Magnetic resonance imaging (MRI) is a diagnostic technique that provides high quality cross-sectional images of organs and structures within the body without x-rays or radiation.
During magnetic resonance imaging (MRI), the patient lies inside a massive, hollow, cylindrical magnet and is exposed to short bursts of a powerful magnetic field.
The nuclei (protons) of the body's hydrogen atoms normally point randomly in different directions, but in a magnetic field, they line up parallel to each other, like rows of tiny magnets. If the hydrogen nuclei are knocked out of alignment by a strong pulse of radio waves, they produce a detectable radio signal as they fall back into alignment. Magnetic coils in the machine detect these signals, and a computer changes them into an image, based on the strength of the signal produced by different types of tissue.
Tissues that contain a lot of hydrogen (such as fat) produce a bright image; those that contain little or no hydrogen (such as bone) appear black.
MRI allows images to be constructed in any plane. It is particularly valuable in studying the brain and spinal cord. The technique reveals tumors vividly, indicating their precise extent (size and shape) and produces impressive images of the internal structure of the eye and ear.
MRI also produces detailed images of the heart and major blood vessels, provides images of blood flow (MRA - magnetic resonance angiogram), and is useful for examining joints and soft tissues, particularly in the knee.
Despite the relative safety of MRI, two basic risks are associated with the procedure:
The first risk relates to the immediate environment of the magnetic resonance imaging suite, where appliances and other magnetic materials may become high-speed projectiles in the presence of powerful magnetic forces. Materials that are dangerous in the radiographic suite include oxygen bottles, stretchers, and wheelchairs made of magnetic materials. Emergency carts also contain such materials. If carts are left in the imaging area, they may act as missiles against either the machine itself or the patient. Stretchers for use in imaging suites should be made of aluminum, plastic or stainless steel, since these materials do not have a magnetic attraction.
The second risk involves patients who have metallic implants, including pacemakers, prosthetic cardiac valves, surgical clips, orthopedic appliances, penile implants, cochlear implants, intravascular filters, stents, coils, dental materials and shunt connectors. This also includes shrapnel.
Metallic implants in patients can contraindicate MRI examinations, and should therefore be documented by careful histories. Cardiac pacemakers, for example, would be dangerous in MRI examinations. Two forces act on the pacemaker - the magnetic field and the gradient field. MRI may result in a synchronous pulsing of the numerous magnetic and electronic parts of the pacemaker when the pacemaker is exposed to these fields.
Surgical clips can also cause problems in MRI. Some clips have no magnetic properties; unfortunately, they cannot be distinguished from the dangerous types by radiographic visualization. Ferromagnetic clips can cause distortion in the MR image and make interpretation impossible. Therefore, a detailed surgical history should be obtained before an MRI is performed. The presence of clips, specifically those used in patients who have undergone an operation for cerebral aneurysms, is a contraindication to MRI screening.
Metallic implants, such as hip prosthesis, have been shown to absorb heat under radiofrequency pulse, but the clinical significance of this finding is unknown. No evidence suggests that significant heating of the metal occurs.
Although not hazardous in MRI, dental brace wire causes image artifacts. Vena cava filters and Gianturco stents are likely to be deflected by magnetic fields and cause artifacts, making interpretation of the MRI by the radiologist more difficult.