Comparing MRI and CT Scans

The brain and nervous system can be visualized with computerized tomography (CT) scans and magnetic resonance imaging (MRI). When faced with a neurologic disorder, an experienced neurologist can often make a diagnosis without the need for additional tests.

At other times, it can be helpful (or even urgent) to order a battery of neuroimaging tests to locate or evaluate disorders that are not so easily identified. Learn how and why these tests are performed.

The term neuroimaging describes methods of visualizing the brain and other parts of the nervous system to either confirm or rule out the neurologist's suspicions. MRIs and CT scans are two such tools a neurologist will regularly turn to.

Metaphorically speaking, an MRI is like an expensive, professional-grade camera while a CT scan is more like a cheap disposable camera. The comparison is even more relevant given that the cost of an MRI far exceeds that of a CT scan.

This doesn’t necessarily mean that one is inherently better than the other. Some people assume that, because the imaging quality of an MRI is higher, it should always be the first choice. But that reflects a general misunderstanding about the technologies, both in terms of their capabilities and shortcomings.

Broadly speaking, an MRI and CT scan differ in three distinct ways:

• When time is of the essence. An MRI can take around 45 minutes to complete whereas a CT scan may only take five to 10 minutes. In the time it takes to get an MRI done (for, say, a serious intracranial hemorrhage) a person could be dead or severely injured. An MRI also requires a person to be very still for a long period of time, which can be difficult in an emergency. A CT scan is often the better choice for such emergencies.
• The types of abnormalities they detect. In some situations, a CT scan can detect abnormalities more easily than an MRI, including acute bleeding and bone fractures. By contrast, an MRI is best at detecting small or subtle lesions such as multiple sclerosis plaques, acoustic neuromas, or low-grade astrocytomas.
• Interference with imaging quality. MRIs create images using strong magnetic waves. Certain metal implants and non-compatible devices can interfere with these waves, causing a distortion of the images. In the same vein, the beams of radiation used by a CT scan can be scattered by dense bone (say, around a brain stem), leading to a picture that is difficult or even impossible to interpret.

Speak with your doctor to fully understand the risks and benefits of CT or MRI scans if pregnant.

CT scans essentially use X-rays to create a rotating image. As such, the amount of radiation involved can be concerning, with some studies suggesting a 1 in 300 chance of getting cancer as a result of a scan.

This is more of a concern in young people since the development of cancer usually takes decades to manifest. For this reason, doctors tend to be more cautious about performing a CT scan on a child than on an older adult.

MRI, by contrast, uses a very powerful magnet to stimulate atoms in a person’s body. Those atoms are then detected by the scanner. The greatest risk of an MRI is that any ferromagnetic metal implant can become a magnetic under the influence the MRI and try to align pole-to-pole This can cause an implant to be displaced or overheat.

In some cases, neurologists will use a contrast dye to better differentiate what is going on inside the brain. Contrast dyes can be useful in highlighting vascular abnormalities such as brain aneurysms or lesions associated with acute MS, hemorrhagic stroke, or cancer.

In both CT scans and MRIs, the contrast agent may cause serious problems:

• CT scans use a contrast agent that may contain iodine. In some rare cases, iodine exposure can cause serious to life-threatening allergic reactions.
• MRI scanners use a contrast agent called gadolinium. In persons with kidney disease, exposure to gadolinium can cause a rare but adverse condition called nephrogenic systemic fibrosis (NSF).

There is a lot that needs to be considered before undergoing a neuroimaging exam. As the patient, it is always important to inform your doctor about any allergy, implant, and health issue (including cancer treatments) you have or may have had.

You should also air any concern you have about the procedure itself, particularly if you have claustrophobia or have had a bad experience in the past. Alternatives may be available.

If an imaging tool is chosen wisely and with the full input of the patient, it can contribute greatly to the ease and accuracy of a diagnosis. Speak with your doctor or get a second opinion, if needed.