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Joseph Park, Andrew Shin, Joseph L Demer; Polarization Sensitive Optical Coherence Tomography (PSOCT) Demonstrates Strain Dependent Birefringence in Ocular Tissues. Invest. Ophthalmol. Vis. Sci. 2016;57(12):3569.
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© ARVO (1962-2015); The Authors (2016-present)
Ex vivo, destructive testing has demonstrated that mechanical properties of the sclera of glaucomatous and myopic eyes are abnormal. A technique for in vivo measurement of these properties would be valuable. Many biological tissues containing regular arrays of collagen exhibit birefringence, an optical property in which refractive index depends on the light's polarization and propagation direction. PSOCT is an interferometric imaging technique employing orthogonal polarization paths that can image local birefringence. Since mechanical loading alters the orientation and geometry of fibrils in collagen, we sought to determine if birefringence changes can be used as a non-invasive optical method to infer local mechanical properties of ocular connective tissues.
An infrared (1300 nm) PSOCT scanner (Thorlabs PSOCT-1300) was mounted over a uniaxial tensile load cell consisting of a linear motor and strain gauge. Ten specimens each of fresh bovine equatorial sclera, extraocular tendon (EOT), and optic nerve sheath (ONS) were elongated to failure at constant rate of 0.1 mm/s while birefringence images were captured every 117 ms. In each frame, average birefringence phase retardation was calculated using MATLAB (Mathworks, Natick, MA), and birefringence values were synchronized with instantaneous strain and tissue tension. We then computed the birefringence modulus, defined as the change of birefringence as a function of strain.
In each tissue, infrared phase retardation was a monotonic function of both strain and stress up to the point of specimen rupture. Mean (±SD) birefringence modulus was highest for sclera at 11.3±4.4×10-4, lower for EOT at 5.5±2.0×10-4, and least for ONS at 1.1±0.9×10-4 (P<0.012).
Sclera, EOT, and ONS have distinct but widely differing birefringence moduli that quantify how birefringence changes with mechanical strain. Measurement of birefringence moduli by PSOCT may enable non-invasive optical monitoring of mechanical strain in these tissues, particularly in sclera where birefringence is most sensitive to strain. In vivo optical strain measurement may be valuable for study of eye diseases such as glaucoma and strabismus.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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