Radioactive inert gases

The most widely used tracer at the present time is the noble gas xenon which has a suitable isotope (xenon-133). Although xenon is not totally inert (at high concentrations it has an anesthetic effect), this is not a significant problem since the isotope has a high specific activity and is administered in low physical concentrations. Importantly, the rate at which the isotope leaves the brain can be determined by monitoring the decay in radioactivity using appropriate externally placed detectors. This technique has two distinct advantages over the Kety-Schmidt technique described earlier. First, no blood samples are required; the flow can be determined from graphical analyses of the learance curves (ideally by computer) and the result obtained rapidly in real time. Second, it is possible to obtain estimates of regional perfusion by increasing the number of suitably collimated detectors. However, as the number of detectors increases, their size and sensitivity decreases such that distinguishing the true washout curve from statistical noise becomes more difficult. A compromise between sensitivity and resolution must be made, and each clinician must decide the most appropriate approach to the situation under investigation.

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