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Editors Selection IGR 10-4

Clinical Examination Methods: Polarization-sensitive OCT

Comment by Qienyuan Zhou on:

53112 Measuring retinal nerve fiber layer birefringence, retardation, and thickness using wide-field, high-speed polarization sensitive spectral domain OCT, Zotter S; Pircher M; Götzinger E et al., Investigative Ophthalmology and Visual Science, 2013; 54: 72-84

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In this paper, Zotter et al. present a novel polarization sensitive optical coherence tomography (PS-OCT) system for measuring retinal nerve fiber layer (RNFL) birefringence, retardation, and thickness, and reported the repeatability of measuring these quantities in vivo based on ten eyes of five healthy volunteer subjects.

Compared to commercially available intensity-based OCT systems which provide the RNFL thickness measurement and the scanning laser polarimeter (SLP), GDx-VCC, which provides the RNFL retardation measurement, the reported PS-OCT provides RNFL retardation and thickness measurements simultaneously, thus, providing ready access to birefringence, the quotient of the two quantities. Birefringence is associated with the intracellular structures of the RNFL which is thought to be disrupted early in the presence of optic neuropathy. The reported PS-OCT system operates at 840 nm, with up to 40° scan angle, A-scan rate of 70 kHz, and axial resolution of 5.8 μ in tissue. In this study, a single dense scan pattern of 1024 x 250 A-scans (acquisition time approximately 4.2 seconds) was used and two different scan angles of 27° x 24° (for comparison with GDx-VCC) and 40° x 40° (for demonstrating large field of view imaging) were acquired.

The sampling density with the PS-OCT in this study was several times higher than that of the commercial OCT systems and that of the GDx-VCC system, producing a detailed retinal nerve fiber layer retardation image showing distinctive nerve fiber bundles, at the expense of longer acquisition time. A qualitative comparison between retardation maps obtained from GDx-VCC and PS-OCT shows good agreement in the pattern and similar shape in circumpapillary retardation curve. However, in the superior and inferior quadrant the GDx-VCC measures retardation values of 19.8° and 21.2°, respectively, while PS-OCT measures a higher retardation value of 28.9° superiorly and 28.2° inferiorly. The difference in measured retardation values between the two devices might be explained by the fact that the GDx-VCC is based on confocal scanning laser ophthalmoscope (cSLO) imaging with axial resolution close to 300 μm, therefore, the retardation values that correspond to light reflected at different depth positions are integrated in GDx-VCC while PS-OCT resolves the retinal layers and provides the RNFL retardation corresponding to appropriate depth.

The PS-OCT system provides information specifically associated with tissue property, such as birefringence and depolarization

The retardation measurement repeatability (in standard deviation) of the PS-OCT over the five consecutive measurements of each subject ranged from 0.16° ~ 0.69°, with mean values of 0.47° superiorly (1.9% of the mean retardation) and 0.36° inferiorly (1.5% of the mean retardation), indicates excellent repeatability in this test group. The thickness measurement repeatability (in standard deviation) over the five consecutive measurements of each subject ranged from 2.10 μm ~ 7.82 μm, with mean values of 4.35 μ and 5.51 μ for the superior and inferior quadrants respectively; these values are comparable to the typical repeatability of 3 μm ~ 4 μm for quadrant thickness measurements with commercial OCT devices.

The PS-OCT system exhibited excellent image quality and measurement repeatability in normal volunteer subjects. It would be of interest to evaluate the image quality and precision of the device in patients. The PS-OCT system provides information specifically associated with tissue property, such as birefringence and depolarization, which may add clinical value to OCT devices.

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