Many ophthalmologists use optical coherence tomography (OCT) to improve diagnosis of retinal diseases. OCT generates cross-sectional and 3D images of retinal structure by detecting the magnitude and echo time delay of light. The newest version of OCT, spectral domain OCT (SD-OCT) [1
], detects all the light echoes in a single axial scan (A-scan) in parallel. Cross-sectional images (B-scans) are created by acquiring a series of A-scans as the OCT beam is scanned in the transverse direction. SD-OCT can image the three-dimensional retinal structure with an axial resolution of up to ≈2µm and an imaging speed of up to 50,000 A-scans per second.
An important application of retinal SD-OCT is the early diagnosis of glaucoma, a progressive blinding disease. Early intervention has been shown to slow progression [2
], but early detection of the disease is challenging. The precise structural measurements available from SD-OCT volumes (such as retinal nerve fiber layer thickness maps [3
]) can be used to detect structural changes before they result in noticeable vision loss.
A typical SD-OCT scan pattern contains 40,000 A-scans and takes between 1 and 1.5 seconds to complete. Any eye movement during imaging will corrupt the data, making structural measurements and glaucoma diagnosis unreliable. Eye movement during imaging is reduced by asking the subject to fixate on a target; however, our eyes remain in motion even during conscious fixation, undergoing fixational eye movements
]. Fixational eye movements encompass three different types of motion: ocular tremor, ocular drift, and microsaccades. Microsaccades, which introduce the most severe motion artifacts, move the fixated point around 30 arcminutes almost instantaneously and occur roughly once per second in normal individuals.
shows a two-dimensional projection of an SD-OCT volume containing motion artifacts. Here, the axial dimension has been collapsed by summing voxel intensities, resulting in a view of the eye (the en face image) analogous to a traditional fundus image or other 2D imaging techniques. Note the discontinuity in the vessels pointed out in the bottom half of the image. These discontinuities indicate that microsaccades occurred during scan acquisition.
Fig. 1 Results of motion artifact correction on an example scan. The en face image from the uncorrected scan (a) shows vessel discontinuities (arrow) resulting from microsaccades. Correcting for tremor and drift (b) fails to correct these discontinuities, but (more ...)
Coupling the imager to a tracker [5
] results in an improvement in the OCT data, but with high production costs and increased scanning time.
We take a complementary approach to eliminating transverse motion artifacts: we correct motion artifacts after imaging is complete via image registration. Immediately after capturing the SD-OCT volume, the OCT system captures an scanning laser ophthalmoscopy (SLO) image of the retina (). Because capture of the SLO image is virtually instantaneous relative to time required for OCT capture, we can treat the SLO image as an artifact-free reference image. The OCT en face image should match the SLO image if the scan is also artifact-free. Any structural misalignment indicates that the OCT volume contains motion artifacts and does not accurately reflect the subject’s anatomy. We remove these artifacts by warping the volume to make the corresponding en face image match the SLO image as closely as possible.
Prior work on correcting motion artifacts in OCT volumes without requiring active tracking registers consecutive B-scans by finding the shifts in the Z
– and X
–dimensions that maximize cross-correlation between adjacent B-scans [6
], correcting transverse and axial artifacts resulting from ocular drift. While our method does not correct axial artifacts, we surpass this method in correcting transverse artifacts by allowing for more complex motions, including fast motion during a single B-scan and motion in the Y