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Ency. home > Test > C > Cranial MRI

Cranial MRI    See images

Overview | Risks | Results

Alternative names:

Nuclear magnetic resonance - cranial; Magnetic resonance imaging - cranial; Head MRI scan; MRI of the head; NMR - cranial

Definition:

A non-invasive procedure that uses powerful magnets and radio waves to construct pictures of the clear, detailed pictures of brain tissues.

Unlike conventional radiography and Computed Tomographic (CT) imaging which make use of ionizing, and hence, potentially harmful, radiation (X-rays) passing through a patient to generate images, MRI imaging is based on the magnetic properties of atoms.  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. The "nuclear" in the original, and now seldom used, name refers to the proton in the nucleus of the hydrogen atom and does not imply radioactivity. When focused short radio wave pulses are  broadcast towards the aligned hydrogen atoms in tissues of interest, they will return a signal of their own. The subtle differing characteristics of that signal from dissimilar tissues combined with complex mathematical formulas solved on modern computers is what enables MRI to differentiate between various organs, and potentially, provide contrast between benign and malignant tissue. Any imaging plane, 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 (artifact).

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. The patient will be asked to 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 to be studied. This is special body coil which send and receive the radio wave pulses, and is designed to improve the quality of the images.  If contrast is to be administered, intravenous access will be placed, usually in 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. A complete scan may take up to one hour or more. Newer scanners with more powerful magnets utilizing updated software and advanced sequences may complete the process in less time.

How to Prepare for the Test

No preparatory tests, diets, or medications are usually needed. You may be asked to fast for 4 to 6 hours prior to the scan. You must sign a consent form before the scan.

Because of the strong magnets, no metallic objects are allowed into the room. Items such as jewelry, watches, credit cards, and hearing aids, can be damaged by the strong magnets. Pins, hairpins, metal zippers, and similar metallic items can distort the images. Frequent exposure to ferrous metals (such as a sheet metal worker has) may make MRI difficult. A hospital gown may be recommended, or you may be allowed to wear "sweats" or similar clothing without metal fasteners. Take out removable dental work just prior to the scan.

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:

How the Test will Feel

There is no pain. The magnetic field and radio waves are not felt. The primary discomfort is the claustrophobic feeling that some people experience 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. Ear plugs are usually given to reduce the noise.

A technologist observes you during the entire procedure and may enter the room to speak to you or may speak with you through an intercom in the scanner.

Excessive movement can blur MRI images. If you have difficulty lying still or are very anxious, you may be given a sedative. The viewing of some brain lesions may require use of an intravenous dye. There is no recovery required (unless you have been sedated). After an MRI scan, you can usually resume normal diet, activity, and medications.

Why the Test is Performed

MRI provides detailed pictures of brain and nerve tissues from multiple planes without obstruction by overlying bone. In fact, approximately 90% of all MRI scans are for cranial or spine disorders.

MRI is the procedure of choice for most brain disorders. MRI is particularly useful in brain and neurologic disorders, because it can clearly show various 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 (disorders such as multiple sclerosis that cause destruction of the myelin sheath 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.

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