Consolidated numerical temperature/pressure modelling to assess the accuracy of optoacoustic temperature determination during 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 temperature dependent thermoelastic pressure waves that can be detected at the cornea. When determining the temperature from the detected pressure waves, the static tissue parameters are assumed to be equal to their mean value that can be found in literature everywhere. However, this is unlikely in a treatment, as the tissue parameters vary from one irradiation site to another. In order to investigate the inaccuracies that are introduced by the assumption of ideal conditions, a numerical model was devised 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 to investigate the behaviour of the temperature profile and the accuracy of the temperature determination under varying conditions such as changes in the irradiation beam profile. It is shown that there is an error of 15% in determining the peak temperature, when the irradiation beam profile changes from a top hat profile to a gaussian profile. 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 there is a difference in pressure amplitude of a factor 2 between a top hat and a gaussian shaped irradiation profile due to the difference in energy deposition in the fundus layers.