CT and MRI scans both generate excellent images of the internal structure and condition of the brain. PET scans can be used to monitor brain activity but are not typically used in the diagnosis of Alzheimer’s. While CT, MRI and PET scans can detect important changes associated with dementia, a diagnosis cannot be based solely on a brain scan. Sometimes brain scans will reveal no significant changes in the brain.
A Computerized Tomography (CT) scanner is a specialized form of X-ray machine. Unlike an ordinary X-ray machine, which sends a single X-ray beam though the body, a CT scanner simultaneously sends several X-ray beams from different angles to produce highly detailed, three dimensional images of the internal structure of the brain.
Magnetic Resonance Imaging (MRI) scans use magnetic and radio waves, instead of X-rays, to provide very clear and detailed images of brain or other internal organs. MRI scans provide static three dimensional images of brain structure. Specialized MRI scans can also be used to monitor brain activity in areas of the brain where speech, sensation, memory and similar functions occur.
Positron Emission Tomography (PET) scans detect special radioactively labeled tracers which are injected into a patient’s body before the imaging procedure starts. PET scans can be used to accurately monitor brain activity while a patient’s memory and cognition are being tested.
PET scans can determine brain activity and function by measuring differences in blood flow and the usage of glucose (sugar), both of which increase when an area of the brain is active. PET scans provide information about brain function and activity as opposed to brain structure, and are more typically used in research.
Single Photon Emission Computed Tomography (SPECT) scans produce a three dimensional picture of blood flow into particular regions of the brain. In the future, SPECT scans may be able to identify characteristic signs of Alzheimer’s disease and distinguish it from other types of dementia. For instance, studies have demonstrated that people with Alzheimer’s disease tend to have reduced blood flow into brain areas responsible for vocabulary and geographical information. SPECT scans are currently being evaluated in a clinical setting and are not generally available for diagnosis.
Functional MRI (FMRI) scans are similar to a typical MRI scan but instead of providing a static image of the brain, they can assess the brain activity and blood flow. FMRI scans are used mainly in a research setting.