Alzheimer’s Disease

Figure 1 - Alzheimer

History: 34-year-old male with clinical suspicion of early-onset Alzheimer disease.

Findings: Axial PET image of the brain (figure1) demonstrates normal activity in the frontal lobes (bright blue), with diminished activity in the parietal, temporal, and occipital lobes. This pattern is typical of Alzheimer disease. As a comparison, figure 2 is of a different patient with normal brain activity.

Figure 2 - normal

Discussion: Conventional imaging with CT and MR is neither sensitive nor specific for the diagnosis of Alzheimer disease (AD). Role of CT and MR in the work-up of dementia is primarily to assess the degree of atrophy and to exclude other causes of dementia such as normal-pressure hydrocephalus, multi-infarct dementia, and intracranial mass.

PET has been used in scanning the brain by evaluating the metabolic uptake of glucose-analogue FDG. High accuracy in differentiating AD from other forms of dementia has been reported. In patients with AD, deficits in temporoparietal metabolism are typically seen. Sensitivity of 94% has been reported in the literature (J Nucl Med 1994; 35:391-398), with high accuracy seen in early and mild forms of the disease. Abnormal temporoparietal uptake has also been demonstrated in asymptomatic patients with family history of early-onset AD (Ann Neurol 1997; 42:85-94). In a multinational consortium study (JAMA 2001; 286:2120-2127), PET accurately predicted cognitive decline in patients with AD, with sensitivity of 93%.

In 2004, Medicare approved coverage for PET scans in the early diagnosis of Alzheimer's Disease. This is an important step in the management of AD patients, as new drugs work best in the early stages to slow the progression of this disease.

PET offers information not available through other types of diagnostic studies. Since PET tracer FDG is taken up at a cellular level, functional images are generated that complement the traditional anatomic images generated through CT and MRI studies.