Dependence of laser photocoagulation on interstitial delivery parameters

Photocoagulation was performed ex vivo between tissue slabs by delivering continuous-wave laser energy from an optical fiber either directly, or by depositing the energy into a 2.4 mm diameter steel sphere at the fiber tip. The dependence of photocoagulation lesions on the following variables was assessed: (1) energy source: Nd:YAG-532 nm, 1,064 nm +/- steel sphere, (2) tissue type: porcine muscle (light), bovine muscle (dark), (3) delivered power: P = 1.5-3.0 W (porcine), 1.0-2.5 W (bovine), (4) exposure duration: T = 300-1500 s. The resulting cross-sectional photocoagulation lesions are summarized as follows: 532 nm: elongated; central charring in all cases; 1,064 nm: circular; central charring only in bovine for P > or = 2.0 W, T > or = 500 s; sphere: circular; central charring in bovine for P > or = 1.5 W and porcine for P > or = 2.0 W. These experiments suggest photocoagulation lesion size decreases as optical penetration increases. The results indicate that interstitial laser photocoagulation lesions > 10 mm diameter can be made without charring in both lightly and heavily pigmented tissues ex vivo by delivering 1,064 nm laser energy at sufficiently low power for at least 1,000 s from well-polished optical fibers.