The validity of an intravenous isotope method for cerebral blood flow measurements. An experimental study

The intravenous isotope method has been examined. The method uses 99Tc as the indicator with recording of the radioactivity with two gamma-dectors placed bilaterally over the skull and oriented over the bifurcation of the middle cerebral artery. 65% of the total curve obtained was found to be generated by activity passing in the ipsilateral internal carotid artery, 25% in the contralateral internal carotid artery and less than 10% by activity passing in the two external carotid arteries. The counting rate efficiency for an intracerebral location was 19.4 +/- 1.8 times the count rate from the same activity placed at 1 m distance in air which in turn was 5 cps/muCi. The variability in efficiency from patient to patient was thus estimated at +/- 9%. The duration of the input bolus had a marked influence on the vurve, which will be of importance since it showed a rather large variability from patient. Using the peak activity as an index of the cerebral blood flow the intravenous method showed a surprisingly good correlation with the 133Xe wash-out method; the correlation coefficient was 0.93 and the variability at slightly subnormal values estimated at +/- 15%. It is concluded that the method can be used in the case of limited demands on the accuracy of the cerebral blood flow determinations.