Brain perfusion studies by xenon-enhanced CT using washin/washout study protocols.

Very short inhalation times and short total examination times are desirable in cerebral blood flow measurements by xenon enhanced CT to minimize the possibility of flow activation and--more importantly for practical purposes--the probability of patient motion due to the effects of xenon. We have investigated washin/washout procedures and have compared them with conventional washin scanning protocols by simulation and in clinical studies. Examination protocols with only 3 min of inhalation and up to eight scans, all taken at 1 min intervals, provide flow estimates with smaller SDs than would be obtained for washin studies taken with the same total radiation dose. Compared with a standard 8 min washin procedure, a 3 min washin/5 min washout study using the same dose yields an SD reduction by a factor of 1.3 for low flow areas and of 1.8 for high flow gray matter. A 3 min washin/3 min washout study, employing only 78% of the dose of an 8 min washin study, will still provide an SD reduction factor of 1.7 in gray matter. These results have been confirmed qualitatively by studies carried out both in volunteers and in patients.