Computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET) all take images of the body. A CT scan uses X-rays and computer technology, an MRI uses magnets and radio waves, and a PET scan uses radioactive tracers. CT and MRI devices produce still images, while a PET scan looks at the metabolic activity of an organ or tissue, which may help distinguish between normal tissue and a tumor, for example.
Neurologists rely mostly on CT and MRI scans to help diagnose and treat many neurologic conditions, including multiple sclerosis, stroke, brain hemorrhage, brain tumor, herniated disc, disease of or damage to the spinal cord, and traumatic brain injury.
CT scans, which can be completed in two minutes, are typically done in emergency departments for suspected head injuries or stroke to immediately detect bleeding, a tumor, or other lesions. Unlike a traditional X-ray of the skull, a CT also shows the brain itself. If the scan shows bleeding in a possible stroke patient, doctors cannot use a clot-buster drug as it may risk more bleeding. If results are inconclusive or concerning, doctors will follow up with an MRI scan.
In evaluating patients with seizures, neurologists may order a CT and, if needed, an MRI to rule out underlying causes such as a tumor, an abscess, bleeding in the brain, or something else. To diagnose epilepsy, neurologists use an electroencephalogram (EEG), in which electrodes are placed on the scalp to measure electrical activity.
To check the spine or brain for injury, disease, or structural problems, neurologists usually order an MRI, which looks at nerves and other soft tissue and provides more detailed images than a CT scan. And since X-rays are not involved, there's no radiation exposure. MRI is the preferred method for diagnosing and monitoring multiple sclerosis. Gadolinium, a contrast dye, may be injected before an MRI to improve the visibility of the brain, spinal cord, and blood vessels. If patients have allergic reactions to the dye, their doctors may prescribe steroids or Benadryl.
PET scans are used mainly to identify cancer but also can help diagnose Alzheimer's disease and sometimes brain tumors. Medicare requires an amyloid PET scan for patients with symptoms of dementia who may be prescribed anti-amyloid treatment for early Alzheimer's disease to detect the amyloid plaques characteristic of the disorder.
CT scans usually take two to 15 minutes. PET scans and MRIs can last between 45 minutes and two hours. All three tests require patients to lie very still on a motorized table that moves into a tubular or donut hole-shaped scanner. Any movement by patients can blur the images and necessitate repeating the scan. Patients wear ear protection for an MRI because it involves loud noises.
Patients getting MRI or CT scans must remove all metal objects such as necklaces, keys, and watches. Metallic objects can be sucked into the MRI's magnetic machinery and damage it, and injure patients in the process. Metal can show up on a CT scan, making it more difficult to interpret the images.
People who have pacemakers, implants, shrapnel, or bullets may not be candidates for MRIs because of the strong magnetic force of the machine. They should discuss any implants they have, including dental ones, with their doctors beforehand.
In rare cases, tattoos have been problematic during an MRI. The iron oxide in the ink can interact with the radio waves of the MRI and cause skin irritation or burning. The location and size of the tattoo may determine whether this type of scan is safe.
People who get claustrophobic in an MRI scanner should speak to their doctors about taking medications to ease their anxiety and ask if someone can accompany them during the test. In addition, technicians can talk to patients or play music during the exam to help them relax.
Before the test, patients should bring medical orders from their doctors and contact information for the physician who will receive the imaging report.
Dr. Józefowicz is professor of neurology and medicine and associate chair for education at the University of Rochester in New York.
