A new study presents a novel method for precisely measuring temperature changes in the retina during laser therapy using phase-sensitive optical coherence tomography (pOCT) in vivo. The researchers demonstrate that Phase-Sensitive OCT can accurately detect temperature rises with a precision of less than 1 °C, crucial for calibrating laser power for patient-specific, non-damaging treatments. Notably, the study reveals a significant difference in how thermal deformations are confined within the retina between healthy and degenerate retinas.
In healthy (wild-type) rats, the researchers observed that thermal deformations induced by a 10-ms laser pulse are localized between the retinal pigment epithelium (RPE) and the photoreceptors’ inner segments (IS). Conversely, in degenerate retinas, these deformations penetrate deeper into the inner retinal layers. This disparity suggests the presence of a structural component within healthy photoreceptors that effectively dampens tissue expansion caused by laser heating of the RPE and choroid. The phase-sensitive OCT study proposes that the thin and soft cilium, connecting the IS and outer segments (OS) of photoreceptors, may play a critical role in absorbing these deformations, preventing further inward tissue expansion.
The researchers utilized a custom-built, high-speed line-scan spectral-domain optical coherence tomography (LS-SD-OCT) system to capture retinal cross-sectional scans at a 10 kHz frame rate. A custom image registration algorithm, phase-restoring subpixel image registration (PRESIR), was employed to correct for bulk tissue motion, enabling precise mapping of laser-induced tissue deformations.
The phase-sensitive OCT study also developed a thermo-mechanical model to simulate the thermally induced optical path length changes (ΔOPL), considering both tissue expansion and refractive index changes. This model, incorporating a soft elastic layer representing the connecting cilium, accurately replicated the observed experimental data, further supporting the hypothesis of the cilium’s role in dampening thermal deformations.
The striking difference in thermal deformation patterns between healthy and degenerate retinas suggests that phase-sensitive OCT could be used as a biomechanical diagnostic tool for characterizing photoreceptor degeneration. The ability to precisely measure retinal temperature changes in vivo has significant implications for optimizing laser therapy and improving patient outcomes. This method could also be used to create more reliable safety regulations for retinal laser treatments.
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