Laser flash photography of cold cavitation-driven ablation in tissues

Abstract

Cold cavitation-driven ablation of collagen gels and porcine corneas by nanosecond laser pulses was visualized using flash photography with photoflash pulses at 650 nm. Initial thermoelastic stress wave propagation, cavitation bubble formation and material ejection were observed and recorded at the microsecond time-scale with 10-ns temporal resolution. Previously, the important role of tensile phase of thermoelastic stress that causes an efficient cavitation in irradiated volume and drives material ejection at temperatures substantially below I 00 °C was demonstrated for aqueous solutions. The experiments reported here have confirmed possibility of similar ablation mechanism for biological tissues. The dynamics of material ejection upon short pulse ablation is dependent on incident laser fluence and tissue optical and mechanical properties. Ablation process duration increases from several microseconds to several hundred microseconds with the increase of water content in tissues and/or decrease of tissue mechanical strength. Duration time of ejection also increases with the rise of the absorbed energy density from the cavitation threshold to the thermal explosion threshold. On the basis of experimental data, cold ablation process in soft tissues upon short laser pulse irradiation is proposed to include two stages of material ejection: (1) initial ejection of subsurface layer due to the expansion and rupture of cavitation bubbles, (2) delayed ejection of bulk material due to the hydrodynamic motions stimulated by initial crater and also by collapse of large coalesced bubbles.

Photomechanical Processes and Effects in Ablation

“…At later times, usually a couple of microseconds after passage of the thermoelastic wave, cavities tend to accumulate in a more or less broad region at some distance from the surface, forming a “spall layer” (Figure ). The fact that this layer is not well-defined contradicts the simple spallation model that assumes a distinct tensile strength but confirms the theoretical prediction of a broad cavitation zone from the more accurate molecular dynamics and hydrodynamic code calculations. ,, Time-resolved photography of cavitation in transparent tissue samples such as cornea or tissue phantoms such as gelatin yielded images of cavitation similar to those in liquids. ,

13 Flash photography of cavitation in water with μ a = 19 cm -1 (a) 1 μs and (b) 10 μs after a laser pulse with a wavelength of 1064 nm, a duration of 8 ns, and a fluence of 3.5 × 10 4 J/m 2 has hit the liquid surface. The ambient pressure is 1 bar.

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“…The dynamics of individual voids strongly influence the temporal evolution of the fracture and the following ablation. Time-resolved imaging has been used in several studies to study cavitation in transparent materials. ,,− Under plane wave conditions cavitation is seen right above the compression−tension transition (Figure ). In liquids or soft materials such as biological tissue or gelatin the cavities first grow under the action of the tensile stress pulse and then collapse under the ambient pressure.…”

Computer Simulations of Laser Ablation of Molecular Substrates

“…14,33,115,116,155,156 Moreover, observations from scattering experiments for laser ablation of polymer targets by Hare et al 157 suggest that photomechanical effects can lead to the ejection of a relatively intact layer of material that maintains its integrity at least on the time scale of tens of nanosecond. These observations can be related to the spallation of a layer of material observed in the simulations performed under conditions of stress confinement, 68,69,71 Figure 10.…”

“…As has been discussed in Section III.C, the contribution of photomechanical effects can lead to the material ejection at energy densities much lower than those required for boiling and vaporization. The condition for the onset of "cold" laser ablation, usually referred as inertial 62,158,160 or stress confinement, 33,68,69,71,74,115,119,155 can be expressed as τ p eτ s ∼ L p /C s , where C s is the speed of sound in the irradiated material and τ s is the characteristic time of the mechanical equilibration of the absorbing volume. 165 In the regime of stress confinement, the laser pulse duration is shorter or comparable to the time that is needed for a mechanical relaxation (expansion) of the absorbing volume and the laser heating takes place at nearly constant volume conditions, causing buildup of a high thermoelastic pressure.…”

Drug delivery with microsecond laser pulses into gelatin

“…Cavitation bubbles play an important role in pulsed laser ablation of tissue and in laser lithotripsy. [13][14][15][16] Van Leeuwen et al demonstrated that cavitation bubbles make it possible to ablate tissue in a noncontact mode through a layer of blood or saline, and the forceful expansion of a cavitation bubble induces mechanical damage in adjacent tissue in the form of dissections. 13 A study by Vogel et al suggested that cavitation-induced dilatation of vessel walls occurring in pulsed laser angioplasty can be prevented by division of the laser pulse energy into a prepulse with low energy and an ablation pulse with high energy.…”