.2mm in length were observed in gels with a Young's modulus of 2.9 x 10(5) Pa at a pulse energy of 650 mJ. An increase of Young's modulus (reduction in water content) led to a monotonic reduction of bubble size, In the softest gels, bubble dimensions exceeded those observed in water. Pressure amplitudes at 3 mm decreased from 100+/- 14 bars to 17+/-6 bars with increasing Young's modulus over the studied range. Theoretical analysis suggested a major influence on bubble dynamics of the mass and energy transfer through the bubble boundary.