Definition of Magnetic resonance imaging

A special radiology technique designed to image internal structures of the body using magnetism, radio waves, and a computer to produce the images of body structures. In magnetic resonance imaging (MRI), the scanner is a tube surrounded by a giant circular magnet. The patient is placed on a moveable bed that is inserted into the magnet. The magnet creates a strong magnetic field that aligns the protons of hydrogen atoms, which are then exposed to a beam of radio waves. This spins the various protons of the body, and they produce a faint signal that is detected by the receiver portion of the MRI scanner. A computer processes the receiver information, and an image is produced. The image and resolution is quite detailed and can detect tiny changes of structures within the body, particularly in the soft tissue, brain and spinal cord, abdomen and joints.

An MRI is painless and has the advantage of avoiding x-ray radiation exposure. There are no known risks of an MRI. The benefits of an MRI relate to its precise accuracy in detecting structural abnormalities of the body. Patients with heart pacemakers, metal implants, or metal chips or clips in or around the eyes cannot be scanned with MRI because of the effect of the magnet. Metallic chips, materials, surgical clips, or foreign material (artificial joints, metallic bone plates, or prosthetic devices, etc.) can significantly distort the images obtained by the MRI scanner. Similarly, patients with artificial heart valves, metallic ear implants, bullet fragments, and chemotherapy or insulin pumps should also not have an MRI. Claustrophobia can be a problem. For an MRI, patients lie in a closed area inside the magnetic tube. Some patients experience a feeling of claustrophobia.

In 2003 the Nobel Prize in Physiology or Medicine was awarded to the American Paul C. Lauterbur (1929-) and the Briton Sir Peter Mansfield (1933-) "for their discoveries concerning magnetic resonance imaging." The presentation speechs was given by Professor Hans Ringertz, Chairman, The Nobel Assembly at Karolinska Institutet, who recounted the history of MRI, as follows.

Felix Block and Edward Mills Purcell first demonstrated the physical phenomenon of nuclear magnetic resonance in 1946. These discoveries were awarded a Nobel Prize in Physics in 1952. Magnetic resonance occurs in magnetic fields between atomic nuclei and electromagnetic waves of radio frequencies. Atomic nuclei have a magnetic moment and in the magnetic field, their spin depends on the strength of the field. The direction of magnetization resulting from the magnetic moments can change. This happens when the nuclei are in resonance with radio waves of the same frequency as the frequency of their own rotation. In the same way the nuclei can send back radio waves, when there is a change in the direction of the magnetic moment.

Initially, magnetic resonance was mostly used for spectroscopy, to study structures of chemical compounds. In the early 1970s Paul Lauterbur discovered the possibility to create a two-dimensional image by introducing gradients in the magnetic field. By analysis of the characteristics of the emitted radio waves, he was able to determine their origin. This made it possible to build up images of structures that could not be visualized with other methods.

Peter Mansfield discovered further possibilities to utilize gradients in the magnetic field. He showed how the radio signals could be mathematically analysed, which made it possible to develop a useful imaging technique. Mansfield also showed how images could be achievable extremely fast using magnetic resonance. This became technically possible in clinical medicine about a decade later.

Using a metaphor, magnetic resonance spectroscopy is like listening to a radio broadcast of a symphony in the 1940s. Imaging would then be like sitting in a concert hall listening to the symphony, and not only hearing but also seeing the instruments, how they play and where they are located, like organs in the human body. And when you hear the violins, you can even recognise, as in a magnetic resonance image, a false note like a disease process in that body.

Last Editorial Review: 6/29/2004