Multi-MHz MEMS-VCSEL swept-source optical coherence tomography for endoscopic structural and angiographic imaging with miniaturized brushless motor probes

Abstract

Swept source optical coherence tomography (SS-OCT) enables volumetric imaging of subsurface structure. However, applications requiring wide fields of view (FOV), rapid imaging, and higher resolutions have been challenging because multi-MHz axial scan (A-scan) rates are needed. We describe a microelectromechanical systems vertical cavity surface-emitting laser (MEMS-VCSEL) SS-OCT technology for A-scan rates of 2.4 and 3.0 MHz. Sweep to sweep calibration and resampling are performed using dual channel acquisition of the OCT signal and a Mach Zehnder interferometer signal, overcoming inherent optical clock limitations and enabling higher performance. We demonstrate ultrahigh speed structural SS-OCT and OCT angiography (OCTA) imaging of the swine gastrointestinal tract using a suite of miniaturized brushless motor probes, including a 3.2 mm diameter micromotor OCT catheter, a 12 mm diameter tethered OCT capsule, and a 12 mm diameter widefield OCTA probe. MEMS-VCSELs promise to enable ultrahigh speed SS-OCT with a scalable, low cost, and manufacturable technology, suitable for a diverse range of imaging applications.

Fig. 1.

Schematic of the SS-OCT system. Sweep to sweep calibration and resampling are performed using dual channel acquisition of the OCT signal and a Mach Zehnder interferometer signal, overcoming inherent optical clock limitations and enabling higher performance. ADC: analog to digital converter, AFG: arbitrary function generator, AMP: 3-phase linear amplifier, BOA: booster optical amplifier, COL: collimator, PC: polarization controller, NI: National Instruments card, OSA: optical spectrum analyzer.

Fig. 2.

Wavelength sweep range of the MEMS-VCSEL measured with an integrating optical spectrum analyzer at 2.4 MHz and 3.0 MHz A-scan rate.

Fig. 3.

3D models of the imaging probes used in this study, including the (A) micromotor OCT catheter (B) tethered OCT capsule, and (C) widefield OCTA probe.

Fig. 4.

(A) Point spread function for 2.4 MHz A-scan rate, shown in linear scale. (B) Corresponding sensitivity roll off in logarithmic scale. Dot-dashed line is −6 dB below the maximum PSF value. Dashed line shows the maximum depth/fringe frequencies (prior to wavenumber calibration) corresponding to the ADC 1 GHz Nyquist limit. (C) Point spread function for 3.0 MHz A-scan rate, shown in linear scale. (D) Corresponding sensitivity roll off in logarithmic scale.

Fig. 5.

Fractional rotational period variation $\delta$T_n/T_n and position variation δS_n versus rotation n (time) for the sleeve, jewel, and ball bearing motors.

Fig. 6.

Histogram of δS_n, the differential displacement along the probe circumference, and standard deviations in µm for the three motor types. The mean of the absolute value <|δS_n|> is also shown.

Fig. 7.

(A) Micromotor OCT catheter structural imaging of the esophagus, covering a 40 mm pullback distance. Pullback is performed rapidly to reduce physiological motion artifacts. Arrowheads denote a large vessel visible as a hypo-scattering contour in the (B) enlarged en face image. (C) Cross sectional image shows the layered appearance of esophageal mucosa and submucosa. Arrow points to a reflection from the catheter sheath. (D) High resolution micromotor OCT catheter structural imaging of swine rectum, covering a longitudinal pullback distance of 30 mm. (E) The enlarged en face region shows columnar epithelial crypts appearing as regular, ovular features. (F) The cross sectional image shows characteristic, highly-scattering vertical projection features from columnar epithelium, marked by arrowheads. Se: squamous epithelium, Lp: lamina propria, Mm: muscularis mucosa, Sm: submucosa.

Fig. 8.

(A) Micromotor probe OCT of the swine esophagus, covering a 14 mm longitudinal pullback distance. En face OCT image shows vessels (arrowheads) and esophageal mucosa with its characteristic smooth appearance. (B) Cross sectional image shows subsurface layered appearance of esophageal mucosa. (C-F) En face OCTA projections over 10 µm at four different depths (300 µm, 650 µm, 1 mm, and 1.35 mm below the surface) show depth resolved vasculature not visible in the structural en face image. (G-J) Enlarged en face OCTA images showing elaborate branching vasculature and appearance of larger vessels at deeper depths.

Fig. 9.

(A) Tethered OCT capsule imaging of the swine upper GI tract, covering a 150 mm longitudinal pullback distance, corresponding to a 5,700 mm² area. The en face image shows gastric and esophageal mucosa separated by the GEJ (black dotted line). The capsule fiducial marks used for NURD correction appear as dark, longitudinal bands spanning the pullback (arrowheads). (B-D) Enlarged regions show a structured appearance of gastric mucosa compared to smooth esophageal mucosa. (E-F) Cross-sections of swine upper GI tract at two different longitudinal positions show structural differences between gastric and esophageal tissue. Fiducials appear as bright, hyper-scattering features (arrowheads) on the capsule window (arrow). Images are vertically cropped to remove the aliased inner surface of the capsule window. GEJ: gastroesophageal junction, Se: squamous epithelium, Lp: lamina propria, Mm: muscularis mucosa, Sm: submucosa

Fig. 10.

(A) Widefield OCT en face image of the swine anorectal region, covering a 20 mm longitudinal pullback distance. The black-dotted line indicates the anorectal junction. The probe fiducial marker appears as a dark horizontal band (arrowhead). (B) En face OCTA image projected over the full axial depth. (C) Enlarged image of the squamous epithelium. (D) Enlarged image of the same region, showing anal canal microvasculature. (E) Cross sectional image spanning columnar epithelium and squamous epithelium. CE: columnar epithelium, SE: squamous epithelium.