RSNA: Amyloid imaging—Not quite a silver bullet for Alzheimer’s diagnosis

CHICAGO—Amyloid imaging shows promise as a predictive tool for Alzheimer’s disease; however, a multi-modal imaging strategy likely will be required to differentiate Alzheimer’s disease from other dementias, Alexander E. Drzezga, MD, of Harvard Medical School in Boston, said during a refresher course Nov. 27 at the annual meeting of the Radiological Society of North America (RSNA).

Drzezga started with an overview of the basic challenge of Alzheimer’s diagnosis, as it is not possible to diagnose Alzheimer’s in vivo, which hampers initiation of early treatment before the patient has suffered irreversible damage

“There is an urgent need for a marker. We believe amyloid imaging may play a role,” he continued. Post-mortem studies have correlated amyloid scans with actual plaque density.

Other studies have shown the predictive value of amyloid imaging. Sensitivity studies of patients with mild cognitive impairment showed that an amyloid-positive scan can predict patients who will convert to Alzheimer’s disease. However, amyloid imaging does not provide a clear idea how soon a patient will convert to Alzheimer’s, said Drzezga.

Recent revisions to diagnostic guidelines recognize the utility of amyloid imaging.

The National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimer's Disease and Related Disorders Association recently revised diagnostic guidelines for Alzheimer’s to indicate that probable Alzheimer’s can be defined in terms of clinical imaging and supportive features, including amyloid imaging.

The National Institute on Aging NIA and the Alzheimer’s Association AA published guidelines for the diagnosis of Alzheimer’s disease in April 2011, which suggests patients with mild cognitive impairment and a positive amyloid scan have an intermediate probability of conversion to Alzheimer’s.

However, approximately 25 percent of cognitively healthy subjects will have positive amyloid scans. “It’s not clear what this means,” said Drzezga. Amyloid imaging also appears to provide limited value in follow-up imaging of patients with Alzheimer’s disease as amyloid remains stable at this stage.

Another challenge is that amyloid imaging does not differentiate Alzheimer’s disease from other dementias, such as dementia with Lewy Bodies.

“It is clear that that amyloid imaging is a sensitive tool with regard to detection of amyloid pathology in the brain and may be useful for diagnosis of Alzheimer’s disease in the mild cognitive impairment stage when patients may have amyloid in brain but may be long way from developing dementia,” said Drzezga. However, amyloid does not answer all  of the diagnostic questions. “A combination of imaging tools may be the perfect way to differentiate dementia disorders.”