Abdominal magnetic resonance imaging (MRI) is a noninvasive procedure that uses powerful magnets and radio waves to produce pictures of the inside of the abdomen without exposure to ionizing radiation (x-rays).
Alternative Names: Nuclear magnetic resonance - abdomen; NMR - abdomen; Magnetic resonance imaging - abdomen; MRI of the abdomen
How the test is performed:
MRI uses radio waves very close in frequency to those of ordinary FM radio stations, so the scanner must be located within a specially shielded room to avoid outside interference.
The patient lies on a narrow table which slides into a large tunnel-like tube within the scanner. Small devices may be placed around the head, arm, or leg, or adjacent to other areas to be studied. These are special body coils which send and receive the radio wave pulses, and are designed to improve the quality of the images.
An IV may be placed in a small vein of the hand or forearm, if contrast medium will be used. A technologist will operate the machine and observe the patient from an adjacent room during the entire study.
Several sets of images are usually required, each taking from 2-15 minutes. A complete scan, depending on the sequences performed, and need for contrast enhancement may take 1 hour or more. Newer scanners complete the process in less time.
How to prepare for the test:
No preparatory tests, diets, or medications are usually needed, unless the colon needs to be cleansed (with preparations such as a laxative or an enema). An MRI can be performed immediately after other imaging studies. Depending on the area of interest, the patient may be asked to fast for 4 to 6 hours prior to the scan.
Because of the strong magnets used in MRI, 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 the magnet is activated and should not accompany the patient into the scanner area.
Strong magnetic fields can displace or disrupt the action of implanted metallic objects, so 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.
Sheet metal workers, or those 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 clothing without metal fasteners.
How the test will feel:
There is no pain. You cannot feel the magnetic field and radio waves. The primary possible discomfort is the claustrophobic feeling that some 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 during normal operation. Ear plugs are usually given to reduce the noise. A technologist observes 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 time pass.
Excessive movement can blur MRI images and cause certain artifacts. If the patient has difficulty lying still or is very anxious, an oral or intravenous sedative may be given. There is no recovery time, 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 soft tissues without obstruction by bone. It is often used to clarify findings from previous x-ray studies or CT scans. It can show or demonstrate wide areas of the abdomen from multiple viewpoints.
MRI can evaluate certain organ functions. It clearly shows lymph nodes and blood vessels, and is a noninvasive imaging method for evaluation of blood flow.
MRI may be used in diagnosing abnormal growths. It can distinguish tumors from normal tissues and can provide information for the staging (determination of the size, extent, and spread) of abdominal tumors. MRI is sometimes used to avoid the dangers of angiography, repeated exposure to radiation, or for patients who cannot receive iodinated contrast dye.
What abnormal results mean:
The sensitivity of MRI depends, in part, on the experience of the radiologist.
Abdominal MRI may reveal many disorders, including:
Additional conditions under which the test may be performed:
What the risks are:
There is no ionizing radiation involved in MRI, and there have been no documented significant side effects of the magnetic fields and radio waves used on the human body to date. The most common MR intravenous contrast agent, gadolinium, is very safe, and although there have been documented allergic reactions to it, this is extremely rare. If sedation is used, there are associated risks of over-sedation. The technologist monitors the patient's vital signs, including heart rate and respiration as needed.
However, because the effects of strong magnetic fields on a fetus are not well documented at this time, pregnant women are usually advised to avoid MRI scans.
MRI is usually not recommended for acute trauma situations, because traction and
life-support equipment cannot safely enter the scanner area and scan times are relatively lengthy.
MRI is more accurate than a CT scan or other tests for certain conditions but less accurate for others. The function of the small and large bowel (intestines) is not readily visible. Disadvantages include the high cost, long duration of the scan, and sensitivity to movement.
People with claustrophobia or who are confused or anxious may have difficulty lying still for the relatively long scan times. MRI is not portable (it cannot be taken to the patient, the patient must come to the scanner) and is incompatible with some metallic implants, life-support devices, traction apparatus, and similar equipment.
MRI is superior in most cases in which differentiation of soft tissues is necessary. It can view organs that may be obscured by bone or foreign bodies on conventional x-rays or CT scans. It is capable of showing the tissues from multiple viewpoints and is a noninvasive way to evaluate blood flow.