Measurement of cerebral blood flow by the stable xenon computerized tomography method

Measurement of cerebral blood flow (CBF) by computerized tomography (CT) is a three-dimensional method with better spatial resolution than the two-dimensional methods. Its principle was first described by Drayer et al. in 1978, with stable xenon (Xes) as CBF indicator. CBF quantitation is based on Fick's principle transformed by Kety and Schmidt when the indicator is a diffusible inert gas. Xes concentrations in cerebral parenchyma and arterial blood are the initial parameters in Kety's equation; they are expressed as variations in attenuation coefficient. Examinations are performed with a Somatom DRH (Siemens) apparatus. Xes (35%) is inhaled from a closed circuit ventilation system which enables xenon to recirculate. From a console connected to the inhalator the operator can command gas preparation, start examination, acquire and transfer data. A reference ("native") section is cut at the site chosen on the topogram. Twelve sections, each 8 mm thick, are then performed while the patient inhales, during 6 minutes, the mixture: air-35% xenon + 65% oxygen. The series of images which enable the CBF parametric image to be calculated is treated in four stages: 1. Xenon concentrations in arterial blood are calculated from the Xes values measured in the air exhaled at the end of expiration. 2. All contrasted sections are visualized on the image monitor after subtraction of the background noise. 3. The CBF parametric image is calculated by Koeppe's optimization method (linear calculation of least squares), using a PDP 11/44 processor. 4. The CBF parametric image is treated to give the CBF value expressed as ml.100 g.min. The method has its limitations: it depends on the limited signal/noise ratio and on the patient's complete immobility; cerebral metabolic rates cannot be measured. But these limitations are largely outweighed by major advantages: the CT/Xes method is non-invasive, safe and reproducible. Owing to its excellent spatial resolution, it provides very accurate maps of superficial and deep regional blood flows. As it measures very low blood flows and can give partition coefficient values, it is of considerable help in the study of ischaemic and degenerative cerebral pathologies.