Model study to investigate the contribution of spallation to pulsed laser ablation of tissue.
OBJECTIVE
Absorption of a short laser pulse produces high thermoelastic stress in the irradiated volume. The relaxation of this stress at a free (tissue-air) surface leads to tensile loading, resulting in mechanical spallation. Using model substances, we investigated the role of this effect in tissue ablation.


METHODS
Stained water and gelatine were irradiated with short pulses (8 ns duration) from a Nd:YAG laser at 1,064 nm wavelength. The dynamics of the induced effects were observed with laser-flash photography and stress wave detection.


RESULTS
Spallation is indicated by the formation of cavitation bubbles below the irradiated surface and is strongly influenced by impurities serving as nucleation sites. Material ejection due to spallation was observed in the liquid sample at a fluence leading to a temperature below the boiling point but needed a temperature in excess of 100 degrees C in gelatine, owing to the small mechanical energy available for this process, estimated to be < 1%.


CONCLUSIONS
The mechanical action of thermoelastic stress waves is characterized by high stress amplitudes but low energetic efficiency. A model combining spallation and vaporization is therefore proposed for efficient tissue ablation.
Paltauf
G
G
Institut für Experimentalphysik, Karl-Franzens Universität Graz, Austria.
Schmidt-Kloiber
H
H
D001696:Biomechanical Phenomena; D053685:Laser Therapy; D008961:Models, Structural