From dynamic to passive elastography, what technique used?

Elastography is a method that allows to quantify mechanical properties of biological tissues. In medical imaging, this last technique has been developed mainly in ultrasound (US) and in magnetic resonance imaging (MRI). Both imaging techniques have advantages and drawbacks depending on the organs and pathologies investigated. In this presentation we focused on two organs, where each technique has its advantage to investigate mechanical properties: brain and muscle. In brain, the preferred technique is MRI since US propagation is very complicated through the skull bone. Taking advantage of 3D acquisition, we investigated brain tumors with a new elastography approach: passive elastography. From the natural vibration, such as cardiac beating or respiration, we locally recover the natural shear wavelength that propagates giving access a stiffness mapping of the brain. Results are compared with US acquire during neurosurgery, showing a good accordance between both techniques. In muscles, US are used to recover not only stiffness but also others mechanical parameters such as viscoelastic anisotropy or shear non-linearity. Special US sequences were developed to quantify these two parameters in different physiological conditions that change in real-time. This presentation shows that depending on the investigated organ, physicists and physicians need to adapt the imaging strategy to better characterize tissues.