Magnetic resonance imaging (MRI) relies on resonance induced in a strong magnetic field. Different tissues will resonate at different frequencies, producing a different image (see Figure 5.7). Images of both the venous and the arterial system can be produced with outstanding quality without the need for any intravenous intra-arterial injections or the use of contrast material. These investigations are slow and expensive and are currently reserved for specific indications. Sensitivity to contrast material, the presence of co-existing renal failure and the investigation of the cerebral circulation are currently the principal indications. Magnetic resonance imaging would not normally be carried out for the assessment of straightforward arterial disease.