Medical imaging includes a broad spectrum of devices and technologies, using a variety of modalities. For example, magnetic
resonance imaging (MRI) uses three kinds of electromagnetic radiation to generate the desired images.
First, a main powerful magnet (normally 0.5 to 3.0 tesla) can be
used to polarize hydrogen atoms in the tissues of interest (brain,
muscle, etc.) because hydrogen has a large magnetic moment. Second, gradient magnets located within the main magnet are switched
on and off rapidly to alter the main magnetic field for the area targeted to create image slices.
Third, radio-frequency (RF) pulses that are specific to hydrogen
are directed at the target tissue perpendicular to the main magnetic
field, causing the protons within the hydrogen atoms to spin in a different direction (resonate) at a particular frequency (i.e., they
become "excited").
When the pulse is removed, the hydrogen atoms "recover" and
return to their natural alignment. In doing so, they emit pent-up
energy in the form of radio waves, producing a signal received with
an RF antenna. The signal is then converted using a discrete Fourier
transform (DFT) into a photo slice.