Physical principles of MRI

In this article the physical principles of MRI are described. The physics underlying each process is followed by a description of the corresponding hardware: nuclear polarization and magnets; resonance and transmit/receive hardware; the action of gradients and gradient hardware. The use of frequency-encoding for slice-selection and 1D imaging is explained. Fourier imaging is explained in terms of complex spatial frequency components, allowing a natural explanation of standard 2D imaging and echo planar imaging (EPI). Relaxation mechanisms and contrast generation for T₁, T₂, T_2', and diffusion-weighted imaging are presented. The effects of static field inhomogeneities on the imaging process are described.