NMR - cardiac

Alternative names
Magnetic resonance imaging - cardiac; Heart MRI; Nuclear magnetic resonance - cardiac; MRI of the heart

Definition
A noninvasive procedure that uses powerful magnets and radio waves to construct pictures of the heart.

Magnetic resonance imaging (MRI) produces pictures of the heart without exposure to ionizing radiation (X-rays). The heart may be scanned by itself, or a heart MRI may be part of a chest MRI.

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, 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. 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, 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.

How to prepare for the test

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, and 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. 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.

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:

     
  • Infant test or procedure preparation (birth to 1 year)  
  • Toddler test or procedure preparation (1 to 3 years)  
  • Preschooler test or procedure preparation (3 to 6 years)  
  • Schoolage test or procedure preparation (6 to 12 years)  
  • Adolescent test or procedure preparation (12 to 18 years)

How the test will feel

There is no pain. 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. After an MRI scan, you can resume normal diet, activity, and medications.

Why the test is performed

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.

What abnormal results mean

The sensitivity of MRI depends, in part, on the experience of the radiologist.

A heart MRI may reveal disorders including:

     
  • heart valve disorders  
  • pericardial effusion  
  • tumor invasion of blood vessels or pericardium (around the heart)  
  • left atrial myxoma  
  • atrial myxoma; right  
  • congenital heart abnormalities (such as pulmonary atresia)  
  • fibrosis or scarring of heart muscle  
  • extent of necrosis (tissue death) of heart muscle (after acute MI)

A heart MRI can show intracellular metabolism and evaluate the response of the heart tissues to treatments.

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. 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.

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 oversedation. The technologist monitors the patient’s vital signs, including heart rate and respiration, as needed.

Special considerations
MRI is more accurate than CT scan or other tests for certain conditions, but less accurate for others. 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 and is incompatible with 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 without obstruction by bone and foreign bodies. It is capable of showing the tissues from multiple viewpoints and is a non invasive way to evaluate blood flow.

Johns Hopkins patient information

Last revised: December 5, 2012
by David A. Scott, M.D.

Medical Encyclopedia

  A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | 0-9

All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.