Visible Light Optical Coherence Tomography: Technology and Biomedical Applications

Abstract

Compared to widely used near-infrared OCT (NIR-OCT) systems, visible light OCT (vis-OCT) is an emerging imaging modality that leverages visible light to achieve high-resolution, high-contrast imaging and enables detailed spectroscopic analysis of biological tissues. In this review, we provide an overview of the state-of-the-art technology development and biomedical applications of vis-OCT. We also discuss limitations and future perspectives for advancing vis-OCT.

Keywords: biomedical imaging; functional imaging; visible light OCT.

Figure 1

OCT system setups. (a) Early TD-OCT setup, (b) typical SD-OCT setup, (c) typical SS-OCT setup, and (d) visible light SS-OCT setup adopted in [16]. PPLN: fanout periodically poled lithium niobate; APD: avalanche photodetector.

Figure 7

Extended-focus in vivo vascular imaging with visible light optical coherence microscopy (xf-visOCM): owing to the high resolution of the xf-visOCM system, the microvasculature can be resolved both in the backscattering tomograms and in the angiograms. (a) A B-scan covering the first ~100 $\mathsf{\mu }$m in depth of the cortex reveals large vessels at the surface and capillaries that can be resolved as dark elongated structures, as pointed by the red arrows. Moreover, a close-up on the large caliber vessel reveals a thin dark cell-free layer below the vessel membrane (blue arrow). Vascular structures can be visualized by either performing a minimum intensity projection (MIP) on the static backscattering, in (b,c), or an MIP on the angiogram, (d,e). Similar features are highlighted between the two visualizations by red arrows in (b,c) and pink arrows in (d,e). Scale bars: 100 $\mathsf{\mu }$m. Reproduced from [133], licensed under CC BY 4.0.

Figure 8

In vivo vis-OCT endoscopic imaging of a mouse brain with a depth of 7.2 mm. (a) Reconstructed 3D image of the mouse brain, showcasing distinguishable brain regions such as isocortex (IC), corpus callosum (CC), and caudate putamen (CP). One major blood vessel (BV) and striatopallidal fibers (SF) can be visualized in the isocortex and caudate putamen, respectively. (b) The corresponding hematoxylin and eosin (H&E) histology image. (c) Coronal (y-z) en face projection image, highlighting the myelinated axon (MA) fibers (red dashed lines) and SF (black arrow). (d) Sagittal (x-z) en face projection image with MA fibers (red dashed lines), a major blood vessel (black arrow), and the SF in the caudate putamen (green triangles). Insets d1-d4 display enlarged boxed regions: neuronal cell bodies (red arrows, d1), MA fibers (red dashed lines, d2), nerve fiber bundles (blue arrows, d3), and SF (green arrows, d4). (e) Transverse (x-y) en face projection image, featuring MA fibers (red dashed lines) and a blood vessel (black arrow, strong OCT signal attenuation). Inset (e1) provides an enlarged view with neuronal cell bodies indicated by the cyan arrows. In the coronal (y-z) plane, the en face projection begins 60 $\mathsf{\mu }$m from the endomicroscope’s glass capillary and extends 60 $\mathsf{\mu }$m deep. In the sagittal (x-z) plane, it starts at a depth of 1696 $\mathsf{\mu }$m and projects 220 $\mathsf{\mu }$m deep. In the transverse (x-y) plane, it initiates at 2000 $\mathsf{\mu }$m from the brain surface and projects 200 $\mathsf{\mu }$m deep. The projection was generated using mean-intensity projection. The scale bars represent 500 $\mathsf{\mu }$m and apply to all the images. Reproduced from [140], licensed under CC BY 4.0.

Figure 9

Sample image of ex vivo porcine anterior segment using a smartphone-integrated OCT. Photograph of the anterior segment of the eye (a) with the red line showing the location of the B-scan. Raw spectrum (b) and 10-frame averaged B-scan (c) of the corneal limbus. Scale bars are 150 $\mathsf{\mu }$m along the y-axis (horizonal) and 50 $\mathsf{\mu }$m along the z-axis (vertical). (d) Optical setup of the smartphone-integrated system. Reproduced with permission from [149] © Optical Society of America.

Figure 2

SLO and vis-OCT images centered at the fovea. (a,b) En face images of SLO and vis-OCT; (c,d) contrast-adjusted images from the squared area in (a) and (b); (e) cross-sectional vis-OCT image from the position highlighted in (b) with all anatomical structures labeled. ILM: inner-limiting membrane; NFL: neural fiber layer; GCL: ganglion cell layer; IPL: inner plexiform layer; INL: inner nuclear layer; OPL: outer plexiform layer; ONL: outer nuclear layer; IS/OS: inner/outer segment junction; OS: outer segment of photoreceptor; RPE: retinal pigmented epithelium; BM: Bruch’s membrane. (f) Single cross-sectional image using a commercial NIR-OCT system. (g) Averaged vis-OCT image from eight consecutive B-scans. The motion artifact was removed by aligning the adjacent B-scans. Reprinted with permission from [23] © Optical Society of America.

Figure 3

Simplified schematic of the concept of OCTA. Signals are sampled from two different locations: A-line 1 passes through a blood vessel, while A-line 2 passes through only static tissue. Dynamic changes in the OCT signal are observed from within the blood vessel over time, while the signal from the static tissue remains steady.

Figure 4

Representative vis-OCT images illustrating postnatal retinal development in mice. (A) P14 (right after eye-opening); (B) P20 (one week post-eye-opening); (C) P30 (two weeks post-eye-opening); (D) P60 (mature adults). Top: en face projections with red lines indicating the location of corresponding non-averaged B-scans (middle). Yellow curves and boxes denote the circumpapillary paths used for resampled B-scan averaging (bottom). Visible light significantly improves layer contrast, especially in early-stage eyes with prominent hyaloid vessel shadows (blue asterisks). Scale bar: 200 $\mathsf{\mu }$m. Reprinted from [103], with permission from Elsevier.

Figure 5

En face and B-scan images of mouse retina. Representative OCT images of (A) control, (B) RP, and (C) glaucoma mouse retina. Scale bar: 200 $\mathsf{\mu }$m. Abbreviations: RP: retinitis pigmentosa; ELM, external limiting membrane; (a1–a3): Red (optic nerve head, (b1–b3)) and green (blood vessel cross sections, c1–c3) dashed lines indicate two distinct B-scan frames, corresponding to the colored borders of their corresponding B-scans below. (d1–d3): Retinal layer segmentation. (e1–e3): Magnified views of yellow solid-line regions in (c1–c3). (e2): Red arrow: lesions. (e3): Blue arrow: GCL. Reproduced with premission from [106], © Optical Society of America.

Figure 6

Vis-OCT fibergraphy (Vis-OCTF) reveals structural differences in the RGC axon bundles of BAX${}^{-/-}$ mice. (A) Example fibergrams (left panels) and circumpapillary B-scans (right panels) of littermate controls (CTRL) and BAX ${}^{-/-}$. The blue arrow highlights the left-most A-line of circumpapillary B-scans, and the red arrows indicate the direction and location of reconstructed circumpapillary B-scans. (B) Magnified view of the boxed regions indicated in panel A. The green arrows exemplify retinal thickness measurements, the blue arrows exemplify the GCIPL measurements, and the orange arrows exemplify RGC axon bundle measurements. (C) Comparing mouse RGC axon bundle organization between CTRL and BAX${}^{-/-}$ retinas using in vivo vis-OCTF and ex vivo confocal microscopy imaging of flat-mounted retinas immunostained for RGC axons (left panel) and magnified view of highlighted areas (red squares) in left panel (right panel). Numbers 1–10 denote different fiber bundles or blood vessels. Red arrows indicate blood vessels, while yellow arrows indicate nerve fiber bundles. The yellow asterisk marks a representative blood vessel. Reprinted from [109], licensed under CC BY 4.0.