Accuracy of real-time 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 optoacoustic pressure waves that can be detected at the cornea. Aim of this work is the investigation of the accuracy of the determined temperatures during a treatment. To calibrate the temperature dependency of the measured pressure, whole enucleated porcine eyes were heated using an infrared laser beam, while probing the retina optoacoustically. The temperatures and the optoacoustic pressure waves were measured simultaneously using thermocouples and a piezoelectric element, respectively. From the deviation of the individual measurements an error of less than 15% in the calibration regime between 37 °C to 55 °C was found. Furthermore, the spatial and temporal temperature course was investigated. Calculations were performed to simulate the temporal and spatial temperature development during photocoagulation. A theoretical model to determine the peak temperature of the irradiated tissue from the mean temperature measured by optoacoustics was developed. The validity of the model was experimentally examined by heating the retina of porcine eyes with a laser beam diameter of 500 µm while successively measuring the temperature optoacoustically with a probe beam diameter of 500 µm and 100 µm at the center of the heated area, respectively. The deviation of the theoretical model and the experimental results were found to be less than 7µ.