Reflectance feedback control of photocoagulation in vivo

Laser induced retinal lesions are used to treat a variety of eye diseases such as diabetic retinopathy and retinal detachment. In this treatment, an argon laser beam is directed into the eye through the pupil onto the fundus where the heat resulting from the absorbed laser light coagulates the retinal tissue. This thermally damaged region is highly scattering and appears as a white disk. The size of the retinal lesions is critical for effective treatment and minimal complications. Currently, laser treatment is accomplished in a ballistic manner. Once an irradiation is begun, no attempt is made to alter the exposure time to correct for the inhomogeneity in the absorption of the tissue being coagulated. Since the size of a lesion that will result from an irradiation cannot be predicted, the lesions should ideally be monitored in real time and the irradiation ceased when a lesion of the appropriate dimension is formed. Lesions form in much less than one second and thus, automated control is required. A real time feedback control system is implemented that monitors lesion growth using twodimensional reflectance images acquired by a CCD camera. The camera views the lesion formation on axis with the coagulating laser beam. The reflectance images are acquired and processed as the lesion forms. When parameters of the reflectance images that are correlated to lesion dimensions meet certain preset thresholds, the laser is shuttered. For example, lesion depth is controlled using light reflected from the center of a lesion - the central reflectance. Results of feedback controlled lesions formed in vivo in pigmented rabbits are presented. An ability to produce uniform lesions despite variation in the tissue absorption or changes in laser power is demonstrated. This lesion control system forms part of a larger automated system for retinal photocoagulation.