An MRI scanner is basically a large magnet, super-cooled, that utilizes radio waves and computers to create digital images on a screen. There is a horizontal tube running through the magnet from front to back (open at both ends). You lie on your back, and slide into the bore on a special table. Whether or not you go in head first or feet first, as well as how far in the magnet you will go, is determined by the type of exam to be performed.
In conjunction with radio wave pulses of energy, the MRI scanner can pick out a very small point inside the patient's body and ask it, essentially, "What type of tissue are you?" The point might be a cube that is half a millimeter on each side. The MRI system goes through the patient's body point by point, building up a 2-D or 3-D map of tissue types.
This is an axial MRA (magnetic resonance angiography) showing the Circle of Willis in the brain
This is an axial MRI (magnetic resonance imaging) cross-sectional image of the brain