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Sabine Kling, Hossein Khodadadi; Optical coherence elastography based corneal strain mapping during low-amplitude intraocular pressure modulation. Invest. Ophthalmol. Vis. Sci. 2019;60(9):1288.
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© ARVO (1962-2015); The Authors (2016-present)
Optical coherence elastography (OCE) is a promising technique for high-resolution strain imaging in ocular tissues. A major strain inducing factor in the eye is intraocular pressure (IOP). Diurnal physiological changes in IOP reach approx. 5mmHg, while in untreated glaucoma IOP changes are even higher. The purpose of the current study was to assess local corneal deformation patterns in response to low-amplitude intraocular pressure changes in ex vivo porcine corneas.
4 freshly-enucleated whole porcine eye globes were obtained and mounted on a customized spherically shaped silicon mold. The initial IOP was set to 15 or 25mmHg. Different levels of corneal strain were induced by two subsequent pressure cycles, in which IOP was first increased and then decreased by 5mmHg, in steps of 1mmHg. For each step, IOP was adjusted by a customized pressure system and controlled by a closed-loop routine written in LabView. Repeated cross-sectional B-scans were recorded at each IOP step. For analysis, the axial and lateral displacement maps were computed from magnitude and phase changes in the raw OCT signal. Strain maps were generated by computing the gradients in axial and lateral direction.
Deformations arising from IOP changes of as little as 1mmHg could be resolved. At 25mmHg, corneas showed a positive axial strain in the 35% most anterior cornea (ε=5.1±2.4%), as well as in the 12% most posterior cornea (ε=3.0±1.2%), indicating axial compression. In between these two regions, a third region - accounting for the remaining 53% of stroma - became visible showing negative axial strain (ε=-4.0±1.5%), indicating axial tension. The lateral strain showed a similar 3-layered strain response. At 15mmHg, tensile axial stress was more pronounced (78% of stroma, ε=-6.4±2.2%), while compressive axial strain was limited to shallower regions (18% most anterior cornea, ε=9.8±4.3% and 4% most posterior cornea, ε=6.4±1.8%).
Low amplitude IOP modulation, similar to diurnal physiologic changes, induced measurable deformations in the corneal tissue, which in turn could visualize mechanical differences between three distinct corneal regions. The derived corneal strain maps permit a localized comparison of biomechanical material properties. Small strain OCE is a promising approach for anterior segment diagnostics, and can likely be extended to the posterior segment.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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