| MadSci Network: Engineering |
Wietse,
That's a good question!
As a matter of fact, an MRI machine actually does use multiple antennas, called "gradient coils," to create an image. During operation, a resonator coil creates a strong field with a single region with sufficient uniformity for use in creating an image. In that region, polar molecules (primarily water) respond to the action of the field by emitting photons which are then picked up by the gradient coils. The signal from the gradient coils is used to determine the spatial location within the imaging region where the photon originated as well as what sort of tissue produced it.
All MRI machines I know of use a single-region resonator and multiple gradient coils; if you wanted multiple images in a single go it is likely you would need to have multiple resonator regions and devise a way to differentiate signals from each of those regions.
A brief, moderately technical overview of how an MRI creates images of different types of tissue is available on Wikipedia (http://en.wikipedia.org/wiki/Mri).
Try the links in the MadSci Library for more information on Engineering.