BACKGROUND: Acoustic shock waves have been detected during pulsed laser irradiation, however, ablation characteristics relevant to the shock waves have not been well elucidated. In this study, the potential role of excimer laser induced shock waves and problems associated with contact-dependent multifiber laser catheter were investigated. METHODS: Normal post-mortem human aortic samples were lased with a xenon chloride excimer laser(308nm, 120ns) using a 1.5mm multifiber laser catheter(40x100microm) at repetition rates of 25-40Hz and coaxial forces of 0.125-1.5Atm in a saline-filled tubing system. A total of 160 craters was evaluated. The operative fluence was 40mJ/mm2 and 125 pulses were delivered at each crater. Laser-induced shock waves were measured with a strain-gauge presssure transducer. The ablation characteristics were evaluated by a stereoscopic microscope(x20). RESULTS: When coaxial force was 0.125Atm, shock waves were less than 0.1mmHg without significant tissue ablation. The magnitude of laser-induced shock wave correlated with coaxial force(r2=0.808, p=0.0001) and with repetition rate at 1.0Atm of coaxial force(r2=0.520, p=0.0001). Assumed pressure relaxation time(or escape time) of laser induced shock wave in the irradiation site was 51.6+/-3.2ms at the lowest repetition rate of 25Hz and 1.0Atm of coaxial force, which was longer than 40ms of the pulse interval(p=0.0001). Repetition rate correlated with the depth of crater(r2> or =0.375, p=0.0001). the area of crater(r2> or =0.121,p<0.05), and the ablated mass of tissue(r2> or =0.351, p=0.0001) when significant coaxial forces were applied. Coaxial force correlated with the depth of crater(r2> or =0.570, p=0.0001), the ablated mass of tissue at repetition rates higher than 30Hz(r2< or =0.265, p<0.001), and had inverse correlation with the area of crate(r2> or =0.677, p=0.0001). Microscopic observation revealed fusion of single fiber lesions and larger crater size at higher repetition rates, which suggested evidences of photomechanical tissue ablation. CONCLUSIONS: 1) Application of significant coaxial force was essential to the effective tissue ablation and high repetition rate enhanced it; 2) effective tissue ablation was always accompanied by the significant magnitude of laser-induced shock wave; 3) photomechanical ablation was considered as a potential mechanism of enhanced tissue ablation; 4) application of coaxial force significantly reduced the area of crater; and 5) although single fiber lesions had precise edge, the final margins created by multifiber laser catheter were very irregular.
Acoustics ; Catheters* ; Humans ; Lasers, Excimer* ; Relaxation ; Shock* ; Transducers ; United Nations ; Xenon