Effect of young's modulus on bubble formation and pressure waves during pulsed holmium ablation of tissue phantoms

Mechanical injury during pulsed laser ablation of tissue is caused by rapid bubble expansions and collapse or by laserinduced pressure waves. In this study the effect of material elasticity on the ablation process has been investigated. Polyacrylamide tissue phantoms with various water concentrations (75 -95%) were made. The Young's moduli of the gels were determined by measuring the stress-strain relationship. An optical fiber (200 or 400 μm) was translated into the clear gel and one pulse of holmium:YAG laser radiation was given. The laser was operated in either the Q-switched mode (τ_p =500 ns, Q_p = 14 ± mJ, 200 μm fiber, H₀ =446 mJ/mm²) or the free-running mode (τ_p = 100 μs, Q_p = 200 ± 5 mJ, 400 μm fiber, H₀ = 1592 mJ:mn²) Bubble formation inside the gels was recorded using a fast flash photography setup while simultaneously recording pressures with a PVDP needle hydrophone (40 ns risetime) positioned in the gel, approximately 2 mm away from the fibertip. A thermo-elastic expansion wave was measured only during Q-switched pulse delivery. The amplitude of this wave (-40 bar at 1 mm from the fiber) did not vary significantly in any of the phantoms investigated. Rapid bubble formation and collapse was observed inside the clear gels. Upon bubble collapse, a pressure transient was emitted; the amplitude of this transient depended strongly on bubble size and geometry. It was found that 1) the bubble was almost spherical for the Q-switched pulse and became more elongated for the free-running pulse, and 2) the maximum bubble size and thus the collapse amplitude decreased with an increase in Young's modulus (from 68 ± 11 bar at 1 mm in 95% water gel to 25 ± 10 bar at 1 mm in 75% water gel).