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.