MRI Scan Overview
Magnetic resonance imaging (MRI scan) is a diagnostic test that uses electromagnetic radio waves (i.e., electrical current and magnetism) to produce two- or three-dimensional (2-D or 3-D) computer images of internal body structures such as organs, muscles, bones, and nerves.
Magnetic resonance imaging can be performed on any part of the body and, unlike other imaging procedures (e.g., CT scan, x-ray), does not involve radiation. Compared to CT scan, MRI produces more detailed images of soft tissue and organs, differentiates between similar tissues more effectively, and produces less detailed images of bone.
MRI scans can be used to accurately detect and locate tumors and to determine if a tumor has spread. MRI scan is often used to examine the following:
- Abdomen (e.g., liver, kidneys, spleen)
- Chest (e.g., heart, lungs, major blood vessels)
- Joints (e.g., shoulders, hips, knees)
- Nervous system (e.g., brain, spinal cord, nerves)
- Pelvis (e.g., reproductive organs)
- Spinal column
A special type of magnetic resonance scan (called magnetic resonance angiography [MRA]) is used to produce images of blood vessels (e.g., arteries, veins). This test is used to detect blood vessel abnormalities such as aneurysms and atherosclerosis (i.e., plaque build-up in arteries) and to evaluate blood flow.
Patients who have any type of implant should notify the radiologist prior to the procedure, because certain types of metal implants (e.g., inner ear [cochlear] implants, brain aneurysm clips, some artificial heart valves, pacemakers, older vascular stents) are not compatible with MRI. Women who have a contraceptive intrauterine device (IUD) also should notify the radiologist, because some of these devices contain copper wire. Patients who work with metal (e.g., welders) should have x-rays taken before undergoing an MRI scan to make sure there are no metal fragments in the body. All metal (e.g., jewelry, watches) must be removed before the procedure.
The image and resolution produced by MRI is very detailed and the test can be used to detect small structural changes in the body. In some cases, a contrast agent (e.g., the element gadolinium) is injected intravenously (i.e., through a vein) prior to MRI scan to increase the accuracy of the images.
During an MRI scan, the patient must remain as still as possible. Sometimes a sedative is administered to help the patient relax during the procedure. If a sedative is used, the patient will be unable to drive home following the procedure and should have a friend or family member with him or her. MRI usually takes between 1 and 2 hours to perform.
The MRI scanner consists of a cylinder surrounded by a magnet, a receiver, and a computer. During the procedure, the patient is placed on a moveable bed, which is inserted into the cylinder.
The MRI scanner creates a strong magnetic field, exposing hydrogen atoms in water molecules in the body to radio waves and causing them to move into different positions. The way these hydrogen atoms move and then move back into their original position once the scanner is turned off provides information about tissue density in the area of the area of the body that has been scanned. A computer processes information about how the molecules move and creates a detailed image of internal body structures. Tissue that contains less water (i.e., hydrogen atoms), such as bone, appears darker and tissue that contains more water appears lighter.
MRI scan of the head can be used to evaluate patients with headaches, seizures, muscle weakness, blurred vision, and hearing loss. The test can also be used to evaluate bleeding and swelling in the brain and to detect spinal cord injury in patients who have experienced traumatic brain injury (TBI).
MRI scan of the brain can also be used to detect strokes, tumors, and aneurysms (widening of a blood vessel).
MRI scan of the spine may be used to evaluate patients with back pain, herniated disks, narrowing of the spinal canal (spinal stenosis), and degeneration of the disc spaces between the vertebrae (cervical spondylosis). It can also be used to detect spinal fractures and tumors.
To evaluate the heart, lungs, liver, reproductive organs, other organs (e.g., spleen, kidneys) and glands (e.g., adrenal glands), MRI scans of the chest, abdomen, and pelvis can be performed. A tear or aneurysm in a major blood vessel (e.g., aorta) can also be detected using MRI scan.
MRI scan can also be used to examine soft tissue, joints, and bones for injury, infection (e.g., brain abscess, meningitis, spinal epidural abscess, osteomyelitis), and tumors. The procedure often is used to evaluate injuries to bones, cartilage, ligaments, muscles, and tendons (e.g., knee and shoulder injuries).
A radiologist (physician who specializes in imaging procedures) interprets the results of an MRI scan.