Purpose
Sub-threshold photocoagulation can reduce excessive laser damage. However, it is difficult to assess the effectiveness and damage during laser treatment without a monitoring technique. Here we describe a method to measure depth-resolved structural and functional changes of retinal tissue during laser irradiation using the correlation map of optical coherence tomography (OCT) signals.
Methods
Spectral-domain OCT with a newly developed image processing method so called correlation-mapping is used for monitoring, which is operated at A-scan rate of 53 kHz and B-scan rate of 50 Hz. A 532-nm coagulation laser is used for photocoagulation. The OCT illumination beam and coagulation laser are coaxially aligned, and a sequence of B-scans is taken at the same region of the retina during laser treatment. In addition to the B-scan structural image, we compute the correlation between successive B-scans to detect temporal changes of the tissue. The motion artifacts between B-scans are numerically corrected with sub-pixel resolution. Then, the motion-corrected correlation map is generated to detect de-correlated area where the alteration of micro-structure induced by photocoagulation occurs and the random motion of the scatterers in blood vessels occurs. The feasibility of the method is examined by measuring three retinal regions of 3 healthy Brown Norway rats before, during and after the laser irradiation. Coagulation laser powers are adjusted to 0.9 W, 0.4 W and 0.35 W, respectively, and the exposure time is 0.2 s.
Results
The decorrelation induced by the coagulation laser was observed in one eye (0.9 W) of three eyes. Figures show the representative OCT B-scan images (left column) and correlation maps (right column) before (first row), during (second row) and after (third row) laser irradiation. The OCT B-scan image shows the disruption of photoreceptors after the laser irradiation as indicated by a red arrow. The correlation maps show the de-correlated area at RPE complex during laser irradiation as indicated by a red circle and the stoppage of choroidal blood flow after the laser irradiation as indicated by a green circle. These findings were not found in the eyes with lower coagulation power.
Conclusions
We have successfully visualized the structural and functional changes of in vivo rat retina induced by the photocoagulation using the correlation map of OCT signals.