Elastography is an imaging technology which measures tissue elastic properties. Ultrasound elastography (UE) was the first technology to perform elastography and is widely studied in clinical diagnostic applications to assess the biomechanical properties of soft tissues. MRI elastography (MRE) has also been successfully applied in the clinical imaging field, especially in diagnosing breast cancer. The elastography method was first applied to OCT technique in 1998. Optical coherence elastography (OCE) has been an emerging technique since then, and to date, the capability of using OCE for diagnosis of various diseases has been demonstrated on both animal and human, ex vivo and/or in vivo.
Resolution and penetration of elastography techniques depend on their imaging modalities. For instance, the typical resolution of UE is 125 – 200 µm, while resolution in MRE is usually of mm-scale (illustrated in the figure below).
Compared to these standard noninvasive clinical imaging modalities, OCE has better resolution (2 – 10 µm). Moreover, by using phase-resolved OCT (equation shown below), an even higher displacement sensitivity (nm-scale) can be achieved.
All the elastography techniques can be classified as static methods and dynamic methods, based on their temporal characteristics of excitation. In static methods, mechanical excitations are considered to be slow and tissue displacements are usually measured as indications of tissue biomechanical properties. Dynamic methods rely on solving wave equations, which in their differential form are local in character. On the other hand, elastography techniques can also be classified as external and internal methods, based on their spatial characteristics of excitation. External excitation elastography methods often apply a stress to deform tissue from outside when imaging, while internal excitation elastography methods apply mechanical excitation directly into the region of interest in tissue. The radiation force of ultrasound is usually used as an internal excitation in elastography techniques.
Magnetomotive OCE (MM-OCE) uses magnetic nanoparticles (MNPs), such as iron oxide, to serve as internal force transducer and enable internal mechanical excitation. To perform MM-OCE, first, the MNPs are introduced into specific tissue of interest. Subsequently, magnetic force was provided by placing an external electromagnetic coil near the MNP-laden tissue, while an alternating magnetic field (AMF) is generated by the coil. Finally, the MNP-laden tissue responds to the modulated magnetic force, and the spatio-temporal characteristics of the responsive tissue displacement can be monitored by OCT imaging beam and quantification of the biomechanical properties can be made. Various techniques have been demonstrated with MM-OCE to assess the viscoelastic properties of biological tissues. For instance, natural frequency, resonant frequency, and elastic-wave propagation speed can all be utilized to infer tissue biomechanical properties. To date, MM-OCE has been applied not only for tissue characterization (e.g. differentiating tumor tissue from normal adipose tissue) but a theranostic function can be enabled as well. For instance, the alteration in tissue stiffness can be probed by MM-OCE, which may indicate whether the MNP-laden tissue has been therapeutically-treated (e.g. by magnetic hyperthermia) or not.