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.
Tissue engineering for regenerative medicine have been developing for a few decades now and the number of applications is increasing to tackle the shortage of organ donors. To date, only few systems can allow both monitoring and 3D characterization of tissue constructs during their growth. In this study, we decided to focus on following the Apparent Diffusion Coefficient (ADC) known to be a marker of cell density and built a MR-Bioreactor to probe the ADC of a growing tissue. In this preliminary work, we were able to follow the cell density of a tumor tissue model using our dedicated MR-bioreactor.
In the last few years, there has been an increasing interest from the scientific community in the fabrication of flexible coils. Several methods can be used for the manufacture of flexible coil, mainly screen-printed coils on flexible substrates. In this work, three different screen printing coils with different layers of silver ink were manufactured and their quality factors were measured on bench. A MR-coil combining screen-printed process with electrodeposition step was also built. The additional manufacture step allowed improving drastically Q factor of our screen-printed coil with more than one order of magnitude while maintaining good flexibility of the substrate.
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