An MRI Wind Tunnel

The subject of last year’s Nobel Prize in Physiology/Medicine, magnetic resonance imaging (MRI) is well known for making beautiful images of the inside of the body. MRI is less recognized for its ability to track movement, such as clinical studies of blood flow.

Now, researchers in Canada (Benedict Newling, University of New Brunswick, bnewling@unb.ca) have invented a new MRI method suitable for measuring much faster fluid speeds, ten times more rapid than the fastest human blood flow. Their approach uses a constant, shorter-than-usual measurement interval (0.6 milliseconds).

In effect, the MRI scanner becomes a new type of practical wind tunnel, one that's non-invasive too. An obstruction of any shape can be placed in the flow at the center of the scanner. The resulting flow of liquid or gas around the object is readily measured.

As an example, the researchers measure gas flow past a wing at realistic speeds (corresponding to a stalled aircraft) and compare them with computer-based calculations of the expected flow (see figure at Physics News Graphics).

The flows they measure are highly turbulent, which means the fluid velocity at every position varies rapidly around some average value. The MRI measurement contains information about both the average velocity and fluctuations.

MRI is naturally three-dimensional and works just as well in opaque or transparent fluids. Furthermore, MRI can measure several positions simultaneously, unlike most conventional wind-tunnel measurements, and therefore has the potential to deliver measurements in substantially reduced times. (Newling et al., Physical Review Letters, 8 October 2004)