Positron emission tomography (PET) scans and combination PET and computed tomography (PET/CT) scans are routinely used to image the brain.
PET involves the use of an imaging device (PET scanner) and a radiotracer that is injected into the patient’s bloodstream. A frequently used PET radiotracer is 18F-fluorodeoxyglucose (FDG), a compound derived from a simple sugar and a small amount of radioactive fluorine.
Once the radiotracer accumulates in the body’s tissues and organs, its natural decay includes emission of tiny particles called positrons that react with electrons in the body. This reaction, known as annihilation, produces energy in the form of a pair of photons. The PET scanner, which is able to detect these photons, creates three-dimensional images that show how the radiotracer is distributed in the area of the body being studied.
Areas where a large amount of radiotracer, such as FDG accumulates, called ‘hot spots’ because they appear more intense than surrounding tissue, indicate that a high level of chemical activity or metabolism is occurring there. Areas of low metabolic activity appear less intense and are sometimes referred to as ‘cold spots.’ Using these images and the information they provide, physicians are able to evaluate how well organs and tissues are working and to detect abnormalities.
Because brain cells affected by dementia are less active, they consume, or metabolize less glucose than normal cells and will appear less bright on FDG-PET scans. Researchers are exploring the use of additional neuroimaging probes, including c-11 PiB, which bind to the abnormal beta-amyloid plaques associated with Alzheimer’s disease and allow them to be visualized on a PET scan.
In addition to dementia, PET scanning is used to diagnose other brain disorders that cause changes in metabolism and blood flow. For example, an area of decreased glucose metabolism may indicate the source of epilepsy. Abnormal patterns of glucose metabolism and an accumulation of amino acid may signal the presence of a brain tumor.
Single-photon emission computed tomography (SPECT) is routinely used to image the brain. SPECT may also be combined with CT for greater accuracy.
SPECT involves the use of an imaging device (a gamma camera) and a radiotracer that is injected into the patient’s bloodstream and accumulates in a target organ or attaches to specific cells. As the camera rotates around the patient, it detects the radiotracer and creates three-dimensional images that show how it is distributed in the body. This distribution pattern reveals information on blood flow and how organs and tissues are functioning.
MR spectroscopy or MRS is a variation of conventional magnetic resonance imaging (MRI). MRS can extract information regarding the concentration of metabolites, or chemical compounds inside the body.
Although MRS has been primarily used in research, it has the potential to provide useful clinical information helpful in the diagnosis and treatment of disease, including: