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Magnetic Resonance Imaging: How It is Performed and the Role of the Neuroradiologist

MRI is an abbreviation for magnetic resonance imaging. The word magnetic in the title refers to the fact that the study is performed with the patient placed inside a large magnet. The word resonance refers to the principle that the magnet causes the hydrogen within the molecules being studied to resonate (ie, vibrate). This vibration occurs on a molecular level and is nearly imperceptable. Radio waves are then used to convert this vibration of the molecules into pictures that radiologists can then study.

MRI is extremely useful for studying the brain and spine. MRI can show fine anatomical details that cannot be well defined by using computed tomography (CT) and other radiological studies. In many cases, MRI is the procedure of choice for studying the brain and spine.

MRI can also be used to study other parts of the body, but is particularly useful in the shoulder and knee joints, in the female reproductive system, and in the heart. New applications of MRI are developed every year, and in many circumstances, MRI provides information that cannot be obtained with other radiological techniques.

Is the procedure safe?

MRI is a safe procedure. Because the images are generated by using a magnet and radio waves, no ionizing radiation is used in the procedure. Furthermore, MRI does not produce any permanent changes in the body. Therefore, in theory, a person could undergo many MRI examinations without any cumulative effects.

Who should not be examined with MRI?

The MRI machine is particularly sensitive to metal. It can cause pieces of metal to move and can also cause metallic devices (such as pacemakers) to malfunction. Furthermore, metal can cause the quality of MRIs to be very poor. For these reasons, MRI is generally not performed in patients with one of the following:

Cardiac pacemaker or other implanted electronic devices
Certain types of cerebral aneurysm clips
Certain types of metallic implants
Metal fragments in the eyes

MRI technologists are trained to routinely ask patients a series of questions to determine whether they can safely undergo an MRI study. When a metal fragment may be present in the body, either X-ray or CT may need to be obtained before the patient is allowed to enter the MRI unit. Therefore, it is very important for patients to notify the MRI technologist when they think they may have metal objects in their body. However, some metal objects are perfectly safe for MRI. After considering the information obtained by the technologist, the radiologist makes the ultimate decision about whether a particular type of metal is safe.

Can pregnant women undergo an MRI examination?

No evidence suggests that MRI is harmful to either a pregnant woman or her fetus. In fact, MRI is often performed in pregnant women to examine the fetus. However, to minimize the risks, many imaging centers prefer that pregnant patients undergo MRI only if they have a serious or life-threatening illness.

Is special preparation needed for the study?

In general, no special preparation is needed. Patients can eat and drink normally and take any regular medication prior to the study. However, some children may need to be sedated before entering the MRI machine to prevent them from moving too much during the study. These children will need to fast for a few hours before receiving the sedatives.

What occurs on the day of the study?

When patients arrive for an MRI examination, they are typically asked to remove all personal items such as watches, wallets, and car keys. Most MRI facilities provide secure lockers where patients can store their items during the procedure. The patient is generally asked to change from street clothes into a hospital gown.

What happens during an MRI study?

The procedure is performed in a room occupied by a large magnet with an examination table in its center. Patients are asked to lie flat on this table, which slides into the center of the magnet. When the machine is in operation, the patient hears a variety of knocking sounds. Earplugs are provided for patients' comfort. In addition, music is sometimes played.

Depending on the body part being studied and the specific type of machine used, the patient's body may be partly or entirely guided into the magnet. In most cases, almost the entire body is within the MRI machine.

The patient must remain as still as possible during parts of the procedure because movement will cause blurring of the final image.

Patients can communicate with the technologist via an intercom system. The technologist generally speaks to the patient during various stages of the examination to see how the patient is doing or to provide instructions.

How long does the examination take?

Typically, an examination takes 45-60 minutes.

Does a needle need to be placed for this procedure?

In some cases, contrast material is injected through a vein during the MRI examination. A gadolinium-based contrast agent is commonly used. This contrast material is extremely safe, and its incidence of adverse effects is very low.

Can claustrophobic patients undergo MRI?

Patients who are claustrophobic can be sedated with medication to reduce their anxiety prior to and during the examination.

Alternatively, some MRI centers have "open" magnets that offer the patient more space and a better view around them. In general, these magnets are less powerful than conventional ones, and they produce images that show slightly less detail.

Are there any special instructions for the patient after the procedure?

None. The patient can resume his or her normal daily activities immediately after the procedure.

Who interprets the findings on brain and spine MRIs?

In general, a physician who has completed four years of training in radiology under the direction of the American Board of Radiology interprets the MRIs. Generally, the physician who ordered the MRI examination receives the results within a few days, and patients obtain results from their personal physician.

If the examination is of the brain or spine, a neuroradiologist who has undergone an additional 1-2 years of training in brain and spine imaging (ie, neuroradiology) often interprets the study.

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