a. MR Elastography

Magnetic Resonance Elastography (MRE) is a non-invasive technique used to quantify the viscoelastic properties (shear modulus and viscosity) of tissues. Because the viscoelastic properties change significantly between healthy and pathological tissues, this technique opens new perspectives in the early detection of diseases such as breast, liver or some visceral cancers. The MRE technique is based on the measurement of the micro-displacements generated by the propagation of mechanical waves in the tissues. This technique usually requires the synchronization of the mechanical driver used to generate the waves within the organ to be explored (frequency in the range of 50 to 200 Hz) with a dedicated MRI sequence encoding the micro-displacements (imaging sequence with the addition of an oscillating gradient, the Gradient Sensitive Motion (GSM), whose duration is conditioned by the period of mechanical excitation). The phase images obtained are then processed, using inversion methods, to locally determine the propagation rate and the attenuation of mechanical shear waves in the organ to be explored. The advantage of this technique is to generate absolute viscoelastic 3D maps without having to know the applied constraint. Main applications targets on humans are the liver in cases of cirrhosis and carcinogenesis.

b. NMR and Brownian Motion. In NMR restricted motion can be visualized using two main approaches: Diffusion techniques or dipolar interaction related techniques. Diffusion techniques gives access to fiber orientation and allows the reconstruction of diffusion tensor maps for fiber tracking. These techniques are well suited and widely used to study slow-moving tissues but becomes very tricky to use in the case of moving organs. Dipolar interaction based techniques like T1ρ, T2ρ and dipolar contrast are less specific and at the present state, gives information limited to anisotropy of tissues but in counterpart they are more robust to movement. Our goal is to be able to study the adantages and drawbacks of these two approach to be able to provide the clinician the best tools for liver or heart investigation.