Polarization optical coherence tomography optoretinography: verifying light-induced photoreceptor outer segment shrinkage and subretinal space expansion

Abstract

Significance: Stimulus-evoked intrinsic optical signal (IOS) changes in retinal photoreceptors are critical for functional optoretinography (ORG). Optical coherence tomography (OCT), with its depth-resolved imaging capability, has been actively explored for IOS imaging of retinal photoreceptors. However, recent OCT studies have reported conflicting results regarding light-induced changes in the photoreceptor outer segments (OSs), with both elongation and shrinkage being observed. These discrepancies may stem from the difficulty in reliably identifying OS boundaries, particularly the inner segment/outer segment (IS/OS) junction and OS tip, as well as potential confusion with subretinal space dynamics. Gaining a better understanding of these light-induced OS changes is essential for accurate interpretation of ORG measurements and for optimizing IOS imaging systems to enhance sensitivity.

Aim: We aim to develop a method for the reliable identification of OS boundaries and to verify light-induced photoreceptor OS shrinkage and subretinal space expansion.

Approach: We employed a polarization-resolved full-field swept-source optical coherence tomography system capable of sequentially capturing parallel-polarization and cross-polarization OCT signals. The parallel-polarization mode is optimized to detect ballistically reflected photons from well-defined retinal boundaries, such as the IS/OS junction and the photoreceptor tips, whereas cross-polarization primarily captures multiply scattered photons. This differentiation enables parallel-polarization OCT to minimize the interference from scattered photons, enhancing the precision of OCT band quantification.

Results: Parallel-polarization OCT revealed photoreceptor OS shrinkage and subretinal space expansion in light conditions compared with dark conditions. Moreover, the overall outer retinal length appeared to swell under light. These observations were consistently confirmed in four healthy adult human subjects.

Conclusions: Parallel-polarization OCT provides a reliable method for identifying the IS/OS junction and OS tip, confirming light-induced photoreceptor OS shrinkage and subretinal space expansion.

Keywords: intrinsic optical signal; optical coherence tomography; optoretinography; photoreceptor; polarization; retina.

Fig. 1

Schematic representation of the FF-SS-OCT system. $L1-L7$, achromatic lens; BS, beam splitter; CL, cylindrical lens; DM, dichroic mirror; LP, linear polarizer; QWP, quarter waveplate.

Fig. 2

Volumetric registration using biological references illustrated in parallel-polarization data. (a) Enface image generated by mean projection across the full depth, indicated by the green arrow in panel (c1). (b) Enface image generated by mean projection from the inner retina, spanning the IPL to Henle’s fiber layer (HFL), indicated by the blue arrow in panel (c1). Notably, the central foveal reflection pattern is only visible in panel (b), making it a reliable reference marker for image registration. (c1) Averaged B-scan generated by spatially averaging 30 B-scans from the region demarcated by the yellow window in panel (b). (c2) Averaged B-scan generated by spatially averaging 30 B-scans from the region demarcated by the orange window in panel (b). These B-scans were obtained from a single volume before volumetric averaging and flattening.

Fig. 3

Representative (a) parallel and (b) cross-polarization OCT mean B-scans displayed in logarithmic scale, obtained to record [(a1), (b1)] 0- and 2-deg nasal eccentricities, and [(a2), (b2)] 10-deg nasal eccentricity. Data were recorded from subject 1 in light conditions. The blue arrow in (a1) and (b1) denotes the central foveal reflection pattern. The 1st, 2nd, 3rd, 3rd_a, 3rd_b, and 4th outer retinal band corresponds to the external limiting membrane (ELM), inner segment/outer segment (IS/OS) junction or ellipsoid zone (EZ), outer segment tip (OST) or interdigitation zone (IZ), cone outer segment tip (COST) or cone interdigitation zone (CIZ); rod outer segment tip (ROST) or rod interdigitation zone (RIZ), retinal pigment epithelium/Bruch’s membrane (RPE/BM) complex, respectively.

Fig. 4

Representative axial intensity profiles (a1), (b1), and (c1) and corresponding band distances (a2), (b2), and (c2) obtained from light- and dark-adapted retinas using parallel and cross-polarization OCT. Data were recorded from subject 1 at (a) 0-deg nasal eccentricity, (b) 2-deg nasal eccentricity, and (c) 10-deg nasal eccentricity.

Fig. 5

Mean and standard error plot of thickness differences between light- and dark-adapted retinas using (a) parallel-polarization and (b) cross-polarization at [(a1), (b1)] 0-deg nasal eccentricity, [(a2), (b2)] 2-deg nasal eccentricity, and [(a3), (b3)] 10-deg nasal eccentricity. Each value was estimated by subtracting the thickness in dark conditions from the thickness in light conditions. Statistical significance was conducted using a paired sample $t$-test with a confidence level of 95%. $P(0.05)$ = *; $P(0.005)$ = **; $P(0.001)$ = ***.