MRI is a test that produces very accurate pictures of the brain and its arteries without x-rays or dyes. This test is useful for detecting a wide variety of brain and blood vessel abnormalities, and can usually determine the area of the brain that is damaged by an ischemic stroke. During this painless test, you lie on a table that moves into the opening of the MRI machine. The machine creates a magnetic field which briefly alters the water molecules in your brain cells. The response to this magnetic field is then detected and used to create an image of the brain. Although MRI scans can be used on any part of the body, the following description applies only to MRI of the head.
Why do doctors use MRI?
MRI is useful for imaging soft tissues such as the brain because it shows great detail. It can detect minute differences, even between areas that are similar (unlike CAT scans, which are useful in imaging bone and soft tissue, but with less detail). MRI can often demonstrate brain abnormalities which are too small or located in regions of the brain that cannot be seen well by CAT scans. Another benefit of MRI is that it can be performed without x-rays or dyes (although many times, an intravenous dye called gadolinium is used to image the brain and its blood vessels). Brain MRI is commonly used to detect and diagnose many kinds of abnormalities of the skull, brain, and spinal cord. In addition to stroke, MRI is used to diagnose abnormal growths such as tumors, blood vessel abnormalities, infections, or disorders such as multiple sclerosis. MRI can provide direct views of the body from almost any direction, while CAT scans only provide images in an axial orientation. Medical images taken of the human body are usually displayed in three orientations:
- Coronal orientation: in a slice dividing the head into front and back halves.
- Sagittal orientation: in a slice dividing the head into left and right halves.
- Axial orientation: in a slice dividing the head into upper and lower halves.
There are several different kinds of MRI scans (called image sequences). Each sequence highlights different aspects of brain tissue, and may be used to answer specific questions. Some sequences (for example, diffusion-weighted MR) are particularly useful for detecting abnormalities in the first few hours after ischemic stroke. MRI can also be used to obtain an image of the blood vessels which supply the brain (magnetic resonance angiography or MRA).
What happens during an MRI?
Before the test, you will be asked a number of questions about previous operations or the presence of any metallic objects in your body. If you have artificial joints, a pacemaker, aneurysm clips, or other metal in your body, consult your doctor before having an MRI. You will be asked to remove all jewelry and metallic objects, and you may be asked to change into a patient gown. The technologist will help you lie down on the scanner table. After you are in the proper place, your head will be put in position and a special radio antenna (called a surface coil) will be placed around your head. The technologist will leave the exam room and go to the control room, where you can still communicate with him or her by intercom. You may have the option of listening to music during the test. While the MR images are being taken, it is important for you to lie still on the table, which will be moving very slowly to image the brain. It is normal for the MRI machine to make a loud knocking noise during the exam, so you should not be alarmed. The table will be moving a few millimeters at a time to obtain images of each “slice” of the brain, until the exam is finished. The test takes between 30 and 90 minutes to complete. A dye (contrast medium) may be given, through intravenous injection, to highlight the area being studied.
What are the risks of an MRI scan?
The MRI does not involve x-rays and there are no side effects. However, if you have any metallic objects in your body, the magnetic field can cause dangerous interactions. It is essential that you tell your doctor or the technologist about any previous surgery, implanted devices such as pacemakers, bullets or shrapnel wounds. You will have to lie flat within a relatively small space for as long as an hour. If you think this may be a problem for you (for example, if you are claustrophobic), you should discuss this with your doctor before you schedule the test. If the MRI test involves contrast dye, you will have an injection by vein (usually in your arm).
How does MRI work?
During the exam, a radio signal is turned on in bursts, and the energy is absorbed differently by the different atoms in the body. This energy is reflected out of the body and detected by the MRI scanner. A digital computer constructs these reflections into a picture of the brain. The switching on and off of the device that measures the reflected MR signals (called the gradient coils) produces the knocking sound heard during the exam.