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 ⩾ 2.0 W, T ⩾ 500 s; sphere : circular; central charring in bovine for P ⩾ 1.5 W and porcine for P ⩾ 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. © 1994 Wiley‐Liss, Inc.