Temperature controlled retinal laser treatment

This research project is carried out in cooperation with  

• Dr. Ralf Brinkmann,
  University Lübeck, Natural Sciences, Institute of Biomedical Optics
• Professor Dr. Karl Worthmann,
  TU Ilmenau, Faculty of Mathematics and Natural Sciences, Institute for Mathematics

Retinal laser photocoagulation has become a standard tool for the treatment of various eye diseases. The main idea is to use intensive laser pulses resulting in a temperature increase and finally in coagulation of the retinal tissue. Photocoagulation is e.g. used to prevent retinal detachment after hole formation, or to reduce the number of highly oxygen consuming photoreceptors in the retinal periphery to preserve the macular function in diabetic patients. Mostly, minimally invasive uniform lesions are demanded: For the treatment of diabetic macular oedema irreversible damages of the neural retina by too strong coagulation must be prevented while guaranteeing the desired therapeutic effect by avoiding underdose.A major challenge in photocoagulation is the adjustment of a proper light dosing owing to strong absorption variations across the retina, changing light scattering within the eye and small involuntary eye movements (microsaccades).

In current clinical practice, the treating physician chooses the laser power for the subsequent lesions according to the visibility of the previous lesions after the typical 50-200 ms irradiation time. However, this is a very cumbersome and time consuming procedure and often leads to quite unsatisfactory results. Hence, a very accurate control of the laser power for the intended temperature rise (induced heat) based on real-time measurements is of great importance.In our previous work a first and so far only method for real-time retinal temperature determination was developed based on the optoacoustic effect. The main goal of the present proposal is to develop closed-loop automatic control strategies based on this methodology, which can guarantee an accurate realization of the desired treatment temperature predetermined by the ophthalmologist. In particular, we plan to develop a new experimental setting getting rid of the previously used combination of treatment and probe lasers by using only one high repetition rate laser, which simultaneously serves to excite optoacoustic pressure waves and produces sufficient heat for coagulation. On the algorithmic side, we will

1. develop models of different granularity suitable for controller design,
2. design observation/estimation strategies to reconstruct the desired quantities (in particular the retinal temperature distribution) from the available measurements (pressure), and
3. develop and evaluate suitable control strategies of different complexity in order to automatically adjust the required laser power.

With respect to the latter point, we will in particular consider different model predictive control approaches allowing for hard guarantees of constraint satisfaction, which is indispensable in medical applications.In conclusion, the proposed project will constitute a crucial step towards significantly improved and safe applications of retinal laser therapy and further helps to investigate short pulsed thermal damage of tissue not fully understood yet.