The heart of MRI is the superconducting magnet. The magnet uses liquid helium to keep the main coil in a superconducting state and therefore requires no external power supply. They align both with the field and against the field. This leads to a slight excess of protons aligned with the field and hence a small net magnetisation.
Spatial information is encoded by means of magnetic field gradients.
The returned signals are processed to reconstruct images. It is possible to run experiments where the images are sensitive to density, velocity and chemical environment of protons. The sample of interest must contain protons in the liquid or near-liquid state such as a gel. Hence MRI is ideal for investigating many opaque and multiphase flows. Applications Magnetic resonance imaging can provide information where traditional fluid mechanics instruments have limited capability.