title: Improving the therapeutic window of retinal photocoagulation by spatial and temporal modulation of the laser beam.
abstract: Decreasing the pulse duration helps confine damage, shorten treatment time, and minimize pain during retinal photocoagulation. However, the safe therapeutic window (TW), the ratio of threshold powers for thermomechanical rupture of Bruch's membrane and mild coagulation, also decreases with shorter exposures. Two potential approaches toward increasing TW are investigated: (a) decreasing the central irradiance of the laser beam and (b) temporally modulating the pulse. An annular beam with adjustable central irradiance was created by coupling a 532-nm laser into a 200-μm core multimode optical fiber at a 4-7 deg angle to normal incidence. Pulse shapes were optimized using a computational model, and a waveform generator was used to drive a PASCAL photocoagulator (532 nm), producing modulated laser pulses. Acute thresholds for mild coagulation and rupture were measured in Dutch-Belted rabbit in vivo with an annular beam (154-163 μm retinal diameter) and modulated pulse (132 μm, uniform irradiance "flat-top" beam) with 2-50 ms pulse durations. Thresholds with conventional constant-power pulse and a flat-top beam were also determined. Both annular beam and modulated pulse provided a 28% increase in TW at 10-ms duration, affording the same TW as 20-ms pulses with conventional parameters.
authors.affiliation: Stanford University, Hansen Experimental Physics Laboratory, 452 Lomita Mall, Stanford, California 94305, USA. csramek@stanford.edu
authors.forename: Christopher
authors.initials: C
authors.lastname: Sramek
authors.forename: Loh-Shan
authors.initials: LS
authors.lastname: Leung
authors.forename: Theodore
authors.initials: T
authors.lastname: Leng
authors.forename: Jefferson
authors.initials: J
authors.lastname: Brown
authors.forename: Yannis M
authors.initials: YM
authors.lastname: Paulus
authors.forename: Georg
authors.initials: G
authors.lastname: Schuele
authors.forename: Daniel
authors.initials: D
authors.lastname: Palanker
mesh_terms: D000818:Animals; D017076:Computer-Aided Design; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005246:Feedback; D005336:Fiber Optic Technology / Q000295:instrumentation*; D017075:Laser Coagulation / Q000295:instrumentation*; D011817:Rabbits; D012161:Retinal Artery / Q000033:anatomy & histology* / Q000601:surgery*; D016896:Treatment Outcome