9(9m)Tc-HMPAO and Tc-99m-ECD perform differently in typically hypoperfused areas in Alzheimer's disease

Technetium-99m hexamethylpropylene amine oxime (HMPAO) and 99mTc-N,N"-1,2-ethylene diylbis-L-cysteine diethyl ester dihydrochloride (ECD) yield significantly different images of cerebral perfusion owing to their particular pharmacokinetics. The aim of this study was to assess the topography, extension and statistical significance of these differences in Alzheimer's disease (AD). Sixty-four patients with mild to moderate AD were retrospectively selected by two European centres. Two series of patients, including 32 studied with 99mTc-HMPAO single-photon emission tomography (SPET) and 32 studied with 99mTc-ECD SPET, were matched for sex, age (±3 years) and severity of cognitive impairment as assessed by the Mini-Mental State Examination (MMSE) (±2 points), following a case-control procedure. SPET data were processed using SPM99 software (uncorrected height threshold: P=0.001). 99mTc-ECD SPET gave significantly higher uptake ratio values than 99mTc-HMPAO SPET in several symmetrical clusters, including the right and left occipital cuneus, the left occipital and parietal precuneus, and the left superior and middle temporal gyri. 99mTc-HMPAO SPET gave significantly higher uptake ratio values than ECD in two smaller clusters, including the hippocampus in both hemispheres. In AD, relative brain uptake of 99mTc-HMPAO and 99mTc-ECD is different in several brain regions, some of which are typically involved in AD, such as the precuneus and the hippocampus. These differences confirm the need for specific normal databases, but their impact on routine SPET reports in AD is not known and deserves an ad hoc investigation.