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A method has been developed for repeat quantitation of physiological parameters from multiple administrations of radiotracers. The background activity distribution due to the previous tracer administration was formulated according to a compartment model, so as to allow initiation of the next scan while residual radioactivity exists, minimizing the intervals between scans, with the minimum enhancement of the statistical noise. Theoretical simulation for repeat administration of H215O demonstrated that the transient tracer distribution was highly weighted on transient rCBF immediately after the previous tracer administration, and that the change of rCBF after a certain period has only small contribution to the estimated tracer distributions. Another set of simulation showed that estimated rCBF was sensitive to the transient rCBF only immediately after the tracer administration, and that the change after a certain period does not contribute to the rCBF estimated by the present approach. This technique was then applied to a clinical study of serial administration of H215O and 15O2, and demonstrated that the yielded CBF and CMRO2 are well agreed with those determined based on the classical 3-step measurements. We thus conclude that quantitation of physiological parameters can be determined by PET with much shorter examination interval as compared with the previous study protocol.

Using positron emission tomography (PET), cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMR02) were measured in 32 healthy volunteers aged from 27 to 67 years. In bilateral putamen, left supratemporal\ left infrafrontal and left parietal cortices, CMR02 showed a significant decline during aging. The age-related decline of CBF was seen only at the left superior temporal cortex. The mean CMR02 was significantly lower in the elder group (over 51 years old) than in the younger group (under 50 years old), whereas no significant difference in mean CBF between the two groups. The poor correlation of CBF to the age could be explained partly by the fact that CBF is easily influenced by the physiological, psychological and/or environmental factors. The age-related changes of CMR02 were more marked in the association cortices of the left hemisphere than in that of the right hemisphere.