Elastography
Elastography is, by definition, an imaging technology which measures tissue elastic properties. Ultrasound elastography was the first technology to perform elastography and is widely studied in clinical diagnostic applications to image the biomechanical properties of soft tissues. MRI elastography has also been successfully applied in the clinical imaging field, especially in diagnosing breast cancer. Acoustic shear waves were commonly used as a mechanical excitation, medical imaging modalities were used for imaging the propagation of these waves, and an algorithm for processing the wave images was used to generate quantitative images depicting tissue stiffness. The elastography method was first applied to OCT technique in 1998. The hardware and theory of operation of OCE were described and internal displacements indicating biomechanical properties of gelatin phantoms, pork meat and in vivo skin were shown by using 2-D cross-correlation speckle tracking algorithm.
Resolution and penetration of elastography techniques depend on their imaging modalities. For instance, the typical resolution of ultrasound elastography is between 125 and 200 µm, while resolution in MRI elastography is usually of millimeter scale. The resolution and penetration of some imaging modalities are shown in the figure below. 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.
