Abstract
Doppler optical coherence tomography (OCT) is widely used for high-resolution mapping of flow velocities and is based on analysis of temporal changes in the phase of an OCT signal (i.e., how fast the OCT signal rotates in the complex plane). Determination of the rate of phase change or rotation speed critically depends on the center of rotation. Here, we demonstrate the bias in high-pass filtering, the current widely used method to determine the center of rotation, and propose two advanced methods for Doppler OCT clutter rejection. The bias in the high-pass filtering method becomes increasingly significant with lower velocities or larger signal noise. Two novel methods based on variance minimization and offset extrapolation can potentially reduce this bias and thereby improve the accuracy of Doppler OCT measurements of flow velocities, even for low-velocity and/or high-noise signals. The two novel methods and the current standard method (high-pass filtering) have been tested in combination with several currently used velocity measurement algorithms: Kasai, autocorrelation function fitting, and maximum likelihood estimation. The newly proposed methods are shown to improve the accuracy in both the center of rotation and resultant velocity by up to 60 percentage points and reduce the flow conservation error by 30% when applied to in vivo cerebral blood flow imaging of the rodent brain cortex.
Highlights
Optical coherence tomography (OCT) is a 3D biomedical imaging technique based on optical interference to achieve depth-revolved mapping with a penetration depth on the order of 1-2 millimeters [1,2,3]
This study describes two novel methods for Doppler OCT clutter rejection and demonstrates the improved accuracy in both numerical tests and in vivo imaging
As can be seen in the numerical test (Figs. 7 and 8), the proposed methods are accurate in lower velocities, improving both
Summary
Optical coherence tomography (OCT) is a 3D biomedical imaging technique based on optical interference to achieve depth-revolved mapping with a penetration depth on the order of 1-2 millimeters [1,2,3]. Through a process called coherence gating, OCT analyzes the interference between light reflected from a specimen and a reference beam to determine the optical properties of a sample in a depth-resolved manner. Doppler OCT is a special application of this technique It uses the time-varying phase of the reflected light from the specimen to determine the velocity component at every voxel in the direction of the probing OCT beam. An idealized noise-free signal rotates about the origin and its phase angle linearly increases in time. Finding the rate of rotation in the ideal signal is simple; one can determine the change in phase or angle that each data point makes about the origin per unit time, often in conjugation with an autocorrelation to reduce noise [4,5,6]; one can determine the mean or maximum frequency of the signal after a frequency decomposition [7,8]
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