Magnetic Resonance Imaging

Magnetic Resonance Imaging (MRI), medical diagnostic technique that combines strong magnetic fields, radio waves, and computer technology to create images of the body using the principles of nuclear magnetic resonance. A versatile, powerful, and sensitive tool, MRI can generate thin-section computerized images of any part of the body—including the heart, arteries, and veins—from any angle and direction, without surgical invasion and in a relatively short period of time. MRI also creates “maps” of biochemical compounds within any cross section of the human body. These maps give basic biomedical and anatomical information that provides new knowledge and may allow early diagnosis of many diseases. In 2003 Paul Lauterbur of the United States and Sir Peter Mansfield of the United Kingdom shared the Nobel Prize in physiology or medicine for their contributions to MRI technology.

MRI is possible in the human body because the body is filled with small biological “magnets,” the most abundant and responsive of which is the proton, the nucleus of the hydrogen atom. The principles of MRI take advantage of the random distribution of protons, which possess fundamental magnetic properties. Once the patient is placed in the cylindrical magnet, the diagnostic process follows three basic steps. First, MRI creates a steady state within the body by placing the body in a steady magnetic field that is 30,000 times stronger than Earth’s magnetic field. Then MRI stimulates the body with radio waves to change the steady-state orientation of protons. It then stops the radio waves and “listens” to the body's electromagnetic transmissions at a selected frequency. The transmitted signal is used to construct internal images of the body using principles similar to those developed for computerized axial tomography, or CAT scanners.

In current medical practice, MRI is preferred for diagnosing most diseases of the brain and central nervous system. MRI scanners provide equivalent anatomical resolution and superior contrast resolution to that of X-ray CAT scanners. They produce functional information similar to that of positron emission tomography (PET) scanners but with superior anatomical detail. MRI scanners also provide imaging complementary to X-ray images because MRI can distinguish soft tissue in both normal and diseased states. Although an MRI scan is relatively expensive, it may actually reduce costs to patients and hospitals by providing diagnostic evaluation to outpatients and thereby frequently limiting more expensive hospitalization. Because it does not use ionizing radiation, MRI is risk free except for patients with cardiac pacemakers, patients who might have iron filings next to their eyes (for example, sheet metal workers), patients with inner ear transplants, and patients with aneurysm clips in their brains.

In the early 2000s open MRI scanners were introduced as an alternative to the standard MRI machine, which encloses the body, requires the patient to lie immobile for 45 minutes, and makes disturbing, loud noises. The open MRI scanners are much less confining and far quieter. While the scans they provide are not as detailed as traditional MRI scans, the open MRI is a highly effective device for patients who fear the loud, dark experience of the closed, cylindrical MRI machine.