How does an MR scanner operate?

The purpose of the magnet is to provide a uniform magnetic field at all points in the bore of the magnet to align mobile protons within the patient. The strength of the magnetic field is measured in units of testa or gauss-1.0 T equals 10 000 G. The magnetic field produced by the magnet is typically 30 000 times stronger than the magnetic field of the earth. Because it is not possible to obtain a perfectly homogeneous magnetic field, adjustments to the magnetic field strength are made by shimming the magnet. Passive shimming is done by strategically placing small pieces of iron in the bore of the magnet to correct for local field inhomogeneities. Active shimming is performed by positioning shim coils in the magnet and passing current through the windings of these coils . Active shim coils have a separate power supply. Resistive magnets are electromagnets in which the magnetic field is produced by continuously passing an electric current through the windings. Resistive magnets produce heat, and this heat is dissipated by a cooling system. The windings of superconducting magnets are made of special metal alloys (eg, niobium-titanium alloy) that lose electrical resistance when cooled to extremely low temperatures. The windings of these magnets are supercooled by liquid helium. Once the electrical current is passed through these supercooled windings , the current will continue almost indefinitely . Because heat eventually will cause the cryogen to boil away, new cryogen must be added occasionally to the system, usually every 4 to 6 weeks. In the case of superconducting magnets , an emergency quenching system is included to Quickly dissipate the magnetic field in the event of an emergency. The magnet and the shims may be resistive, superconducting, or permanent magnets. Resistive magnets require constant direct-current power supplies. Superconducting magnets intermittently require power supplies to maintain the appropriate current in the supercooled windings. There are relatively few clinical systems using permanent magnets. Permanent magnets operate in the lower field -strength range (up to 0.3 T), and they are extremely heavy.