File:Coils (expand the description below for a detailed explanation).webm

English: Magnetic fields and moving electric charges are closely intertwined. A charge moving through a magnetic field will curve, and a charge moving in a curve will create a magnetic field. In fact, all magnetic fields -- including those in MRI, the earth's, and those of refrigerator magnets -- is the result of rotating charges.

Magnetic fields are generated in MRI by electric coils. The shape of the coil forces charges flowing along them to move in a circle, and this produces a magnetic field. The coil to the left has a constant, direct current (DC). Thus, it produces a constant magnetic field. The coil to the right has an alternating current (AC) and its magnetic field oscillates with the current. Notice that when the current equals zero, there is no magnetic field, and when the current changes direction, so does the magnetic field. Likewise, a coil that is exposed to a magnetic field will have a current forced through it. This is how antennas work. Electromagnetic radiation, which is simply a travelling pair of fields, one electric and one magnetic, induces a current in the coil which can be measured.

Both techniques are used in MRI. The patient is exposed to magnetic fields created by coils, and electromagnetic radiation from the patient is measured by antennae.