Posterior rat eye during acute intraocular pressure elevation studied using polarization sensitive optical coherence tomography

Abstract

Polarization sensitive optical coherence tomography (PS-OCT) operating at 840 nm with axial resolution of 3.8 µm in tissue was used for investigating the posterior rat eye during an acute intraocular pressure (IOP) increase experiment. IOP was elevated in the eyes of anesthetized Sprague Dawley rats by cannulation of the anterior chamber. Three dimensional PS-OCT data sets were acquired at IOP levels between 14 mmHg and 105 mmHg. Maps of scleral birefringence, retinal nerve fiber layer (RNFL) retardation and relative RNFL/retina reflectivity were generated in the peripapillary area and quantitatively analyzed. All investigated parameters showed a substantial correlation with IOP. In the low IOP range of 14-45 mmHg only scleral birefringence showed statistically significant correlation. The polarization changes observed in the PS-OCT imaging study presented in this work suggest that birefringence of the sclera may be a promising IOP-related parameter to investigate.

Keywords: (110.4500) Optical coherence tomography; (130.5440) Polarization-selective devices; (170.3880) Medical and biological imaging; (170.5755) Retina scanning.

Fig. 1

Sketch of the experimental setup. (A) Sketch of the OCT system and cannulation of the eye. SLD - superluminescent diode, PC - polarization controller, SMF - single mode fiber, PMF - polarization maintaining fiber, GM - galvanometer mirrors, QWP - quarter wave plate, HWP - half wave plate, NPB - non-polarizing beam splitter, GTP - Glan-Thomson polarizer, PB - polarizing beam splitter, ND filter - neutral density filter. (B) Sketch of the spectrometer used in the OCT system. (C) Core of the Michelson interferometer used in the system. (D) 3D rendering of a data set acquired during the experiment (field of view 30° × 30°).

Fig. 2

Exemplary high resolution PS-OCT images recorded during the experiment. (A) En face reflectivity projection at physiologic IOP, without the cannulation of the eye. (B) En face reflectivity projection at the pressure of 95 mmHg. A change of the intensity can be observed. (C) En face birefringence of the sclera at physiologic IOP and (D) at increased IOP of 95 mmHg. (E-P) Reflectivity and corresponding phase retardation B-scans recorded at the respective IOP levels. The red lines indicate the part of the sclera that was used in the birefringence evaluation. In (J) and (L), the optic nerve canal and optic nerve are visible. Low contrast of RNFL reflectivity is indicated by pink arrows. Extraorbital tissue is indicated by white arrows. ILM – internal limiting membrane, RNFL – retinal nerve fiber layer, IPL – inner plexiform layer, RPE – retinal pigment epithelium.

Fig. 3

ONH depression (cupping). (A) 3D visualization of representative data sets together with en face depression maps. The location of the color bars at the right of each volume rendering indicates the color-coded displacement used in the depression maps. (B) En face depression maps for one animal aligned to the initial measurement with axial motion between B-scans corrected. (C) Average ONH depression measured in the annulus depicted in the first map in (B) in 5 animals (difference between data set at 14 mmHg and the successive data sets, mean ± standard deviation) (D) Individual ONH depression difference for each animal. Change in the depression is more apparent after 45 mmHg.

Fig. 4

Scleral sling visualized at IOP ~95 mmHg. (A) En face reflectivity projection with marked position of cross sectional images. (B) Cross sectional phase retardation image in superior (S) – inferior (I) direction. (C) Cross sectional phase retardation image in nasal (N) - temporal (T) direction. Violet arrows point to the sclera between optic nerve and central retinal vessels (scleral sling).

Fig. 5

Scleral birefringence. (A) 3D visualization of an exemplary reflectivity data set. Scleral birefringence was evaluated in the depicted bluish slab and is shown as a birefringence en face map beneath the volume rendering. (B) En face maps of scleral birefringence for an exemplary animal as response to elevated IOP. (C) Average scleral birefringence measured in the annulus depicted in the middle map in (B) in 5 animals. Scleral birefringence increased with elevated IOP. After lowering back to 14 mmHg, restored scleral birefringence values were observed in just one animal.

Fig. 6

Retinal and scleral changes as a response to IOP elevation. (A) Average scleral birefringence shows an increase as a response to an elevated IOP from 14 to 45 mmHg. (B) The RNFL retardation does not show any strong correlation with IOP variation. (C) The RNFL/retina reflectivity ratio vs. IOP shows a moderate, negative correlation. In the middle column, the mean quantities ( ± standard deviation - STDEV) are plotted. All data points are plotted in the right column, where each dot represents the respective quantity for one animal at a given IOP. Measurements after lowering the IOP back to 14 mmHg are excluded from these plots. Linear regressions are shown for the lower IOP range of 14 – 45 mmHg. Value marked by asterisk was retrieved just from one data set.

Fig. 7

Ex vivo µCT scan of a rat eye and head. (A) Tomogram through the rat eye. Densely packed structures like Harderian gland and muscles are visible. Deformations of the eye are due to preparation artifacts of the sample. (B) Rendered µCT data from the rat head. (C) Rendered µCT data together with sketch of the eye. (D) µCT visualization from the “en face” view similar as used for OCT scans. Color code: Red: oblique and rectus muscles; dark red: retractor bulbi, marked with white asterisk; grey: optic nerve sheath, marked with black asterisk; white: optic nerve, marked with green asterisk. (E) Birefringence and reflectivity en face projections of OCT data corresponding to the location in (D).