Line-field parallel swept source MHz OCT for structural and functional retinal imaging

Abstract

We demonstrate three-dimensional structural and functional retinal imaging with line-field parallel swept source imaging (LPSI) at acquisition speeds of up to 1 MHz equivalent A-scan rate with sensitivity better than 93.5 dB at a central wavelength of 840 nm. The results demonstrate competitive sensitivity, speed, image contrast and penetration depth when compared to conventional point scanning OCT. LPSI allows high-speed retinal imaging of function and morphology with commercially available components. We further demonstrate a method that mitigates the effect of the lateral Gaussian intensity distribution across the line focus and demonstrate and discuss the feasibility of high-speed optical angiography for visualization of the retinal microcirculation.

Keywords: (110.4500) Optical coherence tomography; (170.0110) Imaging systems; (170.3880) Medical and biological imaging; (170.4460) Ophthalmic optics and devices.

Figures

Fig. 1

(a) Schematic of LPSI setup as explained in the text. (b) Ray diagram of the illumination beam in tangential (parallel) direction of sample (black dotted line) and reference arm (red dotted line), respectively. The imaging relation is depicted as green line. (c) Illumination (and imaging) beam of the sagittal (scanning) direction. Numbers without units are in mm. IS and IR are sample and reference arm beam, respectively.

Fig. 2

Principle of LPSI signal processing as explained in the text. Numbers 1-5 correspond to the steps described in the enumeration above.

Fig. 3

Schematic beam geometry for calculating maximum retinal exposures in point focus illumination (top) and anamorphic line focus illumination (bottom). Definition of sagittal and tangential plane as explained in the text. IS is the respective sample illumination intensity in tangential x- direction.

Fig. 4

Stitched widefield 2D retinal images of macula and optic nerve head (ONH). The total field of view is approx. 30°. (a) is obtained without averaging, (b) is obtained by averaging 4 successive tomograms in scanning direction. ILM is internal limiting membrane, ONL is outer nuclear layer, OPL is outer plexiform layer, INL is inner nuclear layer, IPL is inner plexiform layer, GCL is ganglion cell layer, NFL is nerve fiber layer, ELM is external limiting membrane, RPE is retinal pigment epithelium, PJ is photoreceptor junction.

Fig. 5

3D retinal images of parafoveal region. (a) single frame tomogram in transversal (parallel) direction. (b) averaging 4 successive tomograms in scanning (sagittal) direction. (c) and (d) tomograms along the sagittal coordinate. (e), (f), (g) and (h) are enface projections at depth locations indicated in (a). (i) 3D rendering of same data. The arrow points to visible nerve fiber bundles. (GCL- ganglion cell layer, INL - inner nuclear layer, SL - Sattler’s layer, HL - Haller’s layer).

Fig. 6

3D retinal images obtained at an eccentricity of 7° towards the ONH. (a) represents a depth resolved tomogram in transversal direction. No averaging was employed. (b) is obtained after averaging 4 successive tomograms in scanning (sagittal) direction. (c) and (d) are respective tomograms with the abscissa being the sagittal coordinate. (e), (f) and (g) are enface projections at depth locations indicated in (a). (h) 3D rendering of same data. Abbreviations are explained in Fig. 4 and 5.

Fig. 7

Demonstration of the effect of Gaussian weighting on the lateral signal degradation. (a) original retinal tomogram acquired at the periphery of the ONH. The image brightness (B) and contrast (C) was adjusted for optimal examination at the tomogram center. (b) the same tomogram, but B&C adjusted to visualize structures at the periphery. (c) tomogram after lateral Gaussian weighting with curve gˆ1. (d) normalized lateral signal decay (green curve) as a function of sensor pixels, obtained by averaging over 100 successive sagittal tomograms within the indicated green box in (a). The red curve is the respective Gaussian fit (Sect. 2.3). The normalized black and blue dashed curves gˆ1 and gˆ2 are obtained after inverting the Gaussian fit according to Eq. (6) with d = 0 and d = 5 respectively. Sm is the measured lateral sensitivity decay across the sensor pixels. (e) tomogram after Gaussian weighting with curve gˆ2.

Fig. 8

Retinal tomograms acquired at 7° eccentricity from the fovea centralis towards the ONH. (a) was acquired at 600, (b) at 800 and (c) at 1000 equivalent kA-scans/s. The horizontal lines in the tomograms are the remaining DC terms. (d)-(f) were obtained after averaging 4 successive tomograms.

Fig. 9

High resolution retinal imaging at 600kHz. The image was acquired at an eccentricity of 7° from the fovea centralis towards the ONH. (a) enface projection at the depth position indicated with a white dashed line in (b). (b) linearly-scaled retinal tomogram taken at the indicated position (white dashed line on right side) in (a) obtained by averaging over 4 successive sagittal frames.

Fig. 10

Non-invasive, high speed optical angiography of the retinal vasculature network based on speckle variance (Sect. 2.4). (a) tomogram showing the fovea centralis and indicating the depth position (red box) used to obtain the enface- projection of (b). (b) enface OA image obtained after calculating the speckle variance image. The enface image was obtained by maximum intensity projection over a depth range indicated in (a).

Fig. 11

Demonstration of the effect of axial motion artifacts. (a)-(d) are tomograms acquired around the ONH with Config. A1. The blurring emanates from eye motion during acquisition of the spectrum. (e) shows a section acquired at approx. 4° eccentricity from the fovea towards the ONH. The artifact highlighted in the magnified image of (e) and in (e) is a consequence of the blurring induced by the axial blood flow component in the vessel below. The distance between location 1 and 1’ indicated in (b) corresponds to the estimated lateral motion between the tomogram in (a) and the successive tomogram in (b). The axial displacement between tomograms in (a) and (b) might be overestimated, since the sample motion introduces a Doppler frequency causing an additional artificial shift of the sample structure.