Magnetic Resonance Imaging (MRI)
MRI is a fantastic tool. It's a kind of like an x-ray, without the radiation of the x-ray. The picture comes from atoms within cells, arranged in a magnetic field. After they've all lined up in the field, if you bump them and then get out of the way, they all return to the arrangement they had in the magnetic field in the first place. You "bump them" with a pulse of energy, and when they relax back to where they were, they give off some of that energy. Highly sensitive "cameras" pick up that energy.
The atoms will give off that energy in different ways depending on what kind of tissue they are in. If they're in water, such as in blood, they give back a certain signal; but if they're in bone, a different signal; and in brain cells, a different one yet. The cameras, and the computers to which they're connected, sort out those different signals into an image of bone, water and cells. And there you have an image.
It takes a big (we're talking really big) magnet to arrange the atoms in the first place, then a lot of cameras to catch the energy coming off as the atoms relax, then a big computer to assemble that information into an image. So these MRI machines are expensive to build, own, operate, and repair. The cost is part of our increasing national medical budget, as hospitals compete with each other buying bigger and better machines. The next machine "everybody will want to have" is a PET scanner, which is like this MRI -- but instead of the magnet creating the energy that the cameras measure, the cameras measure radiation from an atom that's been joined to sugar injected into the person's blood.
For a spectacular presentation of PET and MRI scans, check out this great teaching site with great scans and pretty plain english: