Phase-stable swept source OCT angiography in human skin using an akinetic source

Abstract

We demonstrate noninvasive structural and microvascular contrast imaging of human skin in vivo, using phase difference swept source OCT angiography (pOCTA). The pOCTA system employs an akinetic, all-semiconductor, highly phase-stable swept laser source which operates at 1340 nm central wavelength, with 37 nm bandwidth (at 0 dB region) and 200 kHz A-scan rate. The phase sensitive detection does not need any external phase stabilizing implementations, due to the outstanding high phase linearity and sweep phase repeatability within 2 mrad. We compare the performance of phase based OCTA to speckle based OCTA for visualizing human vascular networks. pOCTA shows better contrast especially for deeper vascular details as compared to speckle based OCTA. The phase stability of the akinetic source allows the OCTA system to show decent vascular contrast only with 2 B-scans. We compare the performance of using 2 versus 4 B-scans for calculating the vascular contrast. Finally, the performance of a 100 nm bandwidth akinetic laser at 1310 nm is investigated for both OCT and OCTA.

Keywords: (140.3600) Lasers, tunable; (170.0110) Imaging systems; (170.3880) Medical and biological imaging; (170.4500) Optical coherence tomography; (170.4580) Optical diagnostics for medicine.

Fig. 1

(a) Schematic representation of the imaging setup as used in the experiments. 80:20 – fiber-based single mode optical coupler, 80/20 splitting ratio; cr1, cr2 – circulator; p – polarization controller; c – collimator; R – retroreflector; M – mirror; x, y scan – 2-axis galvo-scan unit; L – imaging lens; DBP – dual-balanced photodetector; AMP – pre-amplifier; DAQ –data acquisition card. (b) full width at half maximum (FWHM) represents the lateral resolution of the system by measuring the edge spread function of a resolution target and then Gaussian fitting. (c) FWHM represents the axial resolution in air by measuring the line spread function of a mirror and then Gaussian fitting. (d) phase standard deviation (in 15 seconds) of the whole sweep.

Fig. 2

Tomogram and phase difference with static phantom (rubber) (a) tomogram (512 pixels × 226 pixels, 8 mm × 3.5 mm). (b) phase difference (in radian) of consecutive two tomogram at same position (radian of phase difference is between –π and π). The red rectangle indicates the ROI for the phase stability analysis.

Fig. 3

Tomogram and OCT angiograms based on phase signal at different depths. (a) healthy human palm (b) tomogram (8 × 8mm). (c) structure of skin [39]. (d)-(g) en-face view of z-projection of given depth (8 × 8mm, 512 × 512 pixels). (d) 210-235 μm. (e) 294-587 μm. (f) 587-839 μm. (g) 839-1007 μm. (h) 3-D rendering of angiographic volume (depth range: 294-1007 μm) (movie see Visualization 1).

Fig. 4

Tomogram and OCT angiograms of finger nail region based on phase difference signal at different depths. (a) healthy human finger nail, the direction of the finger is the same as tomogram and angiogram. (b) tomogram. (c)-(f) en-face view of z-projection of given depth from top surface. (c) 295-442 μm. (d) 442-590 μm. (e) 590-885 μm. (f) 885-1007 μm. (g) 3-D rendering of angiographic volume (movie see Visualization 2). (h) fusion of 3-D rendering of angiographic volume and finger.

Fig. 5

Comparison of speckle based angiography vs. phase based angiography of healthy human hand palm. (a), (b), (c) speckle based angiograms at different depths (294-587 μm. 587-839 μm and 839-1007 μm respectively), the size of all angiograms is 8 mm × 8 mm and 512 × 512 pixels. (d), (e), (f), phase based angiograms at the same depths like speckle based angiograms (294-587 μm, 587-839 μm and 839-1007 μm respectively). (g) zoom in the dashed rectangle in (a). (h) zoom in the rectangle in (d). (i) profile plotted along the green line in (g) and (h), where the increase of SNR in case of pOCTA can be verified. (j) improvement of SNR at three depths.

Fig. 6

Comparison of 2 B-scans vs. 4 B–scans based on phase based angiography of healthy human palm. (a), (b), (c) 2 B-scans phase based angiograms at different depths (294-587 μm, 587-839 μm and 839-1007 μm respectively), the size of all angiograms is 8 mm × 8 mm, 512 × 512 pixels. (d), (e), (f), 4 B-scans phase based angiograms at the same depths like 2 B-scans phase based angiograms (294-587 μm, 587-839 μm and 839-1007 μm respectively). (g) zoom in the rectangle in (a). (h) zoom in the rectangle in (d). (i) profile plotted along the green line in (g) and (h), where increase of SNR in case of the 4 B-scans pOCTA can be verified. (j) improvement of SNR at three depths.

Fig. 7

Comparison of phase based OCTA using a narrow bandwidth (37 nm) and a broad bandwidth laser (100 nm). (a) tomogram (8 × 3.6 mm) obtained with the narrow bandwidth laser. (b) tomogram with the broad bandwidth laser (8 × 3.6 mm). (c), (e), (g) phase based angiograms using narrow bandwidth laser at different depths (294-587 μm, 587-839 μm and 839-1007 μm respectively), the size of all angiograms is 6.5 × 4.5 mm, 419 × 281 pixels. (d), (f), (h) phase based angiograms using broad bandwidth laser at different depths (294-587 μm, 587-839 μm and 839-1007 μm respectively).