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 = 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., -5.1 ± 1.5% at 45 mmHg, meridian direction) than lateral strains (e.g., 2.2 ± 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.

Fig. 1

Schematics for sclera shell mounting and ultrasonic measurements

Fig. 2

(a) The experimental protocol of preconditioning and IOP loading; (b) the scanning orientations for ultrasound data acquisition. The circumferential cross-section was about 2 mm from the ONH.

Fig. 3

Displacement vector fields in a posterior porcine sclera obtained from ultrasound speckle tracking. (a) A cross-sectional ultrasound image of the sclera at 5 mmHg; (b) displacement field at 15 mmHg; (c) displacement field at 30 mmHg; (d) displacement field at 45 mmHg

Fig. 4

Strain images of porcine sclera: (a) axial strain at 15 mmHg (dashed rectangle indicates the region of interest at posterior pole); (b) lateral strain at 15 mmHg; (c) axial strain at 30 mmHg; (d) lateral strain at 30 mmHg; (e) axial strain at 45 mmHg; (f) lateral strain at 45 mmHg

Fig. 5

Average scleral strains at different pressure levels

Fig. 6

Comparison of the displacements calculated from speckle tracking and the actuator output: (a) calculated axial displacement versus motor output; (b) calculated lateral displacement versus motor output

Fig. 7

SNR_e versus strain for different kernel sizes

Fig. 8

A simulated ultrasound image of the sclera using the Field II Ultrasound Simulation Program

Fig. 9

The approximated SNR_e curves of the strain maps based on simulated data

Fig. 10

Displacement vector field and strain maps calculated from simulated RF signals for top 1% and bottom 2% strains

Fig. 11

Strain images of a simulated sclera with an inhomogeneous region. Row (a) and (b) are axial and lateral strains, respectively, for an inhomogeneous layer with decreasing thickness: (1) 500 μm, (2) 250 μm, (3) 150 μm, and (4) 50 μm; Row (c) and (d) are axial strains and lateral strains, respectively, for an inhomogeneous zone with decreasing width: (1) 2 mm, (2) 1 mm, (3) 400 μm, and (4) 200 μm.