Tests and studies: Head MRI
MRI of the head
An MRI of the head is a noninvasive procedure that uses powerful magnets and radio waves to construct clear, detailed pictures of brain tissues.
Conventional radiography and computed tomographic (CT) imaging use potentially harmful radiation (x-rays) that passes through a patient to generate images. Magnetic resonance imaging (MRI) is based on the magnetic properties of atoms, and there is no exposure to the same type of radiation used in x-rays and CT scans.
A powerful magnet generates a magnetic field roughly 10,000 times stronger than the Earth's. A very small percentage of hydrogen atoms within the body will align with this field. Radio wave pulses are broadcast towards the aligned hydrogen atoms in tissues of interest, returning a signal of their own. The slight differences of those signals from different tissues enables MRI to tell the difference between various organs, and potentially, provide contrast between benign and malignant tissue.
Any imaging angle, or "slice", can be projected, and then stored in a computer or printed on film. MRI can easily be performed through clothing and bones. However, certain types of metal in or around the area of interest can cause significant errors in the reconstructed images. These errors are called artifacts.
Nuclear magnetic resonance - cranial; Magnetic resonance imaging - cranial; Head MRI scan; MRI - cranial; NMR - cranial; Cranial MRI
How the Test is Performed
Since MRI makes use of radio waves very close in frequency to those of ordinary FM radio stations, the scanner must be located within a specially shielded room to avoid outside interference.
You will lie on a narrow table which slides into a large tunnel-like tube within the scanner. In addition, a small device may be placed around the head. This is a special body coil which sends and receives the radio wave pulses. It is designed to improve the quality of the images.
If contrast dye is used, it will be injected into a small vein of the hand or forearm. A technologist will operate the machine and observe you during the entire study from an adjacent room.
Several sets of images are usually required, each taking from 2 to 15 minutes. Depending on the sequences performed and the possibility of the need for a contrast dye, a complete scan may take up to a hour or more. Newer scanners with more powerful magnets, updated software, and advanced sequences may complete the process in less time.
How to Prepare for the Test
No special tests, diets, or medications are usually needed. An MRI may be performed immediately after other imaging studies.
Because of the strong magnets, certain metallic objects are not allowed into the room. Items such as jewelry, watches, credit cards, and hearing aids can be damaged. Pins, hairpins, metal zippers, and similar metallic items can distort the images. Removable dental work should be taken out just prior to the scan. Pens, pocketknives, and eyeglasses can become dangerous projectiles when magnet is activated and should not accompany the patient into the scanner area.
Because the strong magnetic fields can displace or disrupt the action of implanted metallic objects, people with cardiac pacemakers cannot be scanned and should not enter the MRI area. MRI also should not be used for people with metallic objects in their bodies such as inner ear (cochlear) implants, brain aneurysm clips, some artificial heart valves, older vascular stents, and recently placed artificial joints. The technologist will usually provide you with a questionnaire which lists the potentially dangerous items.
Sheet metal workers, or persons with similar potential exposure to small metal fragments, will first be screened for metal shards within the eyes with x-rays of the skull. The patient will be asked to sign a consent form confirming that none of the above issues apply before the study will be performed. A hospital gown may be recommended, or the patient may be allowed to wear a sweat suit or similar clothing without metal fasteners.
How the Test Will Feel
There is no pain. The magnetic field and radio waves are not felt. Some people experience a claustrophobic feeling from being inside the scanner. The table may be hard or cold, but you can request a blanket or pillow.
The machine produces loud thumping and humming noises during normal operation. Ear plugs are usually given to the patient to reduce the noise. A technologist observes the patient during the entire procedure and may be spoken to through an intercom in the scanner. Some MRI scanners are equipped with televisions and special headphones to help the examination time pass.
Excessive movement can blur MRI images and cause errors in the image. If the patient has difficulty lying still or is very anxious, they may be given medicine to relax them (a sedative), by mouth or through a vein. There is no recovery, unless sedation was necessary. After an MRI scan, you can resume normal diet, activity, and medications.
Why the Test is Performed
MRI provides detailed pictures of the brain and nerve tissues from multiple angles without obstruction by overlying bone. In fact, about 90% of all MRI scans are for brain or spine disorders.
MRI is the procedure of choice for most brain disorders. MRI is particularly useful in brain and neurological disorders, because it can clearly show different types of nerve tissue. It provides clear pictures of the brainstem and posterior brain, which are difficult to view on CT scan. It is also useful for the diagnosis of demyelinating disorders. These are disorders such as multiple sclerosis, which cause destruction of the myelin covering of the nerve.
MRI is a noninvasive procedure that can evaluate blood flow and the flow of cerebrospinal fluid (CSF). MRI can distinguish tumors or other lesions from normal tissues. MRI is sometimes used to avoid the dangers of angiography or of repeated exposure to radiation.
What Abnormal Results Mean
The sensitivity of an MRI depends, in part, on the experience of the radiologist.
An MRI of the head may reveal disorders including:
Additional conditions under which the test may be performed:
There is no ionizing radiation involved in MRI. To date, there have been no documented significant side effects of the magnetic fields and radio waves used on the human body during an MRI scan.
The most common MR intravenous contrast agent, gadolinium, is very safe, and although there have been documented allergic reactions to it, it is an extremely rare occurrence. However, gadolinium should not be given if you are pregnant because of potential harm to the fetus.
If sedation is used, there are associated risks of over-sedation. The technologist monitors the patient's vital signs, including heart rate and breathing as needed.
People have been harmed in MRI machines when they did not remove metal objects from their clothes or when metal objects were left in the room by others.
MRI is superior to computed tomography (CT) in most cases where differentiation of soft tissues is necessary. It can view organs without obstruction by bone and foreign bodies. It is capable of showing the tissues from multiple viewpoints and is a noninvasive way to evaluate blood flow.
A CT scan may be preferred for: