September 15, 2004 / Vol. 29, No. 18 / OPTICS LETTERS 2139

Tracking optical coherence tomography

R. Daniel Ferguson and Daniel X. Hammer

Physical Sciences, Inc., 20 New England Business Center, Andover, Massachusetts 01810

Lelia Adelina Paunescu, Siobahn Beaton,* and Joel S. Schuman*

New England Eye Center, New England Medical Center, 750 Washington Street, Box 450, Boston, Massachusetts 02111

Received December 18, 2003

An experimental tracking optical coherence tomography (OCT) system has been clinically tested. The pro-totype instrument uses a secondary sensing beam and steering mirrors to compensate for eye motion witha closed-loop bandwidth of 1 kHz and tracking accuracy, to within less than the OCT beam diameter. Theretinal tracker improved image registration accuracy to ,1 transverse pixel �,60 mm�. Composite OCT im-ages averaged over multiple scans and visits show a sharp fine structure limited only by transverse pixel size.As the resolution of clinical OCT systems improves, the capability to reproducibly map complex structuresin the living eye at high resolution will lead to improved understanding of disease processes and improvedsensitivity and specificity of diagnostic procedures. © 2004 Optical Society of America

OCIS codes: 170.3880, 170.4500, 170.4470, 170.1610.

Optical coherence tomography (OCT) has emerged overthe past decade as an outstanding tool for imagingbiological structures. Ultrahigh-resolution, Doppler,spectroscopic, polarization-sensitive, parallel (spectraldomain), and three-dimensional (3-D) OCT imagingmodes, among others, have been demonstrated bynumerous researchers.1 –5 These tools have foundimportant applications in quantitative retinal imag-ing and detection and characterization of retinalpathology. Clinical OCT instruments have beenshown to produce highly accurate maps of the retinalnerve fiber layer.6 We have modified a commercialclinical OCT system by integrating a prototype of anactive, hardware-based retinal tracker for OCT imagestabilization. The new experimental system, calledtracking optical coherence tomography (TOCT), uses asecondary sensing beam and steering mirrors to com-pensate for eye motion7 with a closed-loop bandwidthof up to 1 kHz.

Involuntary eye movements during image acquisi-tion, and particularly between scans, currently limitthe multi-image acquisition potential of OCT and thusits feasibility for several advanced clinical applica-tions. To build an undistorted image voxel-by-voxelor co-add multiple images correctly, the absolutecoordinates of each voxel relative to a fixed retinalreference point must be known. Even in patients thatcan fixate well (hold a precise gaze angle), eye-motionamplitude may be several hundred micrometers atthe retina, much larger than the resolution of second-and third-generation OCT instruments [Carl ZeissMeditec, Inc. (CZMI)]. High-speed spectral-domainoptical coherence tomography (SDOCT) systems cancapture images with A-scan rates of tens of kilo-hertz.5 However, even for these systems, retinaltracking may still be useful to lock and return to thesame retinal position from visit to visit, to averagesets of images accurately, and to generate 3-D retinalmaps that will still require many seconds to acquire.As imaged volume and resolution increase, a meansof retinal tracking is increasingly important. Retinalposition tracking permits extended scan periods and

0146-9592/04/182139-03$15.00/0

frame averaging of multiple scans to reduce OCTspeckle with minimal effect on transverse or longi-tudinal resolution. Stabilized two-dimensional (2-D)scans can be concatenated into motion artifact-free3-D image cubes. Because the retinal tracker usesphysical landmarks, precisely aligned scans can becompared months or years apart. Retinal morphologycan thus be accurately recorded and monitored overtime to detect subtle changes. Such capabilities mayprovide greater diagnostic sensitivity and specif icityfor glaucomatous changes in the eye.

The prototype retinal tracking instrument il-lustrated in Fig. 1, employed in the clinical testsdescribed below, projects a low-power tracking beam(,25 mW at 860 nm) onto small retinal features ofmodest light or dark contrast relative to neighboringareas. Such features may include blood vessel junc-tions, the lamina cribrosa (a bright region within theoptical nerve head), retinal lesions, drusen, scars,or irregular pigmentation. The tracking beam iscircularly dithered at 8 kHz and an amplitude of150 mm (0.5±) on the retina with a pair of resonantscanners, detected with a confocal ref lectometer,and processed with a dual-phase lock-in amplifier toproduce directional error signals. A split aperture isused in the confocal ref lectometer to minimize inter-ference from corneal ref lections. The tracking beamis directed onto the retina by a galvanometer-driventracking mirror pair. Real-time digital signal pro-cessing extracts position and orientation informationfrom optical error signals to make precise closed-loopbeam-steering corrections much faster than the high-est possible image-framing rate. Because the OCTscans created with the galvanometer pair are alsoref lected from the tracking mirrors, the OCT beamautomatically follows any movement of the retina withthe same speed and precision as the tracking beam.There was no discernible effect of tracking-inducedfrequency shifts on OCT images. The rms trackingerror of ,0.05± ��15 mm� is typically smaller than theOCT beam diameter ��20 mm�. After this trackingtechnology was integrated into a clinical OCT II

© 2004 Optical Society of America

2140 OPTICS LETTERS / Vol. 29, No. 18 / September 15, 2004

Fig. 1. Optical schematic of TOCT. The 3-D layout isf lattened to two dimensions for clarity. R0 and P0 indi-cate the retinal and pupil planes of the eye, respetively;R1 and P1 are optically conjugate to those planes. OLand SL are the ophthalmoscopic and scan lenses; RT,retinal tracker; CR, confocal ref lectometer; D1, D2,dichroic beam splitters; L1–L3, fiber collimating lenses;TG, tracking galvanometer-driven mirrors; OCT G, OCTbeam galvanometer-driven mirrors; DS, dither resonantscanners; S1, split aperture; NIR, near-infrared.

(CZMI) system, precise eye-motion correction wasdemonstrated within and between 2-D OCT scans inboth normal subjects and glaucoma patients.

The clinical tests consisted of a series of OCT scansperformed on three separate visits of more than 20 sub-jects with normal or glaucomatous eyes. The scanswere collected both with and without retinal track-ing. The tracking feature selected for all scans wasthe lamina cribrosa, which is generally a stable, robusttracking feature. The sequence of OCT scans usedin this protocol for each visit includes peripapillaryscans (nominal angular diameter of 12± and a trans-verse resolution of 110 mm) and sequences of radialscan through the disk and the macula (six scans suc-cessively rotated by 60± with a nominal scan length of20±and transverse resolution of 60 mm). The peripap-illary scan is generally used to measure retinal nervefiber layer thickness around the optic disk to screenor monitor subjects with glaucoma. The radial scan isused to generate a map of a large region of the retinafor measurement of retinal nerve f iber layer or reti-nal thickness. OCT scans were processed and ana-lyzed with custom software designed to register andco-add multiple images within and between visits andto extract quantitative information on the location ofretinal features with sharp edges (e.g., optic disk cup).In the co-added OCT scans, the qualitative criteriafor improvement as a result of eye-motion stabiliza-tion were the ability to resolve blood vessel lumen, thenumber and width of blood vessel shadow edges, andthe clarity of clearly identifiable features such as dis-tinct retinal and subretinal layers. Quantification oftracking performance compared with fixation by mea-

surement of transverse image registration was limitedin this study by the OCT transverse pixel resolution.The typical amplitude of eye motion for a good fixa-tor, including involuntary microsaccades and drift, is�0.5± ��150 mm�, or approximately 1.4 and 2.5 pix-els for peripapillary and radial OCT II scans, respec-tively. The accuracy of a similar tracking system inother higher resolution imaging implementations8 wasfound to be ,0.05± ��15 mm�. In the clinical trials aqualitative improvement in OCT II peripapillary scanswas found in 97% of all subject visits. Analysis of themean variation in disk edge position in OCT II radialdisk scans found an improvement in every subject.

Figure 2 shows average OCT peripapillary imagesco-added from three successive visits to the clinic (�60total scans) with and without tracking. Note theabsence of speckle in the averaged OCT images. Themeasured signal-to-noise ratio for both Figs. 2b and 2cis 5.6 times greater than that of a single OCT imagefrom the same set, implying signif icant decorrelationof speckle between images. Horizontal retinal layersare generally less affected because the shallow gradi-ents of these layers, even in subjects with glaucoma,render the sharpness of such boundaries (once theyare vertically aligned) less sensitive to transversemotion parallel to the layers. Nevertheless, thesharpness of the retinal layers is clearly improved,as presumably is the precision of the delineation ofthese boundaries with automated analysis. It is inthe appearance of nonlayered fine structures thatthe advantage of tracking is immediately apparentin the co-added cross-sectional images. Blood vesselshadows (caused by the relatively high light extinctionby blood at these wavelengths) show good contrastand are precisely aligned. Details of the vessel crosssection, including lumen, emerge with tracking. Fine

Fig. 2. Composite peripapillary OCT images co-addedfrom three visits of 20 scans each: a, fundus image(single frame); b, with tracking; c, without tracking.S, superior; T, temporal; I, inferior; N, nasal. The scalebar is 1.0 mm. The small white circles in a and thearrows in b indicate the location of blood vessels.

September 15, 2004 / Vol. 29, No. 18 / OPTICS LETTERS 2141

Fig. 3. Radial disk OCT scans from one subject on threesuccessive visits: a– i, scans taken with tracking; j, com-posite image created from scans a–i; k–s, scans takenwithout tracking. t, composite image created from scansk–s. The vertical lines indicate the FWHM edge of thedisk cup. V, visit number; R, run number. The scale baris 0.5 mm.

structure within the choroid is evident in the trackingaverage. The limitation in image detail is determinedby the coarse pixelation of the OCT II system ratherthan by motional broadening.

Figure 3 shows a set of three single OCT radialdisk scans for one orientation (of six) acquired onthree successive visits with (left column) and withouttracking (right column). The average OCT image isshown in the f inal frame of each column (Figs. 3j and3t). With tracking, the position and morphology ofthe disk and cup appear to be quite stable across manyframes. Without tracking, not only is the in-planetransverse alignment and distortion problematic butalso out-of-plane motion (normal to the image plane)increases the probability that a different section ofthe disk is scanned. These motions are uncorrectable

by postprocessing and further distort the image whenwe are executing the vertical alignment softwareby coupling transverse alignment error into verticaldistortion. Without tracking, the average, althoughsmoothed, is without diagnostic significance. Themean variance in disk cup edge position for theindividual visits for this subject, a good fixator, was0.35 pixels with tracking and 1.22 pixels withouttracking. One of the significant improvements ex-pected from TOCT is the improved ability to measurethe same retinal location every time a patient is exam-ined to monitor longitudinal progression of a disease ordefect. To test the hypothesis that tracking improvedthe ability to return to a f ixed retinal position, weexamined the variance in disk cup edge position formultiple visits and found it to increase to 0.45 pixelswith tracking and 3.76 pixels without tracking.

In the third-generation clinical OCT system�Stratus OCT � system, there is significant improve-ment in both transverse and longitudinal OCT imageresolution. With the additional capabilities providedby TOCT, retinal boundaries can become even betterdefined by averaging because high image spatialfrequencies are preserved. Direct, high-resolution2-D and 3-D imaging of the disk and other retinalstructures and pathologies with steeper vertical fea-tures will benef it even more dramatically from retinaltracking.

This work was funded by the National Institutes ofHealth (National Eye Institute) under grants EY13036and EY13178. Both Physical Sciences, Inc., and CZMIprovided partial support for the clinical tests. R. D.Ferguson’s e-mail address is ferguson@psicorp.com.

*Present address, The Eye and Ear Institute, Suite816, University of Pittsburgh School of Medicine,203 Lothrop Street, Pittsburgh, Pennsylvania 15213.

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