Introduction
This topic covers the physical principles of magnetic resonance imaging (MRI): how hydrogen nuclei in the body generate signal in a strong magnetic field, how that signal is localised and turned into an image, how tissue contrast is produced, and what the main advantages, limitations and safety rules of MRI are. It forms the basis for understanding MRI sequences and their clinical use.
Physical basis of the signal
MRI uses the magnetic properties of atomic nuclei with an odd number of protons, most importantly the hydrogen nucleus (proton), which is abundant in water and fat. Each proton has spin and a small magnetic moment.
- Without an external field, the spins are randomly oriented and the net magnetisation is zero.
- In a strong static field B0 (typically 1.5 T or 3 T), spins align parallel or antiparallel to B0, with a slight excess in the parallel (lower energy) state. This small excess creates a net longitudinal magnetisation.
- The spins precess around B0 at the Larmor frequency, which is directly proportional to field strength:
Here \gamma is the gyromagnetic ratio, a constant for each nucleus (about 42.58 MHz/T for hydrogen). At 1.5 T the proton frequency is about 64 MHz.