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Neuroimaging of Dementia

Vincent Mathews
Vincent P. Mathews, M.D.

Dementia is an organic mental disorder characterized by a general loss of intellectual abilities involving impaired memory, judgment and abstract thinking, as well as personality changes. Dementia affects approximately 4 million Americans and is projected to affect more than 14 million by 2050. Alzheimer's disease (AD) is the most common cause of dementia and accounts for 50-75% of neurodegenerative dementia cases. The role of imaging in dementia is to exclude "treatable" causes, to clarify diagnostic dilemmas, and to identify early onset cases for possible innovative therapies in clinical trials.

Structural imaging with computed tomography (CT) or magnetic resonance imaging (MRI) are primarily used to exclude treatable causes such as chronic subdural hematoma, normal pressure hydrocephalus, and mass lesions. MRI is also useful in diagnosing cerebrovascular disease that may cause dementia and the prion disease, Creutzfeldt-Jakob disease, which causes a rapidly progressive dementia. CT and MRI typically demonstrate only nonspecific findings in AD.

bilateral hypometabolism

F18 fluorodeoxyglucose PET demonstrates bilateral hypometabolism in the temporal and parietal lobes (arrows) in a 58 year old woman with Alzheimer's disease.

The imaging diagnosis of AD requires physiologic imaging with either positron emission tomography (PET) or single photon emission computed tomography (SPECT). PET more effectively distinguishes AD from normal than does SPECT and is the preferred examination. PET demonstrates a pattern of glucose hypometabolism in AD that involves the temporal, parietal and posterior cingulate regions (see Figure). Requirements for Medicare reimbursement for PET in dementia patients are stated in CMS National Coverage Determination section 220.6.13. Briefly, this requires 1) a recent diagnosis of dementia and documented cognitive decline for more than 6 months, 2) presence of diagnostic criteria for both AD and frontotemporal dementia, and 3) exclusion of alternate neurodegenerative disease or causes.

In summary, dementia is an increasingly important health problem in our aging society. Structural imaging, particularly with MRI, is useful to exclude treatable causes of dementia. PET is the most useful imaging test to diagnose AD and differentiate it from normal and other neurodegenerative diseases.

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