Abstract

Effective pulsed laser angioplasty requires fiber-optic delivery of sufficient energy to remove calcified atheroma without damaging the fiber-optic waveguide. Therefore we studied the ablation thresholds at 532, 355, 308, and 266 nm to define the optimal wavelength for both fiber-optic transmission and tissue removal. To establish tissue ablation thresholds, we recorded the number of pulses to perforation in 788 segments of human atherosclerotic cadaver aorta. 108 sections were stained with hematoxalyn and eosin and occular micrometry was used to evaluate both blast and thermal injury. Ablation threshold for tissue was defined as the minimum energy required to ablate a 1-mm crater with 2000 pulses. Ablation threshold for 532 nm was achieved at 70 mJ/mm² but this wavelength was ineffective against calcified plaque. Ablation thresholds for 355, 308, and 266 were 15, 5, and < 5 mJ/mm², respectively. As energy density increased above ablation thresholds the number of pulses to perforate any given segment decreased asymptotically to a minimum number. Beyond this point thermal injury to the tissue occurred. To study fiber-optic damage thresholds we varied the power density of a 308-nm excimer laser. Fiber-optics were destoryed by 10-ns pulses at all ablation thresholds. In contrast, by employing a magnetically switched excimer laser, operating at 150 ns, it was possible to transmit 100 mJ/mm² through a 5-m long 600-μm core fiber-optic waveguide. This energy was sufficient to recanalize four totally obstructed canine femoral arteries without perforation. We conclude that the ablation threshold for atheroma is lower in the ultraviolet than in the visible. Therefore magnetic switched excimer lasers with pulse durations in excess of 100 ns offer the best source for pulsed angioplasty.

© 1986 Optical Society of America