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

This study aimed to develop and validate a high frequency ultrasound method for measuring distributive, 3D strains in the sclera during elevations of intraocular pressure. A 3D cross-correlation based speckle-tracking algorithm was implemented to compute the 3D displacement vector and strain tensor at each tracking point. Simulated ultrasound radiofrequency data from a sclera-like structure at undeformed and deformed states with known strains were used to evaluate the accuracy and signal-to-noise ratio (SNR) of strain estimation. An experimental high frequency ultrasound (55 MHz) system was built to acquire 3D scans of porcine eyes inflated from 15 to 17 and then 19 mmHg. Simulations confirmed good strain estimation accuracy and SNR (e.g., the axial strains had less than 4.5% error with SNRs greater than 16.5 for strains from 0.005 to 0.05). Experimental data in porcine eyes showed increasing tensile, compressive, and shear strains in the posterior sclera during inflation, with a volume ratio close to one suggesting near-incompressibility. This study established the feasibility of using high frequency ultrasound speckle tracking for measuring 3D tissue strains and its potential to characterize physiological deformations in the posterior eye.

Spatially heterogeneous corneal mechanical responses before and after riboflavin-ultraviolet-A crosslinking.

PURPOSE: To determine the heterogeneous through-thickness strains in the cornea at physiologic intraocular pressures before and after corneal collagen crosslinking (CXL) using noninvasive ultrasound. SETTING: Department of Biomedical Engineering, Ohio State University, Columbus, Ohio, USA. DESIGN: Experimental study. METHODS: Sixteen paired canine corneoscleral shells were divided into 2 groups. The CXL group completed a standard CXL protocol using riboflavin-ultraviolet-A (UVA) irradiation. The control group was given an identical treatment except UVA irradiation. Ultrasound scans (at 55 MHz) of the cornea were obtained before and after treatment as the corneoscleral shell was inflated from 5 mm Hg to 45 mm Hg to calculate the distributive through-thickness strains in the cornea. The mean radial and tangential strains of the whole cornea layer, as well as those of the anterior, middle, and posterior thirds of the cornea, were compared before and after treatment in the control group and CXL group using linear mixed models with repeated measures. RESULTS: Significant reductions in tangential and radial strains occurred in the CXL group (P=.003 and P=.0025, respectively) but not the control group (P=.08 and P=.63, respectively). The anterior third had the smallest strains in all pretreated corneas (P<.001) and posttreated corneas (CXL group, P=.023; control group, P=.01). CONCLUSIONS: Ultrasound speckle tracking showed heterogeneous strain distributions through the cornea and confirmed that CXL results in a stiffer corneal response (ie, smaller strains during physiologic loadings). This technique may provide a clinical tool to quantify the biomechanical effects of CXL. FINANCIAL DISCLOSURE: No author has a financial or proprietary interest in any material or method mentioned.

Biaxial mechanical testing of posterior sclera using high-resolution ultrasound speckle tracking for strain measurements.

This study aimed to characterize the mechanical responses of the sclera, the white outer coat of the eye, under equal-biaxial loading with unrestricted shear. An ultrasound speckle tracking technique was used to measure tissue deformation through sample thickness, expanding the capabilities of surface strain techniques. Eight porcine scleral samples were tested within 72 h postmortem. High resolution ultrasound scans of scleral cross-sections along the two loading axes were acquired at 25 consecutive biaxial load levels. An additional repeat of the biaxial loading cycle was performed to measure a third normal strain emulating a strain gage rosette for calculating the in-plane shear. The repeatability of the strain measurements during identical biaxial ramps was evaluated. A correlation-based ultrasound speckle tracking algorithm was used to compute the displacement field and determine the distributive strains in the sample cross-sections. A Fung type constitutive model including a shear term was used to determine the material constants of each individual specimen by fitting the model parameters to the experimental stress-strain data. A non-linear stress-strain response was observed in all samples. The meridian direction had significantly larger strains than that of the circumferential direction during equal-biaxial loadings (P's<0.05). The stiffness along the two directions was also significantly different (P=0.02) but highly correlated (R(2)=0.8). These results showed that the mechanical properties of the porcine sclera were nonlinear and anisotropic under biaxial loading. This work has also demonstrated the feasibility of using ultrasound speckle tracking for strain measurements during mechanical testing.

Correlation between biomechanical responses of posterior sclera and IOP elevations during micro intraocular volume change.

PURPOSE: This study tested the hypothesis that intraocular pressure (IOP) elevations, induced by controlled increase of intraocular volume, are correlated with the biomechanical responses of the posterior sclera. METHODS: Porcine globes were tested within 48 hours postmortem. The first group of globes (n = 11) was infused with 15 µL of phosphate-buffered saline at three different rates to investigate rate-dependent IOP elevations. The second group (n = 16) was first infused at the fast rate and then underwent inflation tests to investigate the relationship between IOP elevations (ΔIOP) and scleral strains. The strains in the superotemporal region of the posterior sclera were measured by ultrasound speckle tracking. Linear regression was used to examine the association between ΔIOP due to micro-volumetric infusion and the scleral strains at a specific inflation pressure. RESULTS: The average ΔIOP was 14.9 ± 4.3 mm Hg for the infusion of 15 µL in 1 second. The ΔIOP was greater for the faster infusion rates but highly correlated across different rates (P < 0.001). A significant negative association was found between the ΔIOP and the tangential strains in both the circumferential (R(2) = 0.54, P = 0.003) and meridian (R(2) = 0.53, P = 0.002) directions in the posterior sclera. CONCLUSIONS: This study showed a substantial increase in IOP, with a large intersubject variance during micro-volumetric change. A stiffer response of the sclera was associated with larger IOP spikes, providing experimental evidence linking corneoscleral biomechanics to IOP fluctuation. In vivo measurement of corneoscleral biomechanics may help better predict the dynamic profile of IOP.