Most of the time, neurologists can feel confident that they have a good understanding of someone's neurological problem without any need for additional tests. In other situations, though, it can be very helpful, or even urgently necessary, to obtain further testing. The term neuroimaging describes methods of visualizing the brain and other parts of the nervous system, and is often used to confirm a neurologist's suspicions, or to rule out serious problems.
The two main ways that physicians take a picture of the brain are either by using an MRI or a CT scan. I like to compare the two by using a rough allegory. If they were cameras, the CT scanner would be like taking a photo with the camera on a cheap cellular phone: the picture isn't as good as it would be if taken by a professional grade camera, but it's inexpensive, fast and convenient. In contrast, an MRI would be an expensive, professional-grade film camera. The picture quality is great, but the scan is expensive, and it takes a longer time until you can view the pictures.
Indications for CT scan vs MRI
Some people assume that, because the picture quality of an MRI is higher, they should always demand this kind of scan. But this can be a dangerous stance if what's really needed is to quickly rule out a serious problem, such as an intracranial hemorrhage. An MRI can take about 45 minutes to complete, whereas a CT scan may only take 5 to 10 minutes. In the time it takes to take the pictures in the MRI machine, a severe hemorrhage could kill the patient.
Furthermore, in some situations, a CT scan can actually detect abnormalities more easily than an MRI. For example, a CT scan is good at detecting acute hemorrhage and problems in bone, such as fractures. In contrast, an MRI is best at detecting small or subtle lesions, such as multiple sclerosis plaques, acoustic neuromas, or low-grade astrocytomas.
Each type of scan is susceptible to different kinds of artifact: a blurring of the image. For example, at the base of the skull in the region of the brainstem, the beams of radiation used by a CT scan can be scattered by the dense bone in the region, leading to a picture that is very difficult or even impossible to interpret. MRI scans require that the patient stay very still for a long period of time, which may be difficult if the patient is confused or fidgety.
Risks of Neuroimaging
In some cases, the amount of radiation involved with a CT scan is concerning: there is about a one in 300 chance of a CT scan contributing to a cancer. This is most concerning in people who are young, since cancers may take more than one incident of cellular injury to cause a malignancy, and if you're young, there's a longer period of time in which additional doses of radiation or other problems could lead to a cancer. For this reason, physicians tend to be more cautious about giving a CT scan to a child than to an elderly adult.
MRI scanners do not apply radiation to capture their images the same way that a CT scanner does. Instead, an MRI uses a very powerful magnet to stimulate atoms in the patient's body, and the atoms then release a type of energy that can be detected by the scanner. The greatest danger of an MRI is to those with metal in their bodies that could be moved around or heated up by the powerful magnetic force created by the MRI machine.
In some cases, neurologists want to use a contrast dye in order to better understand what is going on inside the brain. For example, contrast can be used to highlight blood vessels inside the brain to look for vascular abnormalities such as aneurysms. In an MRI, contrast can highlight lesions that cause a breakdown in the blood-brain barrier, such as acute MS lesions, hemorrhagic stroke, and some tumors. This not only helps doctors see the lesions — it helps physicians figure out the cause of the abnormality.
CT scans use a contrast agent that may contain iodine. This can cause serious problems in people who are allergic to iodine, even leading to life-threatening allergic reactions. While this is rare, it is critical that you let people know about any such allergy prior to receiving a CT scan with contrast.
MRI scanners use an entirely different type of contrast agent called gadolinium. True allergic reactions to gadolinium are extremely rare. In people with kidney disease, though, another type of serious reaction called nephrogenic systemic fibrosis may occur. This disorder can be extremely dangerous, so doctors should ensure that someone's kidney function has been investigated before they order an MRI with contrast.
As you can see, there's a lot to consider before ordering a neuroimaging study: first is the decision about whether any such study is needed at all. If so, factors to consider include the acuity of the problem, the expected nature of the lesion, and potential risks of radiation, exposure to magnets, and different types of contrast. If studies are chosen carefully, neuroimaging can contribute much to the accurate diagnosis of neurological disorders.
Blumenfeld H, Neuroanatomy through Clinical Cases. Sunderland: Sinauer Associates Publishers 2002.
Robert I. Grossman and David M. Yousem. Neuroradiology: The Requisites 2nd ed. St. Louis, MO: Mosby; 2003.