Modes of shear wave in magnetic resonance elastography

Manual palpation is used for diagnosing change in stiffness of tissues, due to a pathological state. Unfortunately, this diagnosis tool is limited with organs close to the surface of the body. Magnetic resonance elastography (MRE), also known as palpation by magnetic resonance imaging (MRI), can be used in detecting changes in material properties of the heart, liver, muscle, breast and brain. Alteration in stiffness of tissues can be detected by MRE, by simply measuring the wavelength of the induced shear wave by the actuator, from the phase difference images obtained by MR scanner. In addition to wavelength information, dependence of shear wave displacement amplitude to the frequency and excitation direction carry important information about material properties of the tissue. Modes of shear waves in MRE have not been studied previously. Change in material properties of the tissue, may affect modes of shear waves in MRE. Hence, a shift in natural frequencies may indicate a pathological state in the tissue. We propose a novel method to detect change in stiffness of tissues, by analyzing modes of shear waves and detecting frequency shift in peak displacement of shear waves in MRE. Eigenfrequency simulations are computed for a simple geometric object whose eigenfrequencies are known analytically. Validating simulation results with theoretical values, we are encouraged to continue with eigenfrequency analysis of the brain model. For different directions of motions of head, it is demonstrated by eigenfrequency analysis that brain has modes at certain frequencies. Results of frequency domain analysis indicates that modes of shear waves can be observed in brain by exciting head at its eigenfrequencies with correct excitation in that frequency. Results of frequency domain analysis repeated for neurodegenerative brain model are compared with the findings in healthy brain model. Comparing frequencies of peak displacements in neurodegenerative and healthy model, a constant frequency shift is observed in all frequencies of peak displacements. Preliminary results of modes of shear waves in brain MRE are presented, by sweeping mechanical excitation frequency. This method can be used in detecting change in stiffness of tissues for diagnosing diseases by observing shift in frequency of peak displacement and be beneficial for patient follow-up.