Ultrasonic Measurement of Scleral Cross-Sectional Strains During Elevations of Intraocular Pressure: Method Validation and Initial Results in Posterior Porcine Sclera

Abstract

Background. Scleral biomechanical properties may be important in the pathogenesis and progression of glaucoma. The goal of this study is to develop and validate an ultrasound method for measuring cross-sectional distributive strains in the sclera during elevations of intraocular pressure (IOP). Method of Approach. Porcine globes (n ¼ 5) were tested within 24 hs postmortem. The posterior scleral shells were dissected and mounted onto a custom-built pressurization chamber. A high-frequency (55-MHz) ultrasound system (Vevo660, VisualSonics Inc., Toronto) was employed to acquire the radio frequency data during scans of the posterior pole along both circumferential and meridian directions. The IOP was gradually increased from 5 to 45 mmHg. The displacement fields were obtained from correlation-based ultrasound speckle tracking. A least-square strain estimator was used to calculate the strains in both axial and lateral directions. Experimental validation was performed by comparing tissue displacements calculated from ultrasound speckle tracking with those induced by an actuator. Theoretical analysis and simulation experiments were performed to optimize the ultrasound speckle tracking method and evaluate the accuracy and signal-to-noise ratio (SNR) in strain estimation. Results. Porcine sclera exhibited significantly larger axial strains (e.g., À5.1 6 1.5% at 45 mmHg, meridian direction) than lateral strains (e.g., 2.2 6 0.7% at 45 mmHg, meridian direction) during IOP elevations (P's < 0.01). The strain magnitudes increased nonlinearly with pressure increase. The strain maps displayed heterogeneity through the thickness. The lateral strains were significantly smaller in the circumferential direction than the meridian direction at 45 mmHg (P < 0.05). Experimental validation showed that the ultrasound speckle tracking method was capable of tracking displacements at the accuracy of sub-micron to micron. Theoretical analysis predicted the dependence of the strain estimation SNR on the strain level, as well as signal processing parameters such as kernel size. Simulation results showed that ultrasound speckle tracking had a high accuracy for estimating strains of 1-5% and a high SNR for strains of 0.5-5%. Conclusions. A new experimental method based on ultrasound speckle tracking has been developed for obtaining cross-sectional strain maps of the posterior sclera. This method provides a useful tool to examine distributive strains through the thickness of the sclera during elevations of IOP.

Collagen Structure and Mechanical Properties of the Human Sclera: Analysis for the Effects of Age

“…Instead, we directly measured it using WAXS and incorporated this infor mation into our finite element model. Tang and Liu [64] and Mor ris et al [65] recently proposed an alternative method to DIC and ESPI to measure the strain response of scleral tissue using ultra sound imaging. In contrast to ESPI and DIC, which can only mea sure surface deformation, ultrasound imaging can measure the full 3D strain tensor.…”

Biomechanical Properties and Correlation With Collagen Solubility Profile in the Posterior Sclera of Canine Eyes With anADAMTS10Mutation

“…26,54,55 Thus, the mechanical properties reported in our study are most relevant for relative comparisons between the two groups. We recently have developed an inflation testing method based on highresolution ultrasound, 56 which will enable future mechanical testing on whole globes under physiologic loadings. Fourth, the hydroxyproline assay and histology staining were performed on tissue following mechanical testing and flash freezing.…”

Mapping 3D Strains with Ultrasound Speckle Tracking: Method Validation and Initial Results in Porcine Scleral Inflation

“…Several studies have utilized inflation mechanical testing to characterize the deformation of the posterior sclera. These studies have measured the surface 3,8,9 or 2D cross-sectional 19,30 deformation of the sclera while the ocular shell was pressurized to controlled IOP levels. These studies showed a non-linear deformation response to IOP as well as regional/quadrant differences in the deformation profiles of the peripapillary sclera.…”