When a child needs an evaluation involving radiology, parents often ask me, "What is the difference between a CT and a MRI?"

Radiology can be a pandora's box. Donald P. Frush, MD, chief of Duke Radiology’s Division of Pediatric Radiology, tells us what we need to know both as patients, and as physicians trying to explain these procedures to patients.

--Dennis Clements, MD, PhD

Donald P. Frush, MDEither computed tomography (CT or CAT scan) or magnetic resonance imaging (MRI) is useful in the evaluation of children. There are situations in which CT is preferred, and times when MRI should be obtained.

To address these circumstances, it is important to have a basic understanding of how CT and MR imaging work, and the disadvantages of both modalities.

Computed Tomography

CT is a sophisticated x-ray machine that takes detailed pictures of the inside of the body. For a CT examination, the child lays on a bed that will move through the doughnut-shaped gantry containing the x-ray tube, opposite the detectors that capture the x-rays once they pass through the patient.

Often contrast (dye) is needed, which requires an IV. When the IV contrast is administered, your child will have a warm sensation for a minute or so. For many abdomen examinations, oral contrast material (mixed with a liquid such as juice) helps to define the gastrointestinal tract.

The CT examination may take less than one to 20 seconds to perform. Most of the rest of the time is setting up the x-ray machine, and reviewing the pictures. Occasionally, sedation may be necessary. This is usually only required from about one-to-two years of age or older if the child has special needs.

CT can be used in many different situations. It is the best modality we have to look at the lungs such as for complicated infection, trauma, or cancer. Because CT can be performed so quickly, CT scanners are often in the emergency department. In this setting, CT is helpful in evaluation of acute abdominal pain (possible appendicitis or renal stone), or injury to the head, abdomen, and spine. Blood vessel evaluation is excellent with CT (CT angiography).

Disadvantages include radiation. The amount of radiation can vary depending on examination and the size of your child and range to from less than 50 to more than 100 chest x-rays in the amount of radiation. This amount is still low. We are not certain about risks of low level radiation, but if there is a risk, it is extremely small and the information obtained from the examination, even if normal, is far more valuable than the potential risk. It is important, though that the CT examination is adjusted based on the size or age of your child, the region being examined, and the question to be answered.

Magnetic Resonance Imaging

The MR scanner contains a very powerful magnetic and makes pictures from the radiofrequency pulsation (the knocking or buzzing noise) effects on organs and structures inside the body. The child will go into a tunnel. The examination time is longer and ranges from 20 to 60 minutes of actual imaging time, generally 30 to 40 minutes.

Several sequences are performed and the child must be completely still during these sequences. For this reason, children may need to be sedated up to about six years of age.

For some MR examinations, IV contrast material may be necessary. The technologist, as during a CT scan, will sit in the adjoining control room but is always able to see the scanner area. Especially when sedation is used, heart rate, respiratory rate, and blood oxygen is monitored.

Contraindications to MR include internal electronic equipment such as cardiac pacemakers. Other metallic material such as clips for bowel surgery or orthopaedic surgery is generally fine to undergo MRI examination six weeks after the operation.

MRI is superior to CT in evaluation of the brain, spine, and musculoskeletal system (bones, tendons, ligaments, cartilage, and muscles) in the setting of injury, infection, and cancer. MRI is better at evaluation of most sports-related injuries, for example. However, CT is better for evaluation of complex fractures.

MRI is also excellent at looking at intra-abdominal organs, but it does not provide information on lung tissue like CT. MRI can also be used to evaluate the heart and blood vessels. MR tends to give better structural (anatomy) and functional information of the heart than CT but the detail of smaller blood vessels, such as coronary arteries, is better with CT.

Is CT or MR Better?

This will depend on the indication. The benefits are outlined in the above discussion. Understand that sometimes the choice is not straightforward and either would provide useful information. This may be discussed between the health care provider and the patient or parent. In general, if CT and MR provide equal information, MR is preferred due to the amount, even though small, of radiation from CT.

Further information: www.imagegently.org

-- Donald P. Frush, MD, chief of Duke Radiology’s Division of Pediatric Radiology

-- Dennis Clements, MD, PhD, is the chief of primary care pediatrics at Duke Children's Hospital.