Oscillations, Waves, and Interactions - GWDG

Dynamics of pulsed laser tissue ablation 239
Figure 13. Q-switched Er:YAG laser ablation of (a) water at Φ = 1.4 J/cm 2 , (b) liver at
Φ = 1.4 J/cm 2 , and (c) liver at Φ = 5.4 J/cm 2 . The plume consists of water vapour (top)
and a droplet/vapour mixture in (a), tissue fragments in (b), and dissociated biomolecules
(top) and tissue fragments (bottom) in (c). The volumetric energy densities averaged over
the optical penetration depth are ≈ 5.2 kJ/cm 3 in (a), ≈ 4 kJ/cm 3 in (b), and ≈ 9 kJ/cm 3
in (c).
For the ablation of skin at large radiant exposures, a similar sequence of biomolecule
dissociation followed by ejection of tissue fragments was observed [37]. However, in
this case the ejection of tissue fragments occurred over a shorter time interval than
for liver. Ablation ceased when the ablation front reached a depth where the vapour
pressure dropped below the tensile strength of the extracellular tissue matrix. Nevertheless,
fragment ejection was found to continue for several microseconds after the
laser pulse while the tissue matrix is increasingly weakened by thermal denaturation.
Generally, the size of the ejected tissue particles is small at early times after the
laser pulse and increases with time [37,78]. The entire sequence of phase transitions
occurring during water and tissue ablation is summarized in Fig. 14.
Since ablation becomes a volumetric process as soon as the spinodal limit is exceeded
and a phase explosion sets in (Sect. 4.4), it is not self-evident why large
volumetric energy densities sufficient for a vapour explosion and dissociation of
biomolecules should be reached in pulsed laser tissue ablation. However, one needs
to consider that the recoil stress produced by the phase transitions of the uppermost
tissue layers delays the phase transitions in underlying layers because the spinodal
temperature increases with increasing pressure (see Fig. 6). The ongoing absorption
of laser energy into the underlying layers can thus drive the thermodynamic state into
the supercritical regime. Even larger recoil stresses are produced when these layers
are ablated, and the phase transitions in deeper layers are delayed even more. This
“positive-feedback” process continues at least until the intensity peak of the laser
pulse is reached after which a relaxation process resulting in explosive ablation commences
and continues for several microseconds after the end of the laser pulse. The
energy densities generated during the runaway process are in the order of 10 kJ cm −3
[37] and give rise to recoil pressures of several hundred MPa (Sect. 4.3).