Schematic MRI System

MRI scanners are composed of a number of current-carrying coils, as shown in the schematic diagram in Fig. The principal component of an MRI scanner is the main electromagnet. This is usually comprised of a coil of super-conducting material such as Niobium-Titanium (Nb₃Ti), and is used to generate an intense, uniform magnetic field with negligible resistance. Nb₃Ti has a critical superconducting temperature of 10 K. The wire is therefore kept immersed in liquid helium (with a boiling point of 4.2K) to maintain a temperature below the critical temperature. Shim coils are placed within the bore of the magnet to ensure the magnetic field generated by the main electromagnet is as homogeneous as possible . Shim coils may be super-conducting and/or room-temperature resistive coils of wire. Gradient coils are usually positioned inside the shim coils; when energised, magnetic fields in the z-direction that are proportional to the x-, y-, and z-coordinates are generated. These fields are used for spatially encoding the NMR signal for MRI as described in § 2.5.3 to § 2.5.5. The RF coil is located within the gradient coils closest to the object that is to be scanned. It excites the protons at their resonant frequency, and also detects the signal generated by the precessing spins after excitation. A single RF coil may both transmit and receive the RF signals or two coils may be used to transmit and receive separately. The subject is slid into the scanner on a bed so that the region of the body to be scanned is in the centre of the magnet.

Figure : Schematic of a whole-body MRI scanner with cut-away section to show the principal components.

Most of the electronic apparatus is located outside the main magnet usually in a separate room, since the components may be sensitive to the stray magnetic fields from the scanner. The principal component is the computer system that controls the scanner, but there are also other items of electronic equipment, such as amplifiers, pre-amplifiers, frequency synthesizers, and duplexers that drive the scanner hardware.