Abstract
Measurement and imaging of depolarization by polarization-sensitive optical coherence tomography (PS-OCT) requires averaging of Stokes vector elements within two- or three-dimensional (3-D) evaluation windows to obtain the degree of polarization uniformity (DOPU). By use of a PS-OCT system with an integrated retinal tracker, we analyze optimum conditions for depolarization imaging, data processing, and segmentation of depolarizing tissue in the human retina. The trade-offs between figures of merit like DOPU imaging sensitivity, efficiency, and susceptibility are evaluated in terms of 3-D resolution. The results are used for a new, detailed interpretation of PS-OCT high-resolution images of the human retinal pigment epithelium and Bruch’s membrane.
Highlights
According to our first introduction of the parameter, degree of polarization uniformity (DOPU) is calculated from a two-dimensional (2-D) Polarization-sensitive optical coherence tomography (PS-OCT) B-scan frame via the Stokes vectors obtained at each pixel from amplitudes and the relative phase detected by the two orthogonal polarizations, and using a 2-D sliding average window (DOPU evaluation window for averaging the Stokes vectors) in ðx; zÞ, where the xz-plane is parallel to the B-scan frame, and x and z denote scanning and depth directions, respectively.[16]
In-plane retinal position displacement is obtained from the laser ophthalmoscope (LSLO) images [field of view (FOV): 8 × 6 mm2; 800 × 600 pixel2], and correction signals for cancelling the motion artifacts are sent to the OCT imaging scanner so that the same B-scan position on the retina can be kept during the acquisition
In the DOPU and the corresponding segmentation images, it is observed that blood flow in the inner retinal layers cause an increase of depolarization and the segmented pixels around the retinal pigment epithelium (RPE) cell layer, generating a visible difference compared to the adjacent RPE layer, no difference within that layer is expected
Summary
We introduced a new parameter, the degree of polarization uniformity (DOPU), that is formally equivalent to the well-known degree of polarization (DOP), which cannot directly be measured by a coherent imaging technique like OCT. DOPU can be regarded as an averaged DOP and was used to segment depolarizing tissue like RPE16 and hard exudates,[25] as well as various lesions in macular disorders.[25,26,27,28,29,30] According to our first introduction of the parameter, DOPU is calculated from a two-dimensional (2-D) PS-OCT B-scan frame via the Stokes vectors obtained at each pixel from amplitudes and the relative phase detected by the two orthogonal polarizations, and using a 2-D sliding average window (DOPU evaluation window for averaging the Stokes vectors) in ðx; zÞ, where the xz-plane is parallel to the B-scan frame, and x and z denote scanning and depth directions, respectively.[16] A typical window size is configured to be, for example, 8ðxÞ × 10ðzÞ pixels in order to contain a sufficient number of (e.g., 80) Stokes
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