Significance: Selective retina therapy (SRT) selectively targets the retinal pigment epithelium (RPE) and reduces negative side effects by avoiding thermal damages of the adjacent photoreceptors, the neural retina, and the choroid. However, the selection of proper laser energy for the SRT is challenging because of ophthalmoscopically invisible lesions in the RPE and different melanin concentrations among patients or even regions within an eye.
Aim: We propose and demonstrate SRT monitoring based on speckle variance optical coherence tomography (svOCT) for dosimetry control.
Approach: M-scans, time-resolved sequence of A-scans, of ex vivo bovine retina irradiated by 1.7-μs duration laser pulses were obtained by a swept-source OCT. SvOCT images were calculated as interframe intensity variance of the sequence. Spatial and temporal temperature distributions in the retina were numerically calculated in a 2-D retinal model using COMSOL Multiphysics. Microscopic images of treated spots were obtained before and after removing the upper neural retinal layer to assess the damage in both RPE and neural layers.
Results: SvOCT images show abrupt speckle variance changes when the retina is irradiated by laser pulses. The svOCT intensities averaged in RPE and photoreceptor layers along the axial direction show sharp peaks corresponding to each laser pulse, and the peak values were proportional to the laser pulse energy. The calculated temperatures in the neural retina layer and RPE were linearly fitted to the svOCT peak values, and the temperature of each lesion was estimated based on the fitting. The estimated temperatures matched well with previously reported results.
Conclusion: We found a reliable correlation between the svOCT peak values and the degree of retinal lesion formation, which can be used for selecting proper laser energy during SRT.
The most challenging aspect of deep anterior lamellar keratoplasty (DALK), is what’s known as “Big Bubble” technique which injects air/fluid to fully separate the Descemet’s Membrane and stroma with a hydro-dissection needle. Big bubble technique requires micron accuracy to guide the needle to approximately 90% depth of cornea. Here, we developed and tested common-path swept source optical coherence tomography (CP-SSOCT) distal-sensor integrated hydro-dissection needles, which can accurately detect the needle position relative to corneal tissues with micron accuracy. The OCT distal-sensor was put inside a 30-gauge needle, which was also used for hydro dissection. A high-index elliptical epoxy lens was attached to the end of the single mode fiber to increase the signal to noise ratio inside the cornea. To control the position and insertion angle of the sensor, we customized the eye mount with an angular slide and a precise linear motor with Luer-slip. The needle was fixed outside, 100um from epithelium layer, to obtain the A-scan image to identify both epithelium and endothelium membranes at every 10° from 0° to 70° insertion angles. The needle was then inserted into bovine cornea and recorded A-scan images at each step. Freehand insertion test was performed with and without sensor guided needles. The results showed that the position of the epithelium and endothelium membranes were still identified from A-scan even at 70°. Sensor guided freehand test can reach 95% of cornea thickness on average without any perforation. These results are consistent with our hypothesis that CP-SSOCT fiber sensor can guide a needle insertion inside a cornea for Big Bubble technique.