Swept source optical coherence microscopy using a 1310 nm VCSEL light source

Abstract

We demonstrate high speed, swept source optical coherence microscopy (OCM) using a MEMS tunable vertical cavity surface-emitting laser (VCSEL) light source. The light source had a sweep rate of 280 kHz, providing a bidirectional axial scan rate of 560 kHz. The sweep bandwidth was 117 nm centered at 1310 nm, corresponding to an axial resolution of 13.1 µm in air, corresponding to 8.1 µm (9.6 µm spectrally shaped) in tissue. Dispersion mismatch from different objectives was compensated numerically, enabling magnification and field of view to be easily changed. OCM images were acquired with transverse resolutions between 0.86 µm - 3.42 µm using interchangeable 40X, 20X and 10X objectives with ~600 µm x 600 µm, ~1 mm x 1 mm and ~2 mm x 2 mm field-of-view (FOV), respectively. Parasitic variations in path length with beam scanning were corrected numerically. These features enable swept source OCM to be integrated with a wide range of existing scanning microscopes. Large FOV mosaics were generated by serially acquiring adjacent overlapping microscopic fields and combining them in post-processing. Fresh human colon, thyroid and kidney specimens were imaged ex vivo and compared to matching histology sections, demonstrating the ability of OCM to image tissue specimens.

Fig. 1

Schematic of the VCSEL SS-OCM System. Data was acquired using a dual-balanced detector with 200 MHz bandwidth. DAQ: Data acquisition card. DAQ-C: External clock channel. DAQ-I: Acquisition channel. DMG: Dispersion matching glass. IA: Iris attenuator. DBT: Dual balanced detector. PS: Pulse shaper. I: Isolator, PC: Polarization controller. Focal lengths of collimating, scan and tube lenses are f_s = 11 mm, f_s = 37.5 mm and f_t = 85 mm, respectively.

Fig. 2

System characterization. A) OCM image of a USAF 1951 resolution test chart acquired with the 40X water immersion objective. B) Spectrum of the VCSEL measured with an optical spectrum analyzer showing a 117 nm tuning range. C) Mirror fringe signal acquired using optical clocking. D) Spectrally reshaped fringe signal. E) Axial PSF of the raw fringe (blue line) with 11.4 µm resolution in air (~8.1 µm in tissue), and spectrally reshaped fringe (red line) with 13.5 µm resolution in air (~9.6 µm in tissue). F) Sensitivity fall-off of the VCSEL swept source obtained from the raw fringes and spectrally reshaped fringes (G), showing no significant change in the signal sensitivity across the imaging range.

Fig. 3

Correction of delay variation with scanning demonstrated in images of fresh human colon obtained ex vivo. (A) and (B) are from two different depths from the same data set, where (C) is the calibrated image. (D) shows a surface plot of the cover slip surface taken with the 40X/W objective. Dashed lines in (A) and (B) indicate the regions that are in focus for that particular depth. Arrows point to goblet cells.

Fig. 4

Ex vivo OCM images (A-C) and corresponding histology (G) of fresh human colon specimen obtained using the 10X/W, 20X/W and 40X/W objectives. (D-F) Show the corresponding simulated confocal images. Arrows point to goblet cells.

Fig. 5

Ex vivo OCM images (A-C) and corresponding histology (D) of a fresh normal human thyroid specimen obtained using the 10X/W, 20X/W and 40X/W objectives.

Fig. 6

Depth resolved OCM with the 10X/W objective imaging a fresh ex vivo human thyroid specimen. (A) Volume rendering emphasizing that arbitrary planes can be selected for visualization. For the imaging planes indicated by the colored lines, reconstructed cross sectional and en face images are shown in (B-E). (C-E) are en face images from 50 µm, 130 µm and 180 µm below the surface of the specimen, respectively. The cross sectional image in (B) is displayed using logarithmic scale, whereas (C-E) are displayed using square root scale.

Fig. 7

Ex vivo OCM images (A-D) and corresponding histology (E) from fresh thyroid specimen with a histological diagnosis of multinodular goiter, obtained using the 20X/air and 40X/W objectives.

Fig. 8

Ex vivo OCM images (A-C) and corresponding histology (D-F) from fresh normal human kidney (A, B, D, E) and clear cell renal cell carcinoma specimens (C, F) obtained using the 40X/W (A,C) and 20X/W (B) objectives. (A, D, C, F) are from renal cortex, whereas (B, E) are from renal medulla. CT: Convoluted tubules, G: Glomerulus, T: Collecting ducts.

Fig. 9

Large field mosaic OCM image of a normal human kidney specimen. Image is constructed by merging 30 frames taken with the 40X/W objective producing a total field of 1.8 mm x 2.1 mm. (B) shows a zoomed view for the region shown with dashed lines in (A). CT: Convoluted tubules, G: Glomerulus.