Abstract
Since 1995, Magnetic Resonance Elastography (MRE) has been constantly developed as a non-invasive diagnostic tool for quantitative mapping of mechanical properties of biological tissues. Indeed, mechanical properties of tissues vary over five orders of magnitude (the shear stiffness is ranging from 102 Pa for fat to 107 Pa for bones). Additionally, these properties depend on the physiological state which explains the granted benefit of MRE for staging liver fibrosis and its potential in numerous medical and biological domains. In comparison to the other modalities used to perform such measurement, Magnetic Resonance (MR) techniques offer the advantages of acquiring 3D high spatial resolution images at high penetration depth. However, performing MRE tissue characterization requires low frequency shear waves propagating in the tissue. Inducing them is the role of a mechanical actuator specifically designed to operate under Magnetic Resonance Imaging (MRI) specific restrictions in terms of electromagnetic compatibility. Facing these restrictions, many different solutions have been proposed while keeping a common structure: a vibration generator, a coupling device transmitting the vibration and a piston responsible for the mechanical coupling of the actuator with the tissue. The following review details the MRI constraints and how they are shaping the existing actuators. An emphasis is put on piezoelectric solutions as they solve the main issues encountered with other actuator technologies. Finally, flexible electroactive materials are reviewed as they could open great perspectives to build new type of mechanical actuators with better adaptability, greater ease-of-use and more compactness of dedicated actuators for MRE of small soft samples and superficial organs such as skin, muscles or breast.
Highlights
For a long time, physicians have been using palpation to detect diseases revealed by qualitative changes of tissue stiffness
Magnetic Resonance Elastography (MRE) has the potential of characterizing in vivo the mechanical properties of various tissues in 3D and has been approved by the Food and Drug Administration (FDA) for liver fibrosis grading [10]
MRE can be used to quantify for instance neurodegenerative disease [13, 14], brain tumor malignancy [15, 16] or more recently, brain functional activity revealed by fast measurement of mechanical changes induced by neuronal activity [17]
Summary
Physicians have been using palpation to detect diseases revealed by qualitative changes of tissue stiffness. MRE relies on the use of an external mechanical actuator, which is challenging because of the constrained environment of the MRI system This actuator should be able, according to the dynamic approach of MRE (as opposed to the static approach), to generate low frequency (generally between 20 and 1,000 Hz) shear waves. Despite the high diagnosis performance of MRE [4], this technological lock prevents MRE to be democratized in clinic and in preclinic experiment In this challenging context, the question is: can technological progress allow designing an external mechanical actuator, as transparent as the shear wave generation is for users of ultrasound elastography? The accuracy of the MRE measurements depends on the quality of the mechanical excitation source, the acquisition method of the displacement map and the reconstruction algorithm used to obtain the shear modulus
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