Variability study of a non-invasive approach to the absolute quantification of cerebral blood flow with 99Tcm-ECD...

The absolute measurement of regional cerebral blood flow can be made non-invasively if dynamic scanning is performed immediately following the intravenous injection of 99Tcm-labelled HMPAO or ECD. By application of an elementary one-step kinetic model in which the arterial input curve is estimated from a region of interest over the anterior aortic arch projection, a brain perfusion index (BPI) can be calculated which is correlated to the total hemispheric blood flow. The aim of this study was to assess the variability of the BPI approach as applied to the calculation of cerebral blood flow. For 46 patients (30 men, 16 women) aged 23.7-74.4 years (mean 60.3), we calculated hemispheric BPI factors. For two subgroups of 10 patients chosen at random, several data-processing parameters, including the number of noise suppression steps and the linear slope interval for the Patlak fit, were assessed. To determine the influence of region-of-interest selection and to estimate intra- and inter-observer differences, three independent observers conducted a repeat analysis of all 46 patients. From five patients who underwent the same injection procedure twice, an estimate of intra-subject reproducibility was calculated. Temporal noise reduction of the input data significantly decreased the intra-observer variability. Visual estimation of the linear interval in the Patlak curve is much less reproducible than automatic interval selection; very high interval borders in particular significantly decrease the calculated BPI. The intra-observer coefficients of variation for the BPI were 5.3, 5.6 and 7.9% respectively, corresponding to 2.8, 3.0 and 4.1% when converted to hemispheric CBF. The slight inter-observer differences found could be attributed to differing processing parameters. For the five patients in whom the procedure was repeated, the hemispheric BPI reproducibility index was on average 2.6%. Determination of absolute hemispheric blood flow can be performed in a precise manner by means of a dynamic scan during the first 2 min after tracer injection after careful optimization of the processing parameters.