This presentation provides a high-level survey of the rationale, physical basis, and current status of MR Elastography, as well as a perspective on the development of ultrasound and MRI-based elastography technologies over the last several decades.
Many disease processes cause profound changes in the mechanical properties of tissues, yet none of the conventional medical imaging techniques such as CT, MRI, and ultrasound are capable of quantitatively delineating these properties. Magnetic Resonance Elastography (MRE) is an imaging technology that employs a novel MRI-based technique to visualize propagating acoustic shear waves in the body. Inversion algorithms are used to process these wave images to generate quantitative maps of tissue mechanical properties such as stiffness (magnitude of the complex shear modulus), storage modulus, loss modulus, nonlinearity, dispersion, and anisotropy.
Currently, the most important clinical application of MRE is in diagnosing liver disease, and specifically to quantitatively assess the presence of hepatic fibrosis. Extensive clinical experience and published evidence indicates that MRE is at least as accurate as liver biopsy for this diagnosis, while also being safer, more comfortable, and less expensive. MRE is being explored in many other clinical applications including preoperative evaluation of brain tumors, assessement of neurodegenerative disease, functional imaging of skeletal muscle, heart, and lungs, and evaluating breast and prostate cancer.
In research applications, MRE is emerging as a useful quantitative tool for assessing the mechanical properties of organs and tissues in clinical and preclinical studies, and engineered tissue constructs. There is increasing interest in assessing the mechanical properties of the extracellular matrix. Cells sense their mechanical environment and react to the dynamic and static properties of the extracellular matrix environment through mechanotransduction and cytoskeletal remodeling. This process profoundly influences the behavior of cells in diverse areas such as morphogen-mediated cell programming and differentiation in developing embryos, activation of hepatic stellate cells to initiate liver fibrosis, and cell behavior in engineered tissue constructs. Abnormal mechanobiology is now known to be instrumental in many disease processes. For instance, recent research has shown that increased matrix stiffness can drive the onset of malignant transformation in some tissues.
In physics and engineering, elastic properties are described quantitatively as moduli. For instance, Young’s modulus of elasticity describes longitudinal deformation (strain) in response to longitudinal force (stress). The shear modulus relates transverse strain to transverse stress. The bulk modulus describes the change in volume of a material resulting from an applied compressional stress. Another physical property of isotropic Hookean solids is Poisson’s ratio, which is the ratio of transverse contraction per unit breadth divided by longitudinal extension per unit length.
Among these properties, the Young’s modulus and the shear modulus correspond most closely to the tissue characteristics elicited by palpation. In addition, most soft tissues have mechanical properties that are intermediate between those of fluids and solids and as a result, the value of Poisson’s ratio for soft tissues is very much like that of a liquid. This leads to the result that there is a consistent relationship between the Young’s and shear moduli of most soft tissues in that they differ only by a scaling factor of 3. Another characteristic that soft tissues share with liquids is that they are nearly incompressible. In contrast to the many orders of magnitude over which the Young’s and shear moduli are distributed, the bulk moduli of most soft tissues differ by less than 15% from that of water.
Most tissues are viscoelastic, so that the shear modulus is a complex number, with a real part corresponding to elastic behavior (storage modulus) and an imaginary part corresponding to damping behavior (loss modululs). In addition, most tissues are also nonlinear (the modulus changes with the degree of deformation) and many are anisotropic.s hese concepts represent a simplification of the mechanical behavior of soft tissues, which in general are anisotropic and nonlinear. All of these properties are potential diagnostic biomarkers for characterizing tissue.
Over the last several decades, investigators have developed and tested a number of approaches for noninvasively assessing the mechanical properties of tissue with imaging methods. In general, the approach is to apply some form of mechanical stress and to use an imaging technology to observe the resulting strain (deformation). The mechanical stress may be static or dynamic. Many different modalities have been used to observe the resulting static or dynamic strain, including radiography, ultrasonography, CT, and MRI.
An imaging technique that measures stain only may be used to qualitatively assess the relative stiffness of tissues by depicting differences in deformation caused by similar levels of stress. While the term elastography has been used to refer to some these techniques, the term more- properly applies to methods that analyze measurements of the stress and resulting strain to estimate the elastic properties of the imaged material.
This presentation will provide a high-level survey of the rationale, physical basis, and current status of MRE, as well as a perspective on the development of ultrasound and MRI-based elastography technologies over the last several decades.