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Fast 3D in vivo swept-source optical coherence tomography using a two-axis MEMS scanning

micromirror

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2008 J. Opt. A: Pure Appl. Opt. 10 044013

(http://iopscience.iop.org/1464-4258/10/4/044013)

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J. Opt. A: Pure Appl. Opt. 10 (2008) 044013 (7pp) doi:10.1088/1464-4258/10/4/044013

Fast 3D in vivo swept-source opticalcoherence tomography using a two-axisMEMS scanning micromirrorKarthik Kumar1, Jonathan C Condit2, Austin McElroy3,Nate J Kemp3, Kazunori Hoshino2, Thomas E Milner2 andXiaojing Zhang2

1 Department of Electrical and Computer Engineering, University of Texas, Austin, TX 78712,USA2 Department of Biomedical Engineering, University of Texas, Austin, TX 78712, USA3 CardioSpectra, Incorporated, San Antonio, TX 78229, USA

E-mail: john.zhang@engr.utexas.edu

Received 3 October 2007, accepted for publication 8 November 2007Published 3 April 2008Online at stacks.iop.org/JOptA/10/044013

AbstractWe report on a fibre-based forward-imaging swept-source optical coherence tomography systemusing a high-reflectivity two-axis microelectromechanical scanning mirror for high-speed 3D invivo visualization of cellular-scale architecture of biological specimens. The scanningmicromirrors, based on electrostatic staggered vertical comb drive actuators, can provide ±9◦of optical deflection on both rotation axes and uniform reflectivity of greater than 90% over therange of imaging wavelengths (1260–1360 nm), allowing for imaging turbid samples with goodsignal-to-noise ratio. The wavelength-swept laser, scanning over 100 nm spectrum at 20 kHzrate, enables fast image acquisition at 10.2 million voxels s−1 (for 3D imaging) or 40 frames s−1

(for 2D imaging with 500 transverse pixels per image) with 8.6 μm axial resolution. Lateralresolution of 12.5 μm over 3 mm field of view in each lateral direction is obtained usingZEMAX optical simulations for the lateral beam scanning system across the scanning anglerange of the 500 μm × 700 μm micromirror. We successfully acquired en face and tomographicimages of rigid structures (scanning micromirror), in vitro biological samples (onion peels andpickle slices) and in vivo images of human epidermis over 2 × 1 × 4 mm3 imaging volume inreal time at faster-than-video 2D frame rates. The results indicate that our system frameworkmay be suitable for image-guided minimally invasive examination of various diseased tissues.

Keywords: optical coherence tomography (OCT), swept-source, MEMS, scanning micromirror,high-speed 3D imaging

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Optical coherence tomography (OCT) has emerged as a high-resolution biomedical diagnostic imaging technique of choicein situations where biopsy is difficult, for image-guided mi-crosurgery, and for three-dimensional reconstruction of pathol-ogy [1]. OCT has been shown to discriminate architecturallayers and identify morphological changes in highly scatteringtissue. High-resolution 3D imaging can enhance a physician’s

understanding of pathology by providing tomographic, micro-scopic and en face views simultaneously [2]. Miniaturizationof optical diagnostic equipment for clinical implementation ne-cessitates alternative beam-deflection mechanisms to galvano-metric scanning. Proximal scanning mechanisms have been ex-tensively investigated [3–7] wherein a single-mode fibre fusedto a focusing microlens and prism is either translated along thelength of the probe [3], or rotated about the probe axis [4, 6],or operated in a helical scan mode that is a superposition of

1464-4258/08/044013+07$30.00 © 2008 IOP Publishing Ltd Printed in the UK1

J. Opt. A: Pure Appl. Opt. 10 (2008) 044013 K Kumar et al

Figure 1. High-reflectivity two-axis vertical comb-drive scanning micromirrors. (a) Process flow for device fabrication. (b) Scanning electronmicrographs (SEM) of scanning micromirrors depicting mirror and frame design, torsion springs, stator and rotor combs, bond pads andbackside DRIE release window.

these two scans [5]. Proximal scanning systems, besides suf-fering from slow scan rates, poor precision and repeatability,are only useful in sideways-imaging probes for visualizationin tubular structures such as the gastrointestinal tract and oe-sophagus. 3D OCT probes require miniaturized distal scanningmechanisms and advancements in catheter design to image innarrow, sensitive and non-tubular human organs. Forward-imaging configurations are well suited to this task, but are sig-nificantly harder to assemble in compact form factor as com-pared with the sideways-imaging probes [7]. Distal scanningcan be performed by scanning the fibre, objective lens, andby beam steering using paired angle-polished rotating GRINlenses or scanning micromirrors. Fibre and objective scanningcan be performed using piezoelectric actuators [8] and tuningforks [9]. However, these suffer from limited field of view andslow scan rates, respectively. Microelectromechanical system(MEMS) technologies offer the unique capability to packagemicro-optical elements with actuators on a chip scale for imag-ing in in vivo environments. Previous studies have utilized one-dimensional [6, 10–13] and two-dimensional [14, 15] distal-end MEMS scanning systems to perform time- and spectral-domain OCT. MEMS elements have also been used for dy-namic beam focus control in several OCT modalities [16–18]and can enable compact frequency-tuneable lasers [19, 20] forswept-source OCT. Angular [21] and staggered [22–24] ver-tical comb-driven micromirrors, a subset of MEMS scanningsystems, are useful for building forward-imaging probes, pro-viding large actuation torque and scanning angles with smoothoptical surfaces and low dynamic mirror deformation due tohigh mirror layer thickness. Such mirrors have previously beenused in confocal microscopy [25], two-photon microscopy [26]and time-domain optical coherence tomography systems [14].

In this work, we present high-speed 3D swept-sourceoptical coherence tomography employing a high-reflectivity

two-axis vertical comb-drive scanning micromirror that canbe configured into a compact forward-imaging probe. Thescanning micromirror offers the advantages of silver-coatedreflecting surface, two-axis rotation about a single in-planepivot point, and high-speed arbitrary scan patterns for en faceimaging. Spectral domain (i.e. Fourier-domain/swept-source)techniques have been shown to provide significant signal-to-noise ratio and imaging speed advantages over time-domainOCT [27, 28]. Swept-source OCT (SS-OCT), in particular,enables real-time micron-resolution diagnostic imaging overmillimetre depth range due to improvements in spectral range,linewidth and scan speed of swept wavelength lasers. High-speed 3D imaging over 2 × 1 × 4 mm3 volume at ∼12.5 μmlateral and ∼8.6 μm axial resolutions is demonstrated usingthe system. Tomographic, en face and cross-sectional slicesthrough 3D volumetric images of both in vitro and in vivosamples acquired at over 10 million voxels per second arepresented.

2. Experimental methods

2.1. Two-axis MEMS scanning micromirror

We employed a staggered vertical comb-driven two-axisscanning micromirror for distal beam scanning. Staggeredvertical comb drives combine the advantages of large scanningangles, high electrostatic actuating torque, favourable voltagepull-in characteristics, low mirror dynamic deformation underresonant operation and optically smooth mirror surfaces. Themirror is fabricated by a five-mask process employing double-bonded SOI wafers and self-aligned deep reactive ion etching(DRIE) steps. Figure 1(a) outlines the silicon micromachiningprocess used to fabricate the scanning mirrors. Coarse featuresof stationary electrostatic actuating comb drives are etched bydeep reactive ion etching (DRIE) into the 30 μm conductive

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J. Opt. A: Pure Appl. Opt. 10 (2008) 044013 K Kumar et al

device layer of a silicon-on-insulator (SOI) wafer. A separatesilicon wafer is oxidized to form an electrical isolation layer,fusion-bonded on top of the SOI wafer, ground down to 20 μmthickness, and polished to give a smooth (less than 50 nm RMSsurface roughness) optical surface. The mirror and rotor combsare fabricated in this layer. A layer of 1.5 μm thick silicondioxide, serving as a silicon etch hard mask, is deposited bylow pressure chemical vapor deposition (LPCVD). After dryetching through the top device layer to expose alignment marksin the lower silicon layer, electrical bond pad features areetched partially (1.2 μm) and the exact features of the scanningmicromirrors are etched through the oxide layer by reactiveion etching (RIE). The oxide layer is used as an etch maskto perform a DRIE–oxide RIE–DRIE sequence to create rotorcomb features, bond pad vias to the lower silicon layer, andtrim the underlying stator combs to match the lateral position ofthe stator comb features. This self-aligning technique [22, 23]affords a critical mask alignment tolerance of half the combwidth (3.0 or 3.5 μm in our case) for stable and reliable scanneroperation. A backside window is DRIE-etched beneath thedevice to release the micromirror, while simultaneously dicingthe wafer into device chips. The remaining exposed oxide isremoved from the front and backside by RIE. A 〈100〉 siliconwafer is anisotropically wet etched in potassium hydroxideusing a silicon nitride masking layer at 70 ◦C to create a hardmask for sputtering. Using this hard mask, the micromirrorsare selectively coated with metal by electron beam evaporation.The micromirror has design dimensions of 500 μm × 700 μmfor compact oblique (up to 45◦) illumination with a 500 μmdiameter laser spot, and is coated with 125 nm of silver,resulting in greater than 90% uniform reflectance over thesource spectrum. The silver films showed high reflectivityrange during our experimentation period of approximately twoweeks. However, we observed blackening of the films at anextended period of time after the experiments, and reflectivitydropped to <30% at 1310 nm. In future experiments, we planto incorporate gold instead of silver coatings that are inert andmore suitable for long-term use. An important advantage ofthis scanner design is that, in a single plane, the mirror issuspended within a frame by the inner torsion springs and theframe is suspended by the outer torsion springs aligned in theorthogonal direction. This enables two-dimensional rotationabout a single pivot point, reducing optical field distortions.Scanning electron micrographs of the device are shown infigure 1(b).

The micromirror exhibits resonance on the inner and outeraxes at 2.28 kHz and 385 Hz, respectively, and ±9◦ opticaldeflection on both axes for applied voltages of 110 V at lowfrequencies (figure 2). Secondary peaks are also observed athalf and twice the actual resonant frequency of the vibrationmodeshape, as expected [30]. Due to the highly capacitivenature of the electrostatic actuators, the micromirror actuationrequires very low current (usually of the order of 1–10 nA) foroperation in non-resonant mode and around 2.4 μA in resonantmode on either axis.

2.2. Swept-source OCT instrument

We incorporated our system on a 12′′ × 12′′ miniature opticalbreadboard in a forward-imaging configuration for our distal

Figure 2. Scanning micromirror operating characteristics.(a) Optical deflection characteristics for low-frequency voltageapplied to a single vertical comb drive on each rotation axis.(b) Frequency-dependent optical deflection characteristics. Voltageapplied to one vertical comb drive on each rotation axis isV = 18.0 + 9.0 sin(2π f t) V.

beam-scanning mechanism that can be further miniaturizedinto an OCT endoscope suitable for imaging in non-tubularorgans of the human body. Figure 3 illustrates the principleof operation of swept-source OCT with MEMS scanningmicromirrors. Fibre-based instrument layout and swept-sourceconfiguration similar to that used in [31] is adopted due tothe high sensitivity demonstrated by the approach, ease ofalignment, portability and manoeuvrability after incorporationof the scanning micromirror and optical elements into aminiaturized distal probe. The sample and reference armsare designed to have optical path lengths matched to withina few tens of micrometres. Illumination from the swept-wavelength laser is split equally into the sample and referencearms by the 2 × 2 fibre coupler. The phase differencebetween sample and reference arm reflections as a function ofoptical frequency is measured over the 100 nm spectral rangeby sampling the interference signal between the reflectionsfrom the two arms through the infrared photodetector as theillumination wavelength is varied. The Fourier transformof this spectral interference signal provides a map of thereflectivity profile of the sample as a function of depth [32].The two-axis MEMS scanning micromirror moves the beamspot laterally in two dimensions (in raster fashion) and theaxial reflectivity profiling is performed at each point todevelop a 3D image of the sample volume. Optical designsimulations in ZEMAX software indicate that the Steinheiltriplet lenses (JML Optical, TRP14340/100, 0.6 NA, 7.9 mm

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J. Opt. A: Pure Appl. Opt. 10 (2008) 044013 K Kumar et al

Figure 3. MEMS scanning micromirror based fast 3D in vivoswept-source optical coherence tomography. (a) Principle of 3DSS-OCT incorporating a high-reflectivity two-axis MEMS scanningmicromirror in forward-imaging configuration. (b) Opticalcharacteristics of the lateral beam-scanning system—ZEMAXsimulation. A 3 mm linear scan of ∼12.5 μm focused spot isobtained when the micromirror provides ±10◦ of optical deflectionto the 500 μm diameter laser spot.

EFL) provide aberration-free focus of the 500 μm diametercollimated broad-spectrum illumination, resulting in a beamspot size of approximately 12.5 μm on the sample, and a lateralscan range of approximately 3 mm when the micromirror,positioned at the back focal plane of the lens, produces anoptical deflection of ±10◦. The laser performs 20 000 sweepsper second over 100 nm spectral range centred at 1310 nm,theoretically providing λ2

c/2�λ = 8.6 μm axial resolutionand 4 mm imaging depth. In practice, the imaging depth ofoptical coherence tomography is limited to 1.5–2 mm due todegradation of the signal-to-noise ratio by light scattering inturbid samples.

3. Imaging results and discussion

To demonstrate the capabilities of our scanning micromirrorbased swept-source OCT system, we acquired reflectivitydata over a 2 × 1 × 4 mm3 volume of rigid structures, invitro biological samples and in vivo human epidermis. Thesystem acquisition rate of over 10 million volume pixelsper second results in completion of one entire volume scanin approximately 15 s, representing an order-of-magnitudeimprovement in acquisition rate over time-domain opticalcoherence tomographic techniques demonstrated previously.Results of 3D imaging of rigid structures, namely one ofour packaged scanning micromirrors without metal reflectivitycoating, are presented in figure 4. The information presentin the entire 3D volume can be represented in a 2D en face

Figure 4. Results of 3D swept-source OCT imaging of our scanningmicromirror. (a) En face view of the scanning micromirror, depictingmirror surface, torsion rods, bond pads, electrical bonding wires andsubsurface inner stator comb features. Field of view is2 mm × 1 mm. (b) Tomographic image of the scanning micromirrorobtained at the location corresponding to the blue rectangle in (a).(c) Slice images at different planes through the micromirror device,demonstrating volume image acquisition capability.

Figure 5. Tomographic 2D images of in vitro biological samples.Lateral scan width is 2 mm. (a) Single frame from 40 fps video ofpickle slice obtained using the scanning micromirror. (b) Singleframe of 40 fps video of pickle slice from a different location using atraditional galvanometric scanner. (c) Still frame from 40 fps videoof an onion peel using the scanning micromirror.

view by integrating the reflectivity along the axial direction.The micrometre-scale features, such as micromirror torsion

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Figure 6. Results of 3D in vivo imaging of human skin using our scanning micromirror-based swept-source OCT system. (a) Still frame invideo acquired at ∼8 million volume pixels per second. Micrometre-scale subsurface tissue architecture including skin surface, finger ridges,sweat gland grants, stratum corneum (sc), epidermis and dermis are clearly distinguished. (b) Tomographic slices of 3D volumetric imagedata obtained from human skin. Each slice has a lateral extent of 1 mm (1 mm/100 pixels). The variation in epidermis thickness is visiblefrom the parallel slices across different locations of the skin.

springs, electrical bond pads and a 20 μm diameter bondingwire connecting to the top left bond pad, are clearly visiblefrom the en face image in figure 4(a), indicating that thesystem achieves the micrometre resolution predicted by ourZEMAX simulations. The subsurface stator combs for theinner rotation axis are also visible in the image, as evidencedby comparison with the scanning electron micrograph infigure 1(c). Figure 4(c) depicts tomographic cross-sectionalslices across different positions of the micromirror, revealingthe internal structural details of the device. The vertical combdrives typically cause large amounts of laser light scattering,leading to the grainy portions in the image, similar to specklenoise. The mirror section of the device is dark as it scatters verylittle of the incident light due to high transmission of silicon atinfrared wavelengths.

We acquired tomographic images of in vitro biologicalsamples at 40 frames s−1 by operating the scanningmicromirror about only one axis of rotation. Figure 5presents tomographic images of pickle slices obtained usingthe scanning micromirror and a traditional galvanometricscanner from different regions of the sample, and an onion peelusing the micromirror. Subsurface morphology is visible in allthe images.

Finally, we obtained real-time 3D images of in vivo humanfinger skin using our system. Tomographic slices through theimaged volume are shown in figure 6. The micron-scale tissuearchitecture including skin surface, finger ridges, sweat glands,stratum corneum, epidermis and dermis are clearly visible inthe slice images. In some of the images, lens flare artefacts arevisible, but these can be repositioned away from the imagingregion of interest by varying the path length difference betweenthe sample surface and reference reflection.

The lateral and axial resolutions of the instrument aregoverned by independent factors. The axial resolutionis inversely proportional to the spectral bandwidth of theswept-frequency laser. The lateral resolution is determinedpurely by the micromirror and scanning optics. The

diameter of the scanning micromirror limits the maximumbeam diameter incident on the objective lens, and thereforedetermines the effective numerical aperture of the focusinglens. The number of resolvable points of our system can beimproved by increasing the product of the mirror diameter–scanning angle product, which is then transformed into agiven lateral field of view and resolution, depending onthe numerical aperture of the objective [33], which can beselected according to the requirements of the application.In our experiments, some instability in lateral scanning wasobserved, which can be countered by incorporation of angularposition feedback sensors [34] on the scanning micromirrorchip for adaptive control of scan linearity. Miniaturizationof the MEMS scanning optics with the use of fibre-fusedgraded index (GRIN) lens collimators, stationary micro-prisms and monolithic electronics integration such as flip-chip bonding for power supply and signal conditioning willenable possible clinical application of the instrument forapplications such as gastroenterology, urinary/reproductivetract and pulmonary imaging that allow catheter diameters of5 mm [7]. Real-time in vivo volume image acquisition ofsubsurface morphology at micrometre resolution may enableapplication to minimally invasive disease diagnostics, image-guided biopsy and photodynamic therapy.

4. Conclusion

High-speed three-dimensional swept-source optical coherencetomography is demonstrated using a forward-imaging distalbeam-scanning configuration incorporating a high-reflectivitytwo-axis scanning micromirror, which can be miniaturized intoa catheter for in vivo imaging. Tomographic slice imagesand en face views of an imaged 2 × 1 × 4 mm3 volumewere acquired with estimated lateral and axial resolutions of12.5 μm and 8.6 μm, respectively. The imaging capabilityof the instrument was tested on rigid structures, in vitrobiological samples and in vivo imaging of human finger skin.

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The imaging results indicate that the instrument is capableof micrometre-resolution imaging of subsurface morphologyin turbid media. Higher-than-video-rate (40 frames s−1)acquisition of 2D images and 3D imaging at over 10 millionvolume pixels per second in such samples were successfullydemonstrated.

Acknowledgments

We gratefully acknowledge financial support for this researchfrom the Wallace H Coulter Foundation Early Career Award2006–08, and grants from the National Institutes of Health(grant no. R01 EY 016462) and Veterans Administration.The scanning micromirrors were fabricated at StanfordNanofabrication Facility (supported by National ScienceFoundation grant no. 9731293) and University of Texas–AustinMicroelectronics Research Center (supported by NationalScience Foundation grant no. 0335765) under the NationalNanofabrication Infrastructure Network.

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