Ocular pulse induced corneal deformation in healthy subjects

Abstract

Purpose : To quantify corneal displacements and strains in response to ocular pulse in healthy volunteers, using a high-frequency ultrasound elastography method, termed ocular pulse elastography (OPE, Pavlatos et al, IEEE Medical Imaging, 2018).

Methods : Ten human subjects with no known corneal diseases were recruited. Informed consent was obtained according to an approved IRB. Subjects sat at a slit lamp station and were secured to a chin rest with a headstrap to reduce head motion (Fig. 1a). Anesthetic eyedrops were applied to both eyes prior to measurement, and subjects were asked to look at a distant fixation target. A 50 MHz ultrasound probe (MS700, VisualSonics) coated with GenTeal gel for acoustic transduction was gradually advanced toward the measured eye until the gel was in contact with the cornea. One thousand consecutive ultrasound frames of the central 5.7 mm of cornea along the nasal-temporal were obtained at a frame rate of 128 frames per second, while heart rate was simultaneously monitored using an electrocardiogram (ECG, MP36, BIOPAC). The displacement field in the scanned corneal cross-section was calculated using an ultrasound speckle tracking technique (Tang & Liu, JBME 2012). Axial strains were obtained using least-square estimation.

Results : In all measured subjects, corneal displacements exhibited a cyclic pattern in sync with ECG (Fig. 1b). The heart rate calculated from cyclic OPE displacements was highly correlated with that from ECG (R²=0.98, Fig 1c). The in vivo corneal displacements in the anterior-posterior direction were much larger than those previously measured in ex vivo donor eyes (45.8±10.3µm vs. 7.7±4.9µm). Corneal axial strains at peak anterior displacement were negative (Fig 1d), indicative of through-thickness compression. In a subset of volunteers (n=5), axial strains were calculated with magnitudes similar to those observed in human donor eyes (-0.10%±0.07% vs. -0.07%±0.02%).

Conclusions : Corneal displacements measured by OPE were of cardiac origin as confirmed by correlation with ECG. OPE measured in vivo corneal strains were consistent with what we measured in human donor eyes, despite a larger displacement likely resulted from extraocular vascular pulsation.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.

 

Fig. 1. a. In vivo setup; b. cyclic corneal displacements and correspondence to ECG cycles in a healthy subject; c. correlation between OPE and ECG heart rate (HR); d. Ultrasound scan of a cornea and strain map at peak displacement