Imaging Melanin Distribution in the Zebrafish Retina Using Photothermal Optical Coherence Tomography

Purpose: To demonstrate and validate that photothermal optical coherence tomography (PT-OCT) can image melanin in the retinal pigment epithelium (RPE) and can observe light-driven melanosome translocation in the zebrafish retina. Methods: A commercial spectral domain OCT system was modified to perform both OCT and PT-OCT. Four adult tyrosinase-mosaic zebrafish with varying levels of melanin expression across their retinas were imaged, and the PT-OCT signal for pigmented and nonpigmented regions were compared. Wild-type dark-adapted (n = 11 fish) and light-adapted (n = 10 fish) zebrafish were also imaged with OCT and PT- OCT. Longitudinal reflectivity and absorption profiles were generated from B-scans to compare the melanin distribution between the two groups. Results: A significant increase in PT-OCT signal (P < 0.0001, Student's t-test) was observed in pigmented regions of interest (ROI) compared to nonpigmented ROIs in the tyrosinase-mosaic zebrafish, which confirms the PT-OCT signal is specific to melanin in the eye. A significant increase in PT-OCT signal intensity (P < 0.0001, Student's t-test) was also detected in the light-adapted wild-type zebrafish group compared to the dark-adapted group. Additionally, light-adapted zebrafish display more distinct melanin banding patterns than do dark-adapted zebrafish in PT-OCT B-scans. Conclusions: PT-OCT can detect different levels of melanin absorption and characterize pigment distribution in the zebrafish retina, including intracellular changes due to light-driven melanosome translocation within the RPE. Translational Relevance: PT-OCT could quantify changes in pigmentation that occur in retinal diseases. The functional information provided by PT-OCT may also enable a better understanding of the anatomical features within conventional OCT images.