Regression-based algorithm for bulk motion subtraction in optical coherence tomography angiography

Abstract

Abstract:We developed an algorithm to remove decorrelation noise due to bulk motion in optical coherence tomography angiography (OCTA) of the posterior eye. In this algorithm, OCTA B-frames were divided into segments within which the bulk motion velocity could be assumed to be constant. This velocity was recovered using linear regression of decorrelation versus the logarithm of reflectance in axial lines (A-lines) identified as bulk tissue by percentile analysis. The fitting parameters were used to calculate a reflectance-adjusted upper bound threshold for bulk motion decorrelation. Below this threshold, voxels are identified as non-flow tissue, their flow values are set to zeros. Above this threshold, the voxels are identified as flow voxels and bulk motion velocity is subtracted from each using a nonlinear decorrelation-velocity relationship previously established in laboratory flow phantoms. Compared to the simpler median-subtraction method, the regression-based bulk motion subtraction improved angiogram signal-to-noise ratio, contrast, vessel density repeatability, and bulk motion noise cleanup in the foveal avascular zone, while preserving the connectivity of the vascular networks in the angiogram.

Enhanced Quantification of Retinal Perfusion by Improved Discrimination of Blood Flow From Bulk Motion Signal in OCTA

“…1 ). 5 The regression analysis yields a reflectance-adjusted range for bulk-motion–related decorrelation value; voxels within this range are identified as nonvascular and their flow signal values were set to zero. Voxels with decorrelation values above the bulk-motion threshold were identified as vascular voxels.…”

“…The positions of large vessels were detected using their large decorrelation values in the OCTA data and the shadows projected onto the outer retina in the reflectance OCT data. 5 The 0.6-mm diameter central circle was excluded on macular scans to reduce variation due to the size of the foveal avascular zone (FAZ). A 2-mm diameter circle centered (manually centered) on the disc was excluded on disc scans for the evaluation of peripapillary retinal capillary density.…”

“…Two is that because velocity and decorrelation signals are nonlinearly related, subtraction of bulk motion should be done after the nonlinear conversion of decorrelation to velocity. In the first report of this regression-based bulk-motion subtraction (rb-BMS) algorithm, 5 we demonstrated improved visualization and reduced vessel density variability compared with another algorithm that simply subtracted the median decorrelation value (a simplistic estimate of bulk motion signal) within retinal slab. In this paper, we investigate whether the rb-BMS algorithm can improve the reliability of perfusion quantification in commercial OCTA systems by reducing the interscan variability of capillary density as well as its dependence on retinal signal strength (RSS).…”

“…We have recently proposed a fully automated method that separates vascular from nonvascular voxels, and removes the contribution of bulk motion from flow signal in vascular voxels. 5 Our novel algorithm is based on two insights. One is that the decorrelation signal due to bulk motion is dependent on both bulk motion velocity and reflectance signal amplitude, therefore, the velocity could be recovered by regression analysis.…”

Detecting and measuring areas of choriocapillaris low perfusion in intermediate, non-neovascular age-related macular degeneration

“…Four retinal layer boundaries (i.e., the vitreous/inner limiting membrane (ILM), outer plexiform layer (OPL)/outer nuclear layer (ONL), RPE and Bruch's membrane/choriocapillaris) were segmented from structural OCT B-scans by a validated directional graph search method. 18 An iterative regression-based bulk motion subtraction (irb-BMS) algorithm 19 was used to remove the confounding contribution of ocular motion. By application of irb-BMS, nonvascular voxels in the retina and choriocapillaris could be identified and removed in a thresholding scheme that sets a reflectance-adjusted threshold based on the regression analysis of background OCTA signal versus local reflectance, retrieving flow signal in pixels in areas of partial attenuation.…”