MAGNETIC RESONANCE IMAGING PROCEDURE

magnetic resonance imaging procedure
Regardless of the type of MRI planned, or area of the body targeted, the procedure involved is basically the same, and occurs in a special MRI suite. The patient lies back on a narrow table and is made as comfortable as possible. Transmitters are positioned on the body and the cushioned table that the patient is lying on moves into a long tube that houses the magnet. The tube is the length of an average adult lying down, and the tube is narrow and open at both ends. Once the area to be examined has been properly positioned, a radio pulse is applied. Then a two-dimensional image corresponding to one slice through the area is made. The table then moves a fraction of an inch and the next image is made, and so on. Each image exposure takes several seconds, and the entire exam lasts 30–90 minutes. During this time, the patient is not allowed to move. Movement during the scan results in an unclear image.

Depending on the area to be imaged, the radio-wave transmitters are positioned in different locations.
• For the head and neck, a helmet-like hat is worn.
• For the spine, chest, and abdomen, the patient lies down on transmitters known as coils.
• For the knee, shoulder, or other joint, the transmitters are applied directly to the joint.

Additional probes will monitor such vital signs as pulse and respiration.

The process is very noisy and confining. The patient hears a thumping sound for the duration of the procedure. To increase comfort, music supplied via earphones is often provided. Some patients become anxious, or they may panic because they are inside a small, enclosed tube. This is why vital signs are monitored, and the patient and medical team communicate with each other. If a patient has claustrophobia, the physician may prescribe an anti-anxiety drug prior to the procedure. If the chest or abdomen is to be imaged, the patient is asked to hold his or her breath for each exposure. Other instructions may be given as needed.

In many cases, the entire examination will be performed by an MRI operator who is not a physician. However, the supervising radiologist should be available to consult as necessary during the exam, and will view and interpret the results at a later time.

Open MRI units
Many adult patients and, especially children, become extremely claustrophobic when placed inside the confines of a full strength (1.5 Tesla) superconducting magnet. This problem is often severe enough to prevent them from having an MRI scan. In an alternative design, the magnet is comprised of two opposed halves with a large space in between. These units are known as open MRI machines. The advantage is that they can be used for patients who are claustrophobic. The disadvantage is that the field strength of the magnets is lower (usually 0.2–0.5 Tesla) than with standard full-strength machines. Lower strength magnetic fields require more time for image acquisition, increasing the risk of image problems because patients may have difficulty remaining still for longer periods of time.

Preparation
In some cases (such as for MRI brain scanning or MRA), a chemical designed to increase image contrast may be given by the radiologist immediately before the exam. If a patient suffers from anxiety or claustrophobia, drugs may be given to help the patient relax.

The patient must remove all metal objects (i.e., watches, jewelry, eyeglasses, hair clips). Any magnetized objects, such as credit and bank machine cards or audio tapes, should be kept far away from the MRI equipment because they can be erased. The patient cannnot bring a wallet or keys into the MRI machine. He or she may be asked to wear clothing without metal snaps, buckles, or zippers, unless a medical gown is provided. The patient may also be asked to remove any hair spray, hair gel, or cosmetics that could interfere with the scan.

Side effects
The potential side effects of magnetic and electric fields on human health remain a source of debate. In particular, the possible effects on an unborn baby are not well known. Any woman who is, or may be, pregnant should carefully discuss this issue with her physician and radiologist before undergoing a scan.

Chemical agents may be injected to improve the image or allow for the imaging of blood or other fluid flow during MRA. In rare cases, patients may be allergic to or intolerant of these agents, and should not receive them. If chemical agents are to be used, patients should discuss any concerns they have with their physician and radiologist. As in other medical imaging techniques, obesity greatly interferes with the quality of MRI.

Aftercare
No aftercare is necessary, unless the patient received medication or had a reaction to a contrast agent. Normally, patients can return to their daily activities immediately. If the exam reveals a serious condition that requires more testing or treatment, appropriate information and counseling will be needed.

Precautions
MRI scanning should not be used when there is the potential for an interaction between the strong MRI magnetic field and metal objects that might be imbedded in a patient’s body. The force of magnetic attraction on certain types of metal objects (including surgical steel and clips used to pinch off blood vessels) could move them within the body and cause serious injury. The movement would occur when the patient is placed into and out of the magnetic field.

Metal may be imbedded in a person’s body for several reasons:
• Medical. People with implanted cardiac pacemakers, metal aneurysm clips, or who have had broken bones repaired with metal pins, screws, rods, or plates must inform their radiologist prior to having an MRI scan. Generally, a joint replacement or other orthopedic hardware is not a problem if another part of the body is being scanned.

• Injury. Patients must tell their physicians if they have bullet fragments or other metal pieces in their body from old wounds. The suspected presence of metal, whether from an old or recent wound, should be confirmed before scanning.

• Occupational. People with significant work exposure to metal particles (working with a metal grinder, for example) should discuss this with their physician and radiologist. The patient may need prescan testing—usually a single, standard x ray of the eyes to see if any metal is present.

Normal results
A normal MRI, MRA, or MRS result is one that shows that the patient’s physical condition falls within the normal range for the target area scanned. Generally, MRI is prescribed only when serious symptoms or negative results from other tests indicate a need. There often exists strong evidence of a condition that the scan is designed to detect and assess. Thus, the results will often be abnormal, confirming the earlier diagnosis. At that point, further testing and appropriate medical treatment are needed. For example, if the MRI indicates the presence of a brain tumor, an MRS may be prescribed to determine the type of tumor so that aggressive treatment can begin immediately without the need for a surgical biopsy.