OCE and ocular biomechanics
July 23-26, 2017
DYNAMIC OPTICAL COHERENCE ELASTOGRAPHY AND OCULAR BIOMECHANICS
Salavat Aglyamov, University of Houston, Department of Mechanical Engineering
Kirill Larin, University of Houston, Department of Biomedical Engineering
Key Words: Elastography, OCE, OCT, ocular tissue, elasticity
Microscopic changes in biological tissues leading to pathologies often result in macroscopic changes in tissue biomechanical properties, such as tissue elasticity. It is known, that the biomechanical characteristics of ocular tissues have a profound influence on the health, structural integrity, and normal function of the human eye. Such conditions as presbyopia, corneal ectasia and keratoconus correlate with stiffness of the ocular tissues.
Elastography is a general name for a group of diagnostic methods capable of remote evaluation of tissue biomechanical properties based on applying mechanical excitation, measuring tissue response, and interpretation of tissue response to evaluate biomechanical parameters of tissue. Optical Coherence Elastography (OCE) is a branch of elastography that uses Optical Coherence Tomography (OCT) to measure tissue motion in response to external excitation. In comparison with other imaging techniques, such as ultrasound and magnetic resonance imaging, OCT has significant advantages in resolution and accuracy of motion estimation, but limited by the low penetration depth of light. Therefore, OCE is an ideal imaging technique to measure biomechanical properties of the ocular tissues.
We have developed noninvasive OCE approaches to measure elastic properties of the ocular tissues using air puff stimulation and acoustic radiation force. The critical step in evaluating the biomechanical properties of tissues is interpretation of the tissue mechanical response based on the appropriate mechanical model. The model dictates how the measured mechanical response relates to elastic parameters. Biomechanical properties are evaluated based on developed mathematical model of the dynamic deformation of the viscoelastic medium. This approach has been successfully tested in phantom, ex-vivo and animal studies.
The home-built OCE system was composed of a focused air-pulse delivery system or focused single element transducer and a phase-stabilized swept source optical coherence tomography (PhS-SSOCT) system. The system utilized a broadband swept laser source (HSL2000, Santec, Inc., CA) with a central wavelength of ~1310 nm, bandwidth of ~150 nm, scan rate of 30 kHz, and experimentally measured phase stability of ~40 nm during experiments.
We developed a method to assess corneal biomechanics based on the measurements of the elastic wave propagation in the cornea using phase-sensitive OCT imaging system after micro air-puff stimulation of the cornea. The focused air-pulse delivery system was comprised of a controller with a signal input for synchronization, a solenoid-controlled air gate, and an air-pulse port with a flat edge and diameter of ~150 µm. The system is capable of delivering a short duration focused air-pulse (≤1 ms) with a Gaussian profile. The modified Rayleigh-Lamb frequency equation was derived and used to calculate the dispersion of the Lamb wave in cornea. The model was tested in phantom and ex vivo studies, and used to quantify the viscoelasticity of in situ porcine corneas in the whole eye-globe configuration before and after CXL. The viscoelasticity of the untreated and CXL-treated eyes was quantified at various IOPs. The results showed that the stiffness of the cornea increased after CXL and that corneal stiffness is close to linear as a function of IOP. The influence of the corneal thickness and curvature on the propagation on the elastic waves was investigated.
We have developed acoustic radiation force based approach to assess the biomechanical properties of the lens and applied this approach for measuring biomechanical properties of the lens in intact animal eyes in situ for
different ages and IOPs. A 3.7 MHz single element transducer was used to remotely disturb the anterior surface of the animal lenses through the cornea and the aqueous humor. The crystalline lens was modeled as a viscoelastic layer. Ultrasound transducer and OCT system were co-focused, and the measurements of the displacement were performed at the focal point. Results of the measurements demonstrated a significant
difference between elastic properties of the young and the mature lenses, as well as, stiffening lens with IOP.
In summary, the optical coherence elastography was demonstrated as a promising tool for noninvasive assessment of the biomechanical properties of the ocular tissues.
This study was supported by National Institute of Health grant EY022362
Salavat Aglyamov and Kirill Larin, "OCE and ocular biomechanics" in "Advances in Optics for Biotechnology, Medicine and Surgery XV", Peter So, Massachusetts Institute of Technology, USA Kate Bechtel, Triple Ring Technologies, USA Ivo Vellekoop, University of Twente, The Netherlands Michael Choma, Yale University, USA Eds, ECI Symposium Series, (2017). http://dc.engconfintl.org/biotech_med_xv/34
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