Mapping brain activity by imaging blood flow and glucose metabolism

Regional cerebral blood flow and glucose metabolism are indicators of regional neuronal synaptic activity. (6) Radiotracers for these processes, such as H2(15)O to index blood flow, are used to image brain activity in psychiatric illness. Glucose metabolic mapping using [ (18)F]deoxyglucose has some disadvantages over flow mapping. Radiation dosimetry limits for [(18)F]deoxyglucose and the long half-life of the tracer restrict repeated measurement in a subject over a short time-scale. In contrast (15)O-based methods allow, for example, 12 measurements of regional blood flow over a 3-h period in a single subject.

The rapid development of PET cameras and automated data analysis techniques in the last decade has established flow-based functional imaging as a large and active research activity.(4) Early studies examined patients and normal controls while they were at rest in the scanner. Because of the unconstrained nature of subjects' cognitive states at rest some investigators have asked patients to rate certain mental experiences, such as presence of hallucinations whilst being scanned.

As the techniques of regional cerebral blood flow mapping have advanced there has been an equivalent sophistication of experimental design with rest state studies being overshadowed by activation paradigms.(4) In an activation design, subjects are engaged in a specific cognitive task whilst being scanned, for instance generating words, and the blood flow pattern is compared with flow present in a baseline condition such as repeating words. PET activation experiments may involve categorical, correlational, and factorial designs.(4)

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