Effects of irradiance and spot size on pulsed olmium laser ablation of tissue.

Threshold radiant exposures for pulsed holmium laser ablation of tissue were measured with a high-speed framing camera. The effects of laser irradiance and beam size on ablation threshold were investigated. Threshold radiant exposure decreased with increasing laser irradiance over the range of 10(5)-10(7) W/cm(2). Over the range of spot sizes examined (243-970 microm) an increase in the laser beam diameter reduced the laser irradiance required for ablation. At larger spot sizes the ablation event was occasionally initiated from multiple sites. These results suggest that the rate of energy deposition and laser-beam geometry affect the events preceding the onset of ablation and should be considered when pulsed laser ablation of tissue is modeled or quantified.