"How MRI works
Main article: Physics of magnetic resonance imaging
MRI requires a magnetic field that is both strong and uniform. The field strength of the magnet is measured in tesla – and while the majority of systems operate at 1.5T, commercial systems are available between 0.2T–7T. Most clinical magnets are superconducting which requires liquid helium. The lower field strengths can be achieved with permanent magnets, which are often used in "open" MRI scanners for claustrophobic patients.[23]To perform a study, the patient is positioned within an MRI scanner which forms a strong magnetic field around the area to be imaged. In most medical applications, protons (hydrogen atoms) in tissues containing water molecules are used to create a signal that is processed to form an image of the body. First, energy from an oscillating magnetic field is temporarily applied to the patient at the appropriate resonant frequency. The excited hydrogen atoms emit a radio frequency signal which is measured by a receiver coil. The radio signal can be made to encode position information by varying the main magnetic field using gradient coils. As these coils are rapidly switched on and off they create the characteristic repetitive noises of an MRI scan. The contrast between different tissues is determined by the rate at which excited atoms return to the equilibrium state. Exogenous contrast agents may be given intravenously, orally or intra-articularly.
