The most widely used method of intraoperative cerebral blood flow measurement, and the technique used at our institution, is the intraarter-ial injection of 133Xe into the internal carotid artery with extracranial detection of the clearance curve using highly collimated scintillation detectors. 133Xe is a low-energy gamma emitter that diffuses freely through the brain, and thus is an ideal agent for measurement of cerebral blood flow. This radionuclide has an unstable nucleus, continuously emitting beta, gamma, and X-ray photons. The emitted photons are of low energy and, as such, can be absorbed by
0.1 mm of lead to reduce exposure by 90%. It is distributed as a 5% concentration mixed with 95% carbon dioxide, which is then dissolved in 0.9% sodium chloride for injection . 133Xe freely passes through cell membranes, crosses the blood-brain barrier and is not metabolized anywhere in the body. After injection into the internal carotid artery, it is retained momentarily in brain tissue and is then released through normal venous outflow channels. Ninety percent of 133Xe is expelled through the lungs on first-pass, thereby minimizing recirculation of the radionuclide through the cerebral vascu-lature .
It is important to obtain baseline measurements of 133Xe for each patient intraopera-tively. Accuracy of instrumentation and measurement devices must also be verified. The main components of the 133Xe CBF measurement system are a scintillation detector, pulse height amplifier, count-rate meter, power supplies and a strip chart recorder (Fig. 17.5). The two types of scintillation detectors in use today
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