MRE OF THE BRAIN

MRE is a very sensitive technique, capable of recording waves with displacement amplitudes as small as 1 micron (10-6 m). This sensitivity lets us image shear waves propagating in the brain of a healthy subject without any risk of injury.

This image is a standard axial MRI image of a normal brain. The shear waves can be viewed with a color map to aid visualization, and with this map the shear wave pattern typically looks like this.

If the displacements of a shear wave are magnified approximately 10,000 times, they can be used to "warp" a standard axial image to show where the wave propagates relative to brain structures. This is a greatly exaggerated version of the periodic motion of the brain in the S/I direction during the MRE scan.

MRE OF MUSCLE

This is a composite image, with the waves generated in the biceps muscle overlaid on a standard coronal MR image of the same volunteer. This composite shows several features noticeable in in vivo experiments, such as wave attenuation and the effects of boundary conditions.

Intuitively it is obvious that a stretched muscle will be stiffer than a flaccid one. We have been able to demonstrate this with ex vivo passive tension experiments as well as with in vivo experiments in the calf muscles of healthy volunteers. It is also obvious that a flexed or contracted muscle will be stiffer than a relaxed one. We have been able to observe and quantify this effect in the biceps muscles of healthy volunteers.

This image shows the shear waves (in color for easy visualization) generated in the arm of a volunteer under the three different loading conditions indicated.