Ultrasound leads to viscoelastic contrasts in magnetic resonance imaging

Using a diffusion weighted gradient in a standard magnetic resonance (MR) Imaging sequence, one is able to visualize movement inside a sample. The diffusion weighted gradient is split into two parts. The first part dephases the ensembles of the spins according to their location and the second part rephases them. The re phasing works properly only if the location of the spins has not changed during the measurement. This means that a movement of the spins leads to a signal loss whereas the level of the movement is encoded in different gray scale values in the MR image. By coupling a 30 ms long US pulse to our sample, a force along the US beam path is produced (acoustic radiation pressure). This 'static' force leads to a displacement of the spins during the US pulse. By applying the US pulse du ring one part of the diffusion weighted gradient we get images whereof we can calculate the displacement which characterizes the viscoelastic properties of our sample. Measurements which show the feasibility and the great advantages of this new method are presented. Further developments and possible clinical applications are discussed