Oscillations, Waves, and Interactions - GWDG

Dynamics of pulsed laser tissue ablation 245
Figure 17. Stress transients resulting from TEA CO2 laser (tp = 30 ns) irradiation of
porcine dermis for radiant exposures below the ablation threshold (i), at threshold (ii), and
above threshold (iii)–(v). Radiant exposures below threshold produce bipolar thermoelastic
stress transients. For radiant exposures equal to and above threshold, a compressive pulse
is produced by the ablative recoil. (Adapted from Ref. [41] with permission. Copyright 1996
Biophysical Society)
ducers [40,41,108,109], and, for water ablation, through analysis of the speed of the
recoil-induced shock wave [37]. Peak pressures range from a few MPa at the ablation
threshold up to several hundred MPa for radiant exposures well above threshold. For
free-running microsecond laser pulses, average stress values during the laser irradiation
have been determined through measurement of the recoil momentum using the
ballistic pendulum method [65,106], while the peak stress amplitudes produced by
the intensity maxima of the free-running pulses were obtained via transducer measurements
[109]. Peak values of recoil stress produced during cornea ablation using
free-running Er:YSSG laser irradiation at a radiant exposure of 50 J/cm 2 amounted
to 2 MPa [109] while the average pressure value for skin ablation at the same radiant
exposure was only 0.3 MPa [106].
In stress-confined tissue ablation, the compressive recoil stress transient is superimposed
on a bipolar thermoelastic transient [110]. Figure 17 demonstrates the transition
from a bipolar stress transient for radiant exposures below the ablation threshold
to a monopolar compressive transient when the ablation threshold is exceeded. This
transition and the corresponding increase in peak pressure is a sensitive method for
the determination of the ablation threshold [40,41,108].
The recoil stress produced by both vaporization and material ejection in the primary
ablation phase can induce a secondary material expulsion process that leads
to a strong increase of the ablation efficiency [37,105]. Recoil-induced material expulsion
is most pronounced during ablation of liquids and mechanically weak tissues.