Heartbeat elastography allows assessment of cornea condition

12 May 2020

University of Houston uses heartbeat-induced changes in intraocular pressure to measure corneal properties.

Several diseases of the cornea, such as corneal dystrophy and keratoconus, lead to underlying changes in a cornea's intrinsic biomechanical properties as well as the inherent cellular damage.

Measuring those biomechanical markers could be a valuable tool for monitoring and detecting the diseases, especially if the technique can be a non-contact method. Non-invasive measurement methods are inherently likely to be best suited for eventual translation into clinics, and for reducing patient discomfort.

Optical coherence elastography (OCE), in which optical coherence tomography is used to image tissue while it is under some form of transient local mechanical deformation and subsequent recovery, is one promising approach, but it has traditionally required an external force to act upon the cornea.

Kirill Larin's research group at the University of Houston, which has been at the forefront of OCE development for cornea testing, has now demonstrated that the small changes in an eye's interocular pressure (IOP) occurring as a natural response to the heartbeat can be sufficient to generate OCE data. The results were published in Journal of Biomedical Optics.

"Accurate measurement of corneal biomechanics would not only influence our clinical interpretation of diagnostic tests, for example by measuring intraocular pressure or assessing effects of drug therapies, but also predict the onset of posterior eye diseases, such as glaucoma," commented Larin.

The new findings build on existing research into how an animal's heartbeat produces a natural cyclic biomechanical force in the tissues of the cornea, as the blood flow pulses and relaxes, potentially removing the need for an external force in the generation of OCE data.

Speaking to Optics.org in advance of a presentation at SPIE Photonics West in February 2020, Kirill Larin said that OCT plus some additional contrast mechanism is an inherently powerful approach.

"One beauty of OCE is that OCT is already well established in clinics," he said. "OCE can fill a gap in non-destructive imaging techniques, combining field-of-view in the millimeter or centimeter range with spatial resolution from microns to millimeters."

Diabetes and glaucoma cause changes in ocular blood flow

Larin's lab has now developed heartbeat OCE (Hb-OCE), to assess the corneal biomechanical response to varying fluctuations in IOP during the systolic and diastolic phases of the pulse. Initial trials were carried out using ex vivo pig corneas, in which a simulated pulse was created using fluid infusion.

Although the pulse period and pressure amplitude did not precisely represent natural conditions, the experiment confirmed that IOP fluctuations similar to that produced by the ocular pulse can indeed be used as a natural excitation source for OCE measurements, and can distinguish corneas of different stiffness.

Motion compensation and fast imaging operations will be needed for in vivo applications, but the team believes that such live-animal uses are now feasible.

"Currently, Hb-OCE is primarily limited to relative mechanical contrast through strain rather than quantitative measurements of tissue elastic modulus," noted the project in its published paper. "Future work will focus on developing models that can more accurately link the behavior of the cornea to material parameters such as Young's modulus."

This could be particularly useful for assessing the symptoms of custom-crosslinked corneas, created deliberately as a treatment for the condition called keratoconus in which the cornea has become deformed.

It may also be possible to use Hb-OCE to reveal different conditions of the blood vessels and blood flow, which will also be reflected in the biomechanical behavior of the tissues. Diabetes and glaucoma and two conditions known to cause changes in ocular blood flow, noted the project team.

"These results suggest that our method of Hb-OCE utilizing the natural pulsation in the cornea due to the heartbeat may be useful for assessing the stiffness of the cornea in the clinic, using only an OCT system and no additional instruments," said Larin.