The following article was written by Christopher P. Hess, M.D., Ph.D, and Derk Purcell, M.D, Assistant Clinical Professor in the Department of Radiology and Biomedical Imaging at UCSF.
The complexity of the organ that determines how a person thinks, moves, feels, and remembers is overshadowed only by its unique vulnerability. The brain is hidden from direct view by the skull, which not only shields it from injury but also hinders the study of its function in both health and disease. The cells in the arteries that supply the brain are so tightly bound that even most normal cells in the bloodstream are prevented from crossing the so-called “blood-brain barrier,” thereby rendering the normal chemistry of the brain invisible to the routine laboratory blood tests that are often used to evaluate the heart, liver or kidneys.
Computed tomography (CT) and magnetic resonance imaging (MRI) have revolutionized the study of the brain by allowing doctors and researchers to look at the brain noninvasively. These diagnostic imaging techniques have allowed for the first time the noninvasive evaluation of brain structure, allowing doctors to infer causes of abnormal function due to different diseases.
The answer to which imaging modality is better for imaging the brain is dependent on the purpose of the examination. CT and MRI are complementary techniques, each with its own strengths and weaknesses. The choice of which examination is appropriate depends upon how quickly it is necessary to obtain the scan, what part of the head is being examined, and the age of the patient, among other considerations. All imaging studies that are not performed for research should be obtained in close consultation with a physician. Both techniques are designed to examine specific problems. The utility of “screening” CT or MRI, in which a scan is obtained in a healthy patient without any symptoms to look for a brain tumor or any other condition, has not been established.
The advantages of each modality listed below serve as general guidelines that doctors use to decide between head CT and MRI:
Advantages of head CT
- CT is much faster than MRI, making it the study of choice in cases of trauma and other acute neurological emergencies
- CT can be obtained at considerably less cost than MRI, and is sufficient to exclude many neurological disorders
- CT is less sensitive to patient motion during the examination. because the imaging can be performed much more rapidly
- CT may be easier to perform in claustrophobic or very heavy patients
- CT provides detailed evaluation of cortical bone
- CT allows accurate detection of calcification and metal foreign bodies
- CT can be performed at no risk to the patient with implantable medical devices, such as cardiac pacemakers, ferromagnetic vascular clips, and nerve stimulators
Advantages of head MRI
- MRI does not use ionizing radiation, and is thus preferred over CT in children and patients requiring multiple imaging examinations
- MRI has a much greater range of available soft tissue contrast, depicts anatomy in greater detail, and is more sensitive and specific for abnormalities within the brain itself
- MRI scanning can be performed in any imaging plane without having to physically move the patient
- MRI contrast agents have a considerably smaller risk of causing potentially lethal allergic reaction
- MRI allows the evaluation of structures that may be obscured by artifacts from bone in CT images