Synopsis

In recent years, there has been renewed interest in low-field (0.2 T to 0.5 T) and very-low-field (<0.2T) MRI, for many reasons including lower cost, ease of siting, reduced hazard, portability, lower artefacts and enhanced endogenous contrast. The source of signal and its field dependence will be introduced, followed by the sources of noise (coil noise and patient noise), leading to the signal-to-noise ratio (SNR) and its dependence on magnetic field. Finally, the behaviour of contrast as a function of field will be explored, concentrating on contrast between brain grey and white matter.