Melanin concentration and depolarization metrics measurement by polarization-sensitive optical coherence tomography

Abstract

Imaging of melanin in the eye is important as the melanin is structurally associated with some ocular diseases, such as age-related macular degeneration. Although optical coherence tomography (OCT) cannot distinguish tissues containing the melanin from other tissues intrinsically, polarization-sensitive OCT (PS-OCT) can detect the melanin through spatial depolarization of the backscattered light from the melanin granules. Entropy is one of the depolarization metrics that can be used to detect malanin granules in PS-OCT and valuable quantitative information on ocular tissue abnormalities can be retrived by correlating entropy with the melanin concentration. In this study, we investigate a relationship between the melanin concentration and some depolarization metrics including the entropy, and show that the entropy is linearly proportional to the melanin concentration in double logarithmic scale when noise bias is corrected for the entropy. In addition, we also confirm that the entropy does not depend on the incident state of polarization using the experimental data, which is one of important attributes that depolarization metrics should have. The dependence on the incident state of polarization is also analyzed for other depolarization metrics.

Objective evaluation of choroidal melanin loss in patients with Vogt–Koyanagi–Harada disease using polarization-sensitive optical coherence tomography

“…Another important limitation is the polarization state of incident light. DOPU measurements are known to highly dependent on the incident light polarization state 34 – 36 . Previous studies of DOPU, mainly performed at the Medical University of Vienna 14 , 18 , 31 , used a circularly polarized incident beam.…”

Evaluation of choroidal melanin-containing tissue in healthy Japanese subjects by polarization-sensitive optical coherence tomography

“…Another important limitation is the state of polarization of incident light. DOPU measurement is known to highly depend on the incident polarization state 38 – 40 . Our PS-OCT system used single-mode optical fibers in the sample arm 16 , and the polarization state of the incident beam to the eye was not fully stable and may have varied among measurements.…”

Changes in entropy on polarized-sensitive optical coherence tomography images after therapeutic subthreshold micropulse laser for diabetic macular edema: A pilot study

“…The detailed principles of our PS-OCT technology and the associated formula used in this study have previously been reported [ 27 , 28 ]. In brief, the light source used in an optical interferometer of the PS-OCT was a frequency-swept laser (Axsun Technologies, Billerica, MA, USA); its central wavelength, sweep repetition frequency, and wavelength range were 1.05 μm, 100 kHz, and 100 nm respectively.…”

“…As one of the approaches to overcome the limitation in DOPU, Yamanari et al introduced a mathematical framework to compute “entropy” that represents the randomness of the polarization property in an advantageous way, both physically and mathematically [ 27 ]. We have previously reported that Entropy in PS-OCT is a parameter that is independent of the incident polarization state and that in principle it is highly reproducible in retake [ 28 ]. Furthermore, high reproducibility of entropy values in normal eyes also has been reported [ 29 ].…”