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Proceedings Paper

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

Mechanical injury during pulsed laser ablation of tissue is caused by rapid bubble expansions and collapse or by laser-induced 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 micrometers ) 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 (tau) p equals 500 ns, Qp equals 14 +/- 1 mJ, 200 micrometers fiber, Ho equals 446 mJ/mm2) or the free-running mode ((tau) p equals 100 microsecond(s) , Qp equals 200 +/- 5 mJ, 400 micrometers fiber, Ho equals 1592 mJ/mm2). 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 (approximately equals 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).

Paper Details

Date Published: 22 May 1995
PDF: 7 pages
Proc. SPIE 2391, Laser-Tissue Interaction VI, (22 May 1995); doi: 10.1117/12.209906
Show Author Affiliations
E. Duco Jansen, Univ. of Texas/Austin (United States)
Thomas Asshauer, Ecole Polytechnique Federale de Lausanne (Switzerland)
Martin Frenz, Univ. Bern (Switzerland)
Guy P. Delacretaz, Ecole Polytechnique Federale de Lausanne (Switzerland)
Massoud Motamedi, Univ. of Texas Medical Branch/Galveston (United States)
Ashley J. Welch, Univ. of Texas/Austin (United States)

Published in SPIE Proceedings Vol. 2391:
Laser-Tissue Interaction VI
Steven L. Jacques, Editor(s)

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