PET measurement of cerebral acetylcholine esterase activity without blood sampling

Measurement of cerebral acetylcholine esterase (AChE) activity is of clinical interest for the differential diagnosis of memory disorders and dementia. We developed and tested a non-invasive method for quantitation of regional cortical AChE activity with carbon-11-labelled N-methyl-4-piperidyl acetate (11C-MP4A) that does not require arterial blood sampling. AChE activity was measured in terms of the rate constant for hydrolysis of 11C-MP4A (k3). The physiological model is based on the very high AChE activity in the basal ganglia, which are used as a reference structure. Non-invasive k3 was compared with k3 determined with a standard technique by fitting kinetic tissue and metabolite-corrected plasma data in nine subjects with and without dementia. Across all regional values, a very high correlation of 0.92 was found, with a tendency towards moderate underestimation of k3 by 5%-14% with the non-invasive technique as compared to the invasive technique. In addition to its advantages with respect to practicability, the new non-invasive technique overcomes problems of the invasive technique that are related to interindividual variation of delay times between cerebral and peripheral tracer arrival and measurement of very small amounts of non-hydrolysed tracer in plasma samples.