In vivo depth resolved birefringence measurement of the human retinal nerve fiber layer by polarization sensitive OCT

Purpose: To measure the depth resolved retinal nerve fiber layer (RNFL) birefringence using a new Polarization Sensitive Optical Coherence Tomography (PS-OCT) machine that was developed by the authors [1]. Measurement of RNFL optical birefringence enhances specificity in determining RNFL thickness in structural Optical Coherence Tomography (OCT) images. A decrease in birefringence may also be an early sign of glaucomatous atrophy of the RNFL, and therefore PS-OCT may be particularly helpful in the early diagnosis of glaucoma. Methods: With OCT, cross-sectional structural images of the retina can be made in vivo, allowing determination of the RNFL thickness. From light reflected back from the retina, Scanning Laser Polarimetry (SLP) can measure polarization state changes, which are attributed to the birefringence of the RNFL. We have developed a PS-OCT instrument that combines the depth resolution of OCT with the polarization sensitivity of SLP to image the depth resolved optical birefringence of the human RNFL in vivo. By measuring the OCT interference fringes simultaneously in orthogonal polarization channels, the polarization state of light reflected from a structure within the retina can be determined completely. By modulating the input polarization over two states, the optic axis and birefringence of the RNFL can be uniquely determined. The PS-OCT scan location is co-registered by a video camera during data acquisition. Results: Measurements on one volunteer show that the average double pass phase retardation in the RNFL in near the optic nerve head is equal to 0.29 ± 0.02° / mm. Conclusions: For the first time the depth resolved birefringence of the human retinal nerve fiber layer in vivo was measured using Polarization Sensitive Optical Coherence Tomography. [1] B. Cense, T.C. Chen, B.H. Park, M.C. Pierce, J.F. de Boer, "In vivo depth-resolved birefringence measurements of the human retinal nerve fiber layer by polarization-sensitive optical coherence tomography" Optics Letters 27, 1610 (2002)