Numerous methods have been employed to assess both global and regional CBF. This infor mation has been used to evaluate autoregulation and CO2 reactivity, to ensure adequate CBF, to assess the effect of treatments in modifying CBF, and to assess CBF as an outcome predictor. Although many methods are available, as yet there is no functional bedside method for the measurement of CBF that can be performed repetitively in a clinically useful manner. Several semi-quantitative methods are nevertheless clinically useful.
Kety-Schmidt Technique The gold standard for global CBF measurement is the Kety-Schmidt technique of nitrous oxide washin. Although first described in 1945, the technique remains valid today. It measures global hemispheric blood flow, necessitates cannulation of the jugular bulb, thus allowing derivation of cerebral metabolic rate for oxygen, and can be performed at bedside.
Radioactive Xenon Washout The Xenon133 washout method is derived from the Kety-Schmidt technique, and is probably the most commonly used bedside technique today. Xe133 can be administered by inhalation or intraarte-rial injection, but is most commonly given by intravenous injection. Multiple detectors placed next to the head allow measurements of regional CBF. Accuracy may be impaired in low-flow conditions, and areas with no flow cannot be detected (the "look-through phenomenon").
Stable Xenon CT Quantitative CBF can also be determined using stable xenon (30%) by inhalation and CT. Flows at multiple regions of interest can be measured. Transportation to the CT suite is necessary, and stable xenon is expensive and, at 30%, has significant cerebral vasodila-tory effects. Repetitive measurements are possible and evaluation of therapeutic response to treatments aiming to modify CBF may be carried out with this method.
CT Perfusion Scan Quantitative CBF can also be obtained using contrast CT. The computer algorithm examines the transit time of contrast and derives the regional CBF. Compared with stable xenon, only limited slices can be obtained. This technique also allows repetitive measurements, making it possible to assess the patient's response to therapeutic maneuvers such as hyperventilation or augmentation of blood pressure.
Positron Emission Tomography As mentioned above, PET can measure regional CBF, metabolism and volume. This is, however, expensive and, with limited availability, remains primarily an investigative tool.
Laser Doppler Continuous monitoring of CBF is possible using laser Doppler flow probes. This, however, involves implantation of a probe via a small burr hole directly into the brain parenchyma and can only measure local CBF in a volume of 1 mm3.
Semiquantitative/Qualitative Cerebral Blood Flow
Single-Photon Emission Computed Tomography (SPECT) Using Technetium99 isotope administered intravenously, relative regional distribu
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