Cross-sectional image examples of the processed OCT intensity and phase information for several selected stages of the motion contrast data processing are presented in . OCT intensity and phase information available from a single B-scan acquisition are shown in , demonstrating the retinal structure within the intensity image and a lack of any retinal information within this form of the phase image. A set of 10 sequential intensity B-scans were aligned and the linear intensity was averaged to produce the image in . The decision to perform axial alignment only on the entire B-scans as opposed to the individual A-scan realignment did not produce any significant artifacts within the averaged images. For increased B-scan acquisition times from larger retinal scans, bulk motion variations may require individual A-scan axial realignment and transverse B-scan alignment to maintain the quality of the averaged intensity image.
Fig. 2 Example images from selected stages in the motion contrast data processing for OCT data acquired across 1 mm of retina. Initial acquired OCT data presents in the form of a series of (a) intensity and (b) phase images. (c) Averaged intensity image created (more ...)
The effects of bulk motion are observed in the , an image calculated from the phase change between two successive B-scans. The phase-wrapped effects of bulk motion are apparent on the retinal areas of the image; regions without OCT intensity information from the retina appear as a random distribution of phase changes. The bulk motion is observed as vertical lines across the image which changes in value across the image based upon the variations in axial motion during acquisition. Bulk axial motion removal based on a previously detailed method significantly reduces the artifacts within the image, as shown in the example of . Through the phase variance calculation of phase changes across 10 sequential B-scans corrected for bulk motion and the application of a threshold based on the averaged intensity image, the vascular features of the retina become resolvable. Three dimensional smoothing of the data can improve this visualization by reducing specular phase noise appearing within the image while maintaining the smallest motion contrast regions observed.
Summation images produced for a retinal scan area of 1 mm x 0.5 mm at 6 degrees eccentricity are demonstrated in . The OCT data set used to create the images is densely sampled with 1000 B-scans, each containing 125 A-scans for a total acquisition time of approximately 6.6 s. Summation images summed over the entire image depth are presented in for the intensity and motion contrast data sets. Due to the significant motion contrast contribution from the choroidal region, separating the motion contrast data into summation images of the retina and choroidal depth regions significantly improves the visualization of the retinal microvasculature. The vasculature observed in retinal summation image of the motion contrast in demonstrates a very well-defined interconnected structure, with the smallest observed features having sizes on the order of the transverse imaging resolution. Choroidal vascular visualization with this motion contrast method suffers from similar limitations as intensity imaging of the choroid, where absorption of the choroidal vessels impairs visualization. Analysis of the choroidal region based on previously developed approaches [24
] may be used for development of improved choroidal vascular visualization.
Fig. 3 1 mm x 0.5 mm OCT retinal summation images. (a) Intensity summation image summed over entire image depth, analogous to fundus images. (b) Motion contrast summation image summed over the entire image depth. Motion contrast data are separated into summation (more ...)
The data presented in demonstrated the coarse depth separation of motion contrast between the retina and the choroidal regions, but finer depth separation can easily be achieved. With the depth resolution available from OCT, we are able to segment the vasculature from different depths with a resolution limited primarily by the vascular feature sizes. Within the motion contrast data set used to create the summation images in , we have observed three separate layers of microvasculature, located at depths of approximately 60 μm, 100 μm, and 150 μm below the anterior surface of the retina. These correspond to depth regions associated with the posterior part of the ganglion cell layer (GCL), the anterior region of the inner nuclear layer (INL), and the posterior region of the INL within the retina. Segmentation of these individual layers is presented in .
Fig. 4 1 mm x 0.5 mm OCT summation images, using the same data presented in . (a) Intensity and (b) motion contrast B-scans demonstrate the depth regions (yellow boxes) which are summed to create the individual vascular layer images. The motion contrast (more ...)
To optimize the visualization of each vasculature layer, a summation image was created for each layer summing over a 16 μm thick depth region centered at each depth and presented in . This is presented alongside example B-scans of the intensity and motion contrast used to register the depths used for these three separate image layers. As demonstrated in previous work [16
], artifact noise of motion contrast appears below strong flow regions and is only reduced or removed for depth regions where the intensity signal is reduced below threshold levels. For this reason, the first two anterior layers appear to have the same major vasculature and some microvasculature differences. To aid in visualizing the different retinal vascular layers, a composite image of the vasculature observed in the three retinal layers is presented in . In the motion contrast image, the average of the two vascular layers in the GCL and the anterior of the INL are presented in green and overlaid with the posterior INL layer in red. As demonstrated in , the top two vascular layers observed shows minimal spatial correlation to the vascularization in the posterior INL, confirming that motion contrast regions in this image are not artifacts from microvascularization anterior in those retinal locations. The depth sensitivity of the microvascular imaging is a feature that cannot be replicated by clinical fluorescein angiography.
Reducing the transverse sampling by a factor of two in the data set acquisition does not degrade the vascular visualization capabilities with this technique, as demonstrated in . A retinal scan area of 1 mm x 1 mm was acquired in the same acquisition time as the previously shown data set at a similar retinal location. As expected, only the major retinal vasculature is visible within the intensity summation image based upon the blood absorption. Within the retinal depth summation image of the motion contrast data, the major vasculature is observed as well as the network of motion contrast which appears to be microvasculature. To our knowledge, the level of detail observed within this retinal vasculature has not been duplicated within other phase sensitive OCT imaging techniques.
1 mm x 1 mm OCT retinal summation images. (a) Intensity summation image over the entire image depth. (b) Motion contrast summation image over the retinal depth region.
Additional noise removal from the motion contrast data sets is critical to maintain vascular visualization over the entire retinal scan area. While microsaccadic motion can be easily identified within the acquisitions, transverse eye motion caused by slower changes in fixation position also contributes to the noise observed and can be much more difficult to compensate. Fluctuations in this type of transverse eye motion during acquisition can cause a varying level of phase contrast noise across the entire scan area. Due to the relatively short acquisition time of an individual B-scan relative to the expected motion drift variations, the noise experienced due to motion drift is approximated as a constant contribution across each phase variance B-scan image. Histogram-based analysis of the motion contrast over the retinal portion of the image estimated the noise contribution, assuming that the majority of the cross-sectional image contains static retinal layers as opposed to retinal vascular regions and motion artifacts. The motion contrast summation images demonstrated in this paper have implemented this noise removal technique to maximize vascular visualization.
Applying this motion contrast imaging technique to the foveal region of the retina demonstrates the extent of the vascular imaging capabilities, since the fovea contains a significant avascular region. Within this region, there is expected to be no motion contrast within the retinal depth. contains 1 mm x 1 mm retinal data acquired over the foveal region for motion contrast calculations. The 3 mm x 3 mm intensity summation image presented in was used only for context to identify the approximate foveal region imaged and does not have any associated motion contrast information. The motion contrast observed in provides a valuable tool to visualize the avascular region boundary within the fovea. While the major retinal vasculature which can be observed within the intensity images only extends over the right half of the imaged region, the motion contrast images demonstrate a network of microvasculature extending further towards the center of the fovea. The edge of the observed microvasculature structure is associated with the initial thinning of the retina within the OCT images at the fovea.
Fig. 6 OCT summation images over the foveal region of the retina. (a) 3mm x 3mm intensity summation image identifying approximate location of 1mm x 1mm scan area. (b) 1 mm x 1 mm intensity summation image. Motion contrast retinal summation images (c) before (more ...)
demonstrates the retinal motion contrast summation image before the additional noise removal. Horizontal black lines are associated with contrast data which was eliminated from the image due to thresholds imposed on the processing of the motion contrast data. Each of these lines is associated with a microsaccade occurring during the image acquisition. The horizontal lines on the image which appear to have high contrast have been identified as motion contrast artifacts which have not been compensated with bulk axial motion noise removal and thresholding. The motion contrast image after additional noise processing removes almost all of these artifacts and improves vascular visualization across the entire image.
Improvements to the demonstrated motion contrast technique focus around increasing the retinal scan area while maintaining or improving vascular visualization capabilities. Due to the fixation limitations of typical subjects, acquisition times should remain within a few seconds or retinal tracking is required. This contrast method can be adapted to faster OCT systems without significant change in vascular visualization, allowing for larger retinal scan areas in the same acquisition time. Further improvements to increasing the scan area can be achieved through stitching together multiple data acquisitions guided by retinal tracking information or retinal features [26
presents the comparison of the motion contrast images with a fluorescein angiography image of the same subject cropped over a selected region of interest. A retinal area of the fluorescein angiography image of approximately 2 mm x 2 mm was chosen which includes the OCT data scan regions corresponding to the data presented in and . and were overlaid over the fluorescein angiography image based on the approximate scan location. Distortions caused by transverse motion during the acquisitions were not compensated for in this comparison, which accounts for position discrepancies of the major vascular features between the images. Comparison of microvascular imaging between the OCT motion contrast images and the fluorescein angiography image shows comparable vascular imaging capabilities, with similar feature sizes and spacing between vasculature within the images. Direct comparison between specific microvascular events is difficult due to distortions in the OCT image which limit the accuracy of the vascular location, and by the contrast limitations within regions of the angiography image but the visualization capabilities appear to be analogous.
2 mm x 2mm region of a fluorescein angiography image. OCT motion contrast images presented in and are overlaid over the approximate scan locations (right)
Detailed analysis in comparing the microvascular visualization capabilities between fluorescein angiography and the demonstrated phase variance motion contrast technique requires an accurate mapping of retinal locations, currently limited by transverse motion during data acquisition. Retinal tracking and faster data acquisitions are both options available to reduce the effect of these motions and improve mapping of the vasculature. The demonstrated motion contrast images in this paper have shown resolution-limited features independent of vascular orientation or flow velocity, and with improvements to the retinal scan area while maintaining this visualization, this technique is expected to continue to produce comparable vascular visualizations to fluorescein angiography over larger retinal scan areas.