The various types of brain scans give different kinds of information about the condition of the brain:
An FDG PET scan shows activity in the brain by measuring glucose metabolism levels. Since the brain’s primary energy source is glucose, greater activity by brain cells (neurons) is associated with greater levels of glucose metabolism. The neurons in a normal adult are typically very active, and so a normal FDG PET scan will show strong activity throughout the neuron-rich regions of the brain. However, when the brain is facing a challenge, such as dementia, underactive thyroid, or depression, the activity in certain parts of the brain is reduced. Different types of dementia affect different, but overlapping, regions of the brain. ADMdx analyzes this pattern to determine which dementia type is most likely indicated by a patient's scan. It is important to note that strong medications or even drowsiness can affect an FDG scan.
MRI is a type of imaging that can do many different things, but it is used most often to show the structure of the brain (unlike FDG PET, which shows the brain's function). With aging, brain tissue can shrink and die, in a process called atrophy. Certain dementias, can cause the brain tissue to shrink more rapidly, and the pattern of atrophy can be a characteristic signature of the disease. Atrophy typically occurs in the same brain regions that are shown to have reduced activity in the FDG PET scan. We can use this information to help determine which type of dementia is the most likely diagnosis for a given patient. An MRI scan can also be used to rule out other disorders, such as a brain tumor, stroke, or vascular disease.
A CT scan also shows the brain structure, but typically with less clarity than an MRI scan. CT scans are often used to check for bleeding in the brain after a patient experiences a fall. CT scans are frequently used in conjunction with FDG PET scans, so that both the structure and function of brain can be viewed together.
An amyloid PET scan shows whether there are accumulations of amyloid plaque in the brain, which may indicate either Alzheimer’s disease pathology or Lewy body disease (present in some, but not, all patients). Amyloid PET can help to differentiate Alzheimer’s disease from frontotemporal dementia (FTD), but not from Lewy body disease.
Tau proteins are a naturally occurring substance in the brain. In Alzheimer's disease and some other dementias, tau proteins form abnormal structures called "neurofibrillary tangles" in the brain. A tau PET scan shows whether there is an accumulation of tau and neurofibrillary tangles in the brain. Lewy body disease cannot be confirmed by a tau PET scan. Currently, tau PET imaging is approved for research use only.
A DaTscan is an imaging test in which small amounts of a radioactive drug are used to measure the brain's levels of dopamine (a molecule used in the brain to communicate between nerve cells). Dopamine levels are below normal in Parkinson’s disease and in Lewy body disease, but not in Alzheimer’s disease. Thus, a DaTscan can help to differentiate between Alzheimer’s disease and Lewy body disease, and/or to tell if there is Lewy body disease in addition to Alzheimer’s disease.