Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is defined by the National Institute of Health as a non-invasive procedure that uses powerful magnets and radio waves to construct pictures of the body. Unlike conventional radiography and Computed Tomographic (CT) imaging, which make use of radiation (X-rays), MRI imaging is based on the magnetic properties of atoms.

A powerful magnet generates a magnetic field roughly 10,000 times stronger than the natural background magnetism from the earth. A very small percentage of hydrogen atoms within a human body will align with this field. When focused radio wave pulses are broadcast towards the aligned hydrogen atoms in tissues of interest, they will return a signal. The subtle differences in that signal from various body tissues enables MRI to differentiate organs, and potentially contrast benign and malignant tissue.

Any imaging plane (or "slice") can be projected, stored in a computer, or printed on film. MRI can easily be performed through clothing and bones. However, certain types of metal in the area of interest can cause significant errors in the reconstructed images.

An MRI can provide additional imaging information for the clinician based upon it's superior tissue contrast resolution. Combined with other imaging modalities, a more definitive diagnosis can be given in the work up of a patient's disease.

Sequences performed with intravenous contrast may provide additional data regarding the vascular nature of masses. An MRA, or magnetic resonance angiogram, is a special type of MR which creates three-dimensional reconstructions of vessels containing flowing blood and is often utilized when conventional angiography cannot be performed due to renal failure or other contraindications.

What to Expect

There is no pain during an MRI. The magnetic field and radio waves are not felt.

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 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 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. 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.

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.

After an MRI scan, you can resume normal diet, activity, and medications.

How to Prepare for an MRI

No preparatory tests, diets, or medications are usually needed. 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, 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.

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, recently placed artificial joints.

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.

Because the effects of strong magnetic fields on a fetus are not well documented at this time, pregnant women areusually 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.

The patient will be asked to sign a consent form confirming that none of the above issues apply before the study will be performed.

Chest MRI

Purpose: MRI provides detailed pictures of tissues within the chest cavity, without obstruction by overlying bone. It may be used to clarify findings from previous X-rays or CT scans. It can show the structures of the chest from multiple planes. It is useful in the diagnosis of abnormal growths, and can provide information for the staging (such as the size, extent, and spread) of thoracic tumors. MRI clearly shows lymph nodes and blood vessels, and is a noninvasive procedure that can evaluate blood flow. 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.

Prior Preparation: None

Administration: The patient will be asked to lie on a narrow table which slides into a large tunnel-like tube within the scanner. If contrast is to be administered, intravenous access will be placed, usually in a small vein of the hand or forearm.

Scan Time: Several sets of images are usually required, each taking from 2 to 15 minutes. A complete scan, depending on the sequences performed,and need for contrast enhancement, 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.

Abdominal MRI

Purpose: MRI provides detailed pictures of soft tissues without obstruction by overlying 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 planes. 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 or other lesions 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 that cannot receive iodinated contrast dye.

Prior Preparation: 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.

Administration: The patient will be asked to lie on a narrow table which slides into a large tunnel-like tube within the scanner. In addition, 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. If contrast is to be administered, an IV will be placed, usually in a small vein of the hand or forearm.

Scan Time: Several sets of images are usually required, each taking from 2 to 15 minutes. A complete scan, depending on the sequences performed, and need for contrast enhancement 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.

Heart MRI

Purpose: MRI provides detailed pictures of the heart and blood vessels and can distinguish tissues from moving blood. It can differentiate between heart muscle and surrounding tissues and can clarify findings from previous X-rays or CT scans. MRI excels at showing the heart from multiple imaging planes. MRI is useful in the diagnosis of congenital abnormalities, abnormal growths, and tumors.
MRI is sometimes used to avoid the dangers of angiography, repeated exposure to radiation, or exposure to iodinated contrast dye. It may provide additional information when an echocardiogram is unclear, since bone and air do not interfere with imaging. In addition, cardiac MRI provides a wider view and better spatial resolution (detail) than echocardiogram.

Prior Preparation: No preparatory tests, diets, or medications are usually needed. 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.

Scan Time: Several sets of images are usually required, each taking from 2 to 15 minutes. A complete scan, depending on the sequences performed, and need for contrast enhancement 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.

Cranial MRI

Purpose: 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.

Prior Preparation: 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.

Administration: 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.

Scan Time: 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.

Lumbosacral Spine MRI

Purpose: After plain radiographs of the lower spine, MRI is usually the next non-invasive imaging examination in the work-up of back pain and radiating leg pain. MRI is excellent at visualizing degenerative changes, such as from arthritis, which may be narrowing the spaces through which the spinal nerves travel. As well, MRI can demonstrate disease of the lumbar dics between vertebral levels which may be bulging and compression the spinal cord.

An MRI may be performed in a trauma setting to rule out acute compression of the spinal cord in the setting of weakness or paralysis. MRI is also superior to Computed Tomography (CT) in the evaluation of abscesses, tumors, or other masses near the spinal cord. While CT is better at detecting fractures of the vertebrae, MRI can detect subtle changes in the bone which may be due to infection or tumor.

Prior Preparation: None

Scan Time: Several sets of images are usually required, each taking from 2 to 15 minutes. A complete scan may take up to one hour. Newer scanners with more powerful magnets utilizing updated software and advanced sequences may complete the process in less time.

Spine MRI

Purpose: MRI may show the exact location of tumors or other lesions of the spine, spinal cord, or intervertebral disks. It provides detailed pictures of hard-to-view areas of the spine, including the spinal canal, bony segments, and soft tissue. MRI is sometimes used to avoid the dangers of exposure to contrast dye or radiation.

Prior Preparation: None

Additional Links

Mount Sinai Department of Radiology
Mount Sinai Positron Emission Tomography (PET) Scan Program
RadiologyInfo.org
Radiological Society of North America

Some of the material on this site is courtesy of the National Institute of Health