Optical coherence elastography (OCE) is an emerging imaging technique used in biomedical imaging to form pictures of biological tissue in micron and submicron level and maps the biomechanical property of tissue.
Introduction
Elastography was first used in 1979 and subsequent progress in the field has been extensive, based largely on ultrasound, and magnetic resonance imaging. Optical techniques have also been proposed for elastography to probe mechanical properties of tissues dates back to at least the 1950s. In 1998, Schmitt first proposed optical coherence elastography (OCE), in employing optical coherence tomography (OCT) detect depth-resolved sample deformation induced by quasi-static compression. But the term optical coherence elastography was first coined in a 2004 paper with Brett Bouma.
Requiring no injections, OCE is a non-invasive imaging method can gives more details than ultrasound or MRI. Using light source to image biological tissue, OCE is considered generally safe compared to CT scan and other radiographic imaging modalities which involve with ionizing radiation. And it is also more affordable and time efficient compared to MRI. However, OCE can also cause tissue damage in some handling. For example, surface acoustic wave OCE can cause to a tissue is primarily concentrated on temperature effects. In the focus point, the temperature of the tissue will rise locally. Therefore, it is necessary to take this into account in certain circumstances.
OCE is characterized by its niche in intermediate spatial resolution (10s–100s μm) and degree of depth penetration and, by exploiting optical interferometry, its high sensitivity to small mechanical changes—at the microstrain level. However, it's still in the early stage of development and needs more clinical practice before enters into the market. But the term optical coherence elastography was first coined in a 2004 paper with Brett Bouma.
Theory
Although optical coherence tomography (OCT) provide crucial information for the diagnosis, they often are insufficient for early diagnosis, before structural changes occur. Elastography, the display of the elastic properties of soft tissues, may be performed using ultrasound, magnetic resonance imaging (MRI) or OCT. Elastography has been proven feasible for characterization of ocular tissues.
Tissue exhibits varying degrees of viscoelasticity (time-dependent response to a load), poroelasticity (presence of fluid-filled pores or channels), and anisotropy, as well as a nonlinear relationship between elasticity and the applied load. As a starting point in establishing the link between elasticity and displacement, a number of simplifying assumptions are usually made about tissue behavior and structure. Most commonly, tissue is approximated as a linear elastic solid with isotropic mechanical properties. The assumption of linearity is commonly valid for the level of strain (typically <10%) applied in elastography.
Selected applications
Optical coherence elastography holds great promise for detecting and monitoring the altered mechanical properties that accompany many clinical conditions and pathologies, particularly in cancer, cardiovascular disease and eye disease.
Ophthalmology
In ophthalmology, OCE could be utilized to characterize the mechanical properties of the cornea in order to diagnose related ocular disease: especially to diagnose and assess the properties of keratoconus, which provides necessary information to establish adaptive biomechanical models of the cornea for the optimization of individual laser ablation procedure. It also helps improve the management of collagen cross-linking therapy. The methods utilizing OCE to study these properties are still under investigation and some particular ones listed below seem promising to be developed into clinical use. Methods could be divided into two main categories based on required interactions which are in-contact and non-contact detections.
Contact detection
In contact detection using a standard clinical gonioscopy, direct contact with the cornea using gonioscopy creates static compression and resultant displacement amplitude inside the cornea responding to the compression indicates the heterogeneous mechanical properties of the cornea.