Xenon CT cerebral blood flow in patients with head injury: influence of pulmonary trauma on the input function

The noninvasive xenon-enhanced CT (Xe CT) cerebral blood flow (CBF) method has been used in patients with severe traumatic brain injury (TBI) to ident ify the blood-flow thresholds for the development of irreversible ischaemia or infarction following severe TBI. Quantitative regional CBF (rCBF) estim ates are based on the assumption of identity between the end-tidal xenon co ncentration curve, used as the input function, and the arterial xenon conce ntration curve, being the true input function to the brain. Accordingly, rC BF data addressing the issue of ischaemia should be viewed in relation to p ossible deviations between the end-tidal and arterial xenon concentration c urves. To evaluate this possible source of error, we studied five patients with severe TBI (Glasgow coma score less than or equal to 7) who also bad p ulmonary trauma. CBF was studied with the Xe CT CBF method and flow rates w ere determined, by fitting the Kety equation to each CT voxel using either the end-tidal or the arterial xenon curve as input function. In all patient s rCBF estimates were lower using the end-tidal xenon curve than with the a rterial xenon curve; the mean underestimation was 20.3% in gray metter and 17.3% in white matter. The deviation between the end-tidal and arterial xen on concentration curves should be considered as a source of error when defi ning critical flow values according to the flow thresholds of tissue viabil ity.