Magnetic Resonance Elastography of the Brain.

In clinical medicine, assessment of mechanical properties of the biological tissue is an age-old practice to diagnose various diseases. It is a well-known fact that the biomechanical properties of specific tissue types vary significantly at different stages of physiological and pathological conditions. For instance, a malignant tumor in the breast is stiffer than a benign tumor or the normal tissue[1]. Palpation remains the time-tested and gold standard diagnostic tool to feel the difference between normal and abnormal tissues. However, non-invasive palpation is only applicable to superficial organs and cannot be applied to internal organs without surgery. Especially palpation is impossible to apply to the brain due to the presence of skull bone. Another limitation of palpation is that it is qualitative and may potentially bias the diagnosis. However, radiology medicine has technologically advanced, making it possible to detect tumors in the brain using computed tomography (CT), Magnetic resonance imaging (MRI), and Ultrasound Imaging (US) non-invasively. Unfortunately, none of these conventional techniques provide any information about their biomechanical properties, which is very useful while planning for surgery. Decades of research on brain development in the pediatric and adolescent population using MRI have provided us rich information regarding changes in Grey matter (GM) and white matter (WM) volume [2], CSF content ([3], and myelination [4] and functional parameters such as cerebral blood flow during different activities and in resting condition, etc. In humans, the neural and behavioral changes during childhood and adolescent years are as rapid as the early postnatal development [5]–[7]. So far, these structural and functional changes and their correlation with respect to biomechanical properties of brain tissues were not fully explained. Extensive studies on biomechanical properties could potentially lead to new biomarkers for pathological conditions that could not be explained with the help of existing techniques. For example, neurodevelopmental disorders such as cerebral palsy (CP) caused by brain trauma during fetal development or shortly after birth may not display characteristic lesions in their structural MRI scans[8]. Therefore, the structural or functional images of conventional imaging are not well correlated to the functional ability of the organs [9], [10] and make them insufficient to diagnose and fully understand. Therefore, a detailed understanding of mechanical properties, such as stiffness of the pediatric brain, plays paramount importance in clinical and diagnostic medicine. However, the availability of such data in the literature is very limited, and only a few studies are conducted with a limited age group and number of subjects [10]–[12]. The most established way to assess internal organs’ stiffness in a non-invasive method is based on elastographic techniques with the help of MRI and US systems. These methods rely on the propagation of mechanical motion waves in the tissues. In particular, the MR-based method called magnetic resonance elastography (MRE) is gaining a lot of attention from the research community due to its ability to make quantitative measurements of tissue elasticity non-invasively and three-dimensionally (3D). Elasticity of Soft Tissue In material science, an object/body’s elasticity is defined as “its ability to resist deformation when subjected to a mechanical force and return to its original shape and size when the force is removed.” The force applied to the object is called stress, and the displacement of an object in response is called strain. The slope of the stress-strain curve is the elastic modulus, a physical parameter that reflects the stiffness of a material. According to Hook’s law, pure elastic material has linear stress and strain relation and thereby behaves like a spring. Soft tissues in the human body usually have an inhomogeneous structure that is viscoelastic in nature and displays both elastic and viscous properties. Therefore, the elastic modulus of these tissues comprises “a storage modulus” representing the elasticity component and a “loss modulus” representing the viscosity component. The human brain is a soft tissue composed of neurons, neural stem cells, glial cells, and blood vessels, and its viscous property arises from the geometric organization of these tissue components and their network complexity.