Magnetic resonance imaging (MRI) - also called magnetic resonance tomography (MRT) - is a method of creating images of the inside of opaque organs in living organisms as well as detecting the amount of bound water in geological structures. It is primarily used to demonstrate pathological or other physiological alterations of living tissues and is a commonly used form of medical imaging. MRI has also found many niche applications outside of the medical and biological fields such as rock permeability to hydrocarbons and certain non-destructive testing methods such as produce and timber quality characterization. Medical MRI most frequently relies on the relaxation properties of excited hydrogen nuclei in water.

In clinical practice, MRI is used to distinguish pathologic tissue (such as a brain tumor) from normal tissue. One of the advantages of an MRI scan is that, according to current medical knowledge, it is harmless to the patient. It utilizes strong magnetic fields and non-ionizing radiation in the radio frequency range. Compare this to CT scans and traditional X-rays which involve doses of ionizing radiation and may increase the chance of malignancy, especially in children receiving multiple examinations.

While CT provides good spatial resolution (the ability to distinguish two structures an arbitrarily small distance from each other as separate), MRI provides comparable resolution with far better contrast resolution (the ability to distinguish the differences between two arbitrarily similar but not identical tissues).

The presence of a ferromagnetic foreign body (such as shell fragments) in the subject, or a metallic implant (like surgical prostheses, or pacemakers) can present a (relative or absolute) contraindication towards MRI scanning: interaction of the magnetic and radiofrequency fields with such an object can lead to: trauma due to shifting of the object in the magnetic field, thermal injury from radiofrequency induction of heating of the object, or failure of an implanted device.

1. Diffusion MRI measures the diffusion of water molecules in biological tissues.

2. Magnetic resonance angiography (MRA) is used to generate pictures of the arteries, in order to evaluate them for stenosis (abnormal narrowing) or aneurysms (vessel wall dilatations, at risk of rupture). The main use of MRA is to evaluate the arteries of the neck and brain, the thoracic and abdominal aorta, and the kidneys.

3. Magnetic resonance spectroscopy (MRS), also known as MRSI (MRS Imaging) and Volume Selective NMR Spectroscopy, is a technique which combines the spatially-addressable nature of MRI with the spectroscopically-rich information obtainable from nuclear magnetic resonance (NMR).

4. Functional MRI (fMRI) measures signal changes in the brain that are due to changing neural activity. The brain is scanned at low resolution but at a rapid rate (typically once every 2-3 seconds).

5. Interventional MRI - Because of the lack of harmful effects on the patient and the operator, MR is well suited for "interventional radiology", where the images produced by an MRI scanner are used to guide a minimally invasive procedure intraoperatively and/or interactively.