The effect of pulse structure on soft tissue laser ablation at mid-infrared wavelengths

A series of experimental investigations have demonstrated that targeting a mid-infrared Mark-III Free-Electron Laser to wavelengths near 6.45 mum results in tissue ablation with minimal collateral damage and substantial efficiency useful for human surgery. Thermodynamic reasoning suggests that the minimal collateral damage at this wavelength is due to the differential absorption of protein and water; which causes compromised tissue integrity by laser heating of the non-aqueous components prior to explosive vaporization. These properties are advantageous for surgery because they reduce the structural integrity of the tissue, thus reducing amount of energy needed for ablation. While the FEL, based on these findings, has been used successfully in eight human surgeries to date, it is unlikely that this laser will become broadly used clinically due to its expense and difficult implementation. Recent developments in conventional laser technology have provided access to this wavelength. While the average and peak power of these sources are still not equivalent to the FEL, recent data indicates that ablation studies are feasible. The research described here investigates the role of pulse structure with regards to soft tissue ablation to determine the feasibility of these sources as potential FEL replacements for clinical applications. Relevant parameters including the threshold radiant exposure and ablated crater depth were studied for comparison of the native FEL micropulse with a stretched FEL micropulse and a ZnGeP2 OPO. Brightfield imaging was used to analyze the effect of pulse structure on the dynamics of ablation, while histology on cornea and dermis was performed to study pulse effects on thermal damage. Mass spectrometry was also used to investigate whether non-linear effects are involved with the FEL micropulse in changing the chemical structure of proteins prior to ablation. The results of this analysis show that the micropulse structure of the FEL does not play a role in the efficient ablation of soft tissue with minimal collateral damage that has been shown previously, and alternative sources remain viable alternatives to the FEL.