Acoustic Signal Characteristics during FEL Ablation of Soft Tissue

(TH-Wrk-1-02)

J. David Mongin, Mark A. Mackanos, Calley A. Hardin, Stephen R. Uhlhorn, E. Duco Jansen  

Department of Biomedical Engineering, Vanderbilt University, Nashville, TN

Background: IR laser ablation of soft tissue is accompanied by acoustic signals which are closely related to the ablation dynamics. Previous work showed that the fraction of the energy that is coupled into acoustic energy is a function of the mechanical strength of the ablated material. We hypothesize that the acoustic signals would be able to provide insight into the role of the FEL wavelength and pulse structure on the mechanical properties and laser-induced alterations thereof. Methods: Acoustic signals associated with ablation were measured with a piezoelectric transducer (frequency response: 5kHz-1MHz). Samples used were gelatin (70% and 90% water [w/v]) and rat skin. The FEL was tuned from 2.5 to 8 $\mu$m. At each wavelength, the laser energy was set to twice the ablation threshold of water. The spot diameter = 1 mm at each wavelength. Additionally, at wavelengths of 2.8, 2.94, 3.36, and 6.45 $\mu$m, ablation noise was measured and analyzed for energy levels from 0 to 25 mJ per macropulse (radiant exposure = 0 to 3.2 J/cm²). Results: The acoustic energy at twice threshold closely followed the water absorption curve. At 2.8 and 2.94 $\mu$m, the acoustic energy was significantly greater in the 90% gel compared to the 70% gel for radiant exposures greater than 0.6 J/cm² (P<0.0005). At 3.36 $\mu$m, the acoustic energy was significantly lower in the 90% gel as compared to the 70% gel for radiant exposures greater than 1.5 J/cm² (P<0.0005). At 6.45 $\mu$m, the acoustic energy was not statistically different at any radiant exposure level. Conclusion: The acoustic energy measurement can distinguish between samples with a water concentration difference of 20%. Increases in acoustic energy by a factor of 4 in 90% gel compared to 70% gel, as reported in Erbium:YAG laser experiments were not observed at the same wavelength with the FEL. The duration and ultrastructure of the FEL pulse is thought to be responsible for this difference.