How MRI Works

The Other Parts of an MRI Machine

MRI Developments

MRI machines are evolving so that they're more patient-friendly. For example, many claustrophobic people simply can't stand the cramped confines, and the bore may not accommodate obese people. There are more open scanners, which allow for greater space, but these machines have weaker magnetic fields, meaning it may be easier to miss abnormal tissue. Very small scanners for imaging specific body parts are also being developed.

Other advancements are being made in the field of MRI. Functional MRI (fMRI), for example, creates brain maps of nerve cell activity second by second and is helping researchers better understand how the brain works. Magnetic resonance angiography (MRA) creates images of flowing blood, arteries and veins in virtually any part of the body.

Two other magnets are used in MRI systems to a much lesser extent. Resistive magnets are structurally like superconducting magnets, but they lack the liquid helium. This difference means they require a huge amount of electricity, making it prohibitively expensive to operate above a 0.3 tesla level. Permanent magnets have a constant magnetic field, but they're so heavy that it would be difficult to construct one that could sustain a large magnetic field.

There are also three gradient magnets inside the MRI machine. These magnets are much lower strength compared to the main magnetic field; they may range in strength from 180 gauss to 270 gauss. While the main magnet creates an intense, stable magnetic field around the patient, the gradient magnets create a variable field, which allows different parts of the body to be scanned.

Another part of the MRI system is a set of coils that transmit radiofrequency waves into the patient's body. There are different coils for different parts of the body: knees, shoulders, wrists, heads, necks and so on. These coils usually conform to the contour of the body part being imaged, or at least reside very close to it during the exam. Other parts of the machine include a very powerful computer system and a patient table, which slides the patient into the bore. Whether the patient goes in head or feet first is determined by what part of the body needs examining. Once the body part to be scanned is in the exact center, or isocenter, of the magnetic field, the scan can begin.

What goes on during a scan? Find out on the next page.