A numerical model for heat and pressure propagation for temperature controlled retinal photocoagulation

Retinal photocoagulation is an established treatment for various retinal diseases. The temperature development during a treatment can be monitored by applying short laser pulses in addition to the treatment laser light. The laser pulses induce thermoelastic pressure waves that can be detected at the cornea. We present a numerical model to examine the temperature development during the treatment as well as the formation and propagation of the ultrasonic waves. Using the model, it is possible to determine the peak temperature during retinal photocoagulation from the measured signal, and investigate the behaviour of the temperature profile and the accuracy of the temperature determination under varying conditions such as inhomogeneous pigmentation or change in irradiation parameters. It was shown that there is an uncertainty of 2.5 -9% in the determination of the peak temperature when the absorption coefficient between the absorbing layers is varied by a factor of 2. Furthermore the model was extended in order to incorporate the photoacoustic pressure generation and wave propagation. It was shown that for an irradiation pulse duration of 75 ns the resulting pressure wave energy is attenuated by 76 % due to frequency dependent attenuation in water.