Diagnostic Imaging
June 2004

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MRI, PET compete for gold standard in Alzheimer's

Following many 'one-dimensional' studies, researchers are calling for cross-modality comparisons

While many studies attest to the efficacy of PET and MR for diagnosing Alzheimer's disease, none directly compare the two modalities against each other.

"The literature is full of 'one-dimensional' studies proving the efficacy of MRI or PET to predict who will get AD. What's missing, however, are good solid comparisons across modalities," said Dr. Clifford R. Jack, a professor of diagnostic radiology at the Mayo Clinic in Rochester, MN.

It's a timely discussion, he said, as the National Institute on Aging recently unveiled a neuroimaging initiative to compare MR and PET in diagnosing and following patients with AD.

According to Jack, imaging studies must provide three clinically useful pieces of information:

- Will a scan improve the differential diagnosis over that provided by a clinical exam?

- Can a single scan predict when a person with mild cognitive impairment will convert to AD?

- Can serial scans measure the rate of change indicative of pathology?

Proponents on both sides will contend that their modality passes the test, he said, and there is truth to their assertions. But which imaging modality a clinician should choose for a particular patient at a particular time has not been studied.

Investigators at the University of California, Los Angeles, a hub for PET research, have shown that FDG patterns of hypometabolism in the parietotemporal cortex are highly suggestive of AD. While PET imaging is costly at $700 to $1200 per scan and is not yet reimbursable, UCLA researchers suggest the use of PET for this patient population would save money (J Nuc Med 2002;43[2]:253-266).

Researchers at the Massachusetts General Hospital Institute for Technology Assessment, however, claim that adding FDG-PET to the standard strategy used at specialized centers to diagnose AD would not be cost-effective (Radiology 2003;228:515-522).

Several recent MR studies have shown that patterns of brain atrophy correlate with clinical memory decline. Jack and colleagues at the Mayo Clinic found that atrophy rates of the hippocampus, entorhinal cortex, whole brain, and ventricle in patients with AD were more consistent than specific clinical tests to assess cognitive decline (Neurology 2004;62:591-600).

Mony J. de Leon, Ed.D., and colleagues at New York University found that volume changes in the medial temporal lobe were the best predictors of future memory decline (Radiology 2003;229:691-696). Dr. Ranga Krishnan and colleagues at Duke University showed that patients taking the drug donepezil had increased hippocampal volumes and increased levels of N-acetylaspartate (Am J Psychiatry 2003;160:2003-2011).

"We have a need for rigorous head-to-head cross-modality comparisons," Jack said. "These will have to be big, expensive studies, but that's what the field needs."