Cyclophotocoagulation: experimental investigations of dosage problems

During cyclophotocoagulation, transsclerally applied laser light reduces the aqueous-producing structures of the eye. One problem using this therapy is patient-specific dosage of the applied laser energy. The aim of our investigations was to obtain information about intensity and distribution of tissue destruction in the coagulation area. This may provide a basis for further on-line control of cyclophotocoagulation by ultrasound-controlled engineering of the diode laser.

To visualize the process of cyclophotocoagulation, a multifunctional measurement set-up was developed. It allowed the visualization of structural changes in the coagulation area using a common light microscope and comparison in the first set-up to the results detected by high-resolution ultrasound, applied in different working modes (B-mode, M-mode and RF signal analyses). In a second set-up an infrared thermography system showing temperature distribution on the scleral surface at the contact point of the laser probe was used.

High resolution working in B- and M-mode was unsuitable to visualize structural changes within the therapeutic width. By analyzing RF ultrasound date, structural changes within the therapeutic width could be detected. Surface temperatures measured by infrared thermography correlated with visible structural changes when long exposure times and low laser power were applied.

In certain cases the visualization of coagulation effects was possible with the help of either high-resolution ultrasound or infrared thermography. Spectrum analysis of RF ultrasound signals seems to be a potential method for successful control of cyclophotocoagulation.