American Medical Association Family Medical Guide - American Medical Association [91]
THE PROCEDURE
Because the MRI scanner creates a very strong magnetic field, you cannot carry or wear any metal objects (such as jewelry, eyeglasses, or hair clips) during the examination. Make sure you let your doctor know if you have any metal implants (such as artificial joints, plates, screws, or clips), metal attachments (such as dental braces), or electrical devices (such as a hearing aid), which could be affected by the magnet. It is especially important to tell your doctor if you have a pacemaker because the magnet can make the pacemaker stop working. Although MRI is not known to pose any health risks to a fetus, tell your doctor before having the procedure if you are (or could be) pregnant.
Prostate cancer
This MRI scan of a male pelvis shows an enlarged prostate gland (dark brown) with a cancerous tumor (black kidney-shaped area). The bones of the pelvis (on either side of the prostate) are yellow. The rectum (blue) is below the prostate. The reddish brown areas are muscle.
Lung cancer
This side view of a lung shows a cancerous tumor (yellow area) that is extending into the neck.
For an MRI, you lie on your back on a narrow padded table with a cushion under your knees. You are given a signal button to alert the technician if you begin to feel uncomfortable during the test. (If you don’t like confined spaces, ask your doctor for a sedative before the test.) Because the scanner is very noisy, you are given earplugs or headphones to help block the noise. You may be given a contrast medium through a needle inserted into a vein. You must lie still during the MRI, and you may be asked to hold your breath occasionally. In closed MRI, the table slides into a narrow tunnel inside the scanner. In open MRI, the scanner is quieter and less confining. MRI is painless, and the test can last from 20 to 90 minutes or longer.
Knee joint
MRI can provide clear images of the structures in a joint. This is a side view of the knee joint; the gray areas are the two long bones that meet in the joint (the femur and tibia) and the kneecap (at right on the image).
Nuclear Medicine Imaging
Nuclear medicine imaging (also called radionuclide scanning) shows not only what organs or other structures of the body look like but also evaluates how they function. In nuclear medicine imaging, a small amount of a radioactive substance (called a radionuclide or radioisotope—or tracer) is either swallowed or injected into a blood vessel. The doctor chooses a tracer that confines itself to the organ being examined (for example, iodine concentrates in the thyroid gland). After entering the bloodstream, the tracer travels to the target organ, emitting very small amounts of gamma rays (similar to X-rays). The gamma rays are detected by an instrument called a gamma camera. Depending on the type of tissue, an abnormality may absorb more or less radiation than healthy tissue. For example, a cancerous tumor may absorb more radiation, while dead heart tissue will absorb less. The information is then analyzed by computer and constructed into an image of the organ called a gamma scan (also called a scintiscan or scintigram).
Brain during a dream
In this image, PET scanning has produced a slice image of the brain during a dream. In the REM (rapid eye movement) sleep phase (when dreaming occurs), the brain is active (red areas). The blue areas indicate low brain activity.
Brain
In this image, PET data (brightly colored areas) are combined with three-dimensional MRI scans of the left and right sides of the brain to show the activity in the brain when a person hears a language he or she knows