SPIE Proceedings BIOS 2010, Ophthalmic Technologies XX, Vol. 7550 Improved safety of retinal photocoagulation with a shaped beam and modulated pulse Christopher Sramek *1 , Jefferson Brown 1 , Yannis M. Paulus 2 , Hiroyuki Nomoto 2 , Daniel Palanker 1, 2 1 Hansen Experimental Physics Laboratory, 2 Department of Ophthalmology Stanford University, Stanford, CA ABSTRACT Shorter pulse durations help confine thermal damage during retinal photocoagulation, decrease treatment time and minimize pain. However, safe therapeutic window (the ratio of threshold powers for rupture and mild coagulation) decreases with shorter exposures. A ring-shaped beam enables safer photocoagulation than conventional beams by reducing the maximum temperature in the center of the spot. Similarly, a temporal pulse modulation decreasing its power over time improves safety by maintaining constant temperature for a significant portion of the pulse. Optimization of the beam and pulse shapes was performed using a computational model. In vivo experiments were performed to verify the predicted improvement. With each of these approaches, the pulse duration can be decreased by a factor of two, from 20 ms down to 10 ms while maintaining the same therapeutic window. Keywords: retinal photocoagulation, damage threshold, retinal thermal damage 1. INTRODUCTION Laser photocoagulation, the standard of care for several retinopathies 1-4 , involves the application of pulses of 100 – 500 ms in duration, often resulting in collateral thermal damage to the inner retina 5 . In patterned scanning laser photocoagulation, patterns of 4 to 50 exposures are delivered sequentially within the eye fixation time, with pulse durations in the range of 20 ms 6 . These shorter exposures have been shown to be less painful and as efficacious as traditional retinal photocoagulation 5,7 , while targeting the retinal pigment epithelium (RPE) and outer retina more selectively 8 . However, coagulation at shorter pulse durations requires higher peak temperatures, increasing the potential for photomechanical injury due to vaporization and subsequent rupture of Bruch’s membrane 9 . This leads to narrowing of the safe therapeutic window (TW), defined as the ratio of power for producing a rupture to that of mild coagulation, which approaches unity at 1 ms 10 . It is thus desirable to increase the therapeutic window in order to allow for shorter duration pulses to be used in photocoagulation. One approach to this end is to modify the shape of the treatment beam. With conventional flat-top or Gaussian radial beam profiles, heat diffusion during the pulse results in an elevated temperature at the beam center. Such over-heating results in a higher maximum temperature than necessary to produce the desired retinal coagulation, and increases the probability of rupture. A beam shape with a lower intensity in the center compensates for the effects of thermal diffusion, resulting in a more uniform temperature profile and thermal damage zone. This absence of central overheating is expected to result in a wider therapeutic window. A second approach to increasing the safe therapeutic window is to modify the temporal structure of the pulse. Conventional pulses of constant power (square pulse shape) result in an increasing temperature during the pulse, asymptotically approaching a steady-state value for long exposures. Thermal cellular damage in the millisecond regime is often described using the Arrhenius model 11,12 . It assumes a decrease in concentration of viable cells D(τ)/D 0 as an exponential integral of the temperature time-course: * csramek@stanford.edu; phone 1 650 723-0130; http://www.stanford.edu/~palanker/lab/