Excellent description by Liz.
There is nothing really to add unless you want to get technical. The magnets are currently up to about 4 Tesla (about 30'000 times stronger than the earth magnetic field). Hydrogen atoms are little magnets that either point up or down when exposed to an external field. (A testament to the quantum nature of the world). You measure the signal when they flip back after a radio frequency pulse. Why the noise.... In order to localize the signal (i.e. get a picture) one applies strong currents to a coil for a short time. (Hence the woodpecker noise).

Seriously cool stuff ....

A drawback is that it takes relatively long to take a scan and that these things are very expensive to maintain. The powerful ones are all superconducting and contain liquid helium (about 4.2 K, or -269 deg C). Unless you have an electronic implant or a ferromagnetic material there is no real problem.


M