Tissue ablation by holmium:YSGG laser pulses through saline and blood

The use of 2.09 micrometers Ho-YSGG laser pulses for intra-vascular non- contact ablation of tissue has been investigated. Therefore the transmission and the temporal shape of the laser pulse transmitted through saline was measured. Also the interaction between the laser pulses (200 microsecond(s) FWHM) and saline was studied by time resolved flash photography. Finally, porcine aorta was ablated (in vitro) through either blood or saline. The lesions and adjacent tissue were examined histologically. The penetration depth (the depth for a decrease to 1/e of the transmitted energy) of the laser pulses in saline depended on the power density (0.01 to 12.4 J/mm2) and varied from 0.33 to 2.2 mm, respectively. The photography showed the development of a transparent water vapor cavity around the fiber tip (320 micrometers ) during the laser pulse. The maximum dimensions of the cavity varied as function of the intensity. Within the vapor cavity the laser pulse was undisturbed. Due to this 'Moses effect in the microsecond region' porcine aorta could be ablated through up to 3 mm of saline and blood. Especially after successive laser pulses, histology showed large fissures in adjacent tissue, presumably due to the expanding vapor cavity and the layered structure of the aorta. In conclusion, the formation of a vapor cavity during Holmium laser irradiation in physiological media enables non- contact tissue ablation and induces fissures into adjacent tissue, that may be undesirable.