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Simultaneous measurements of cerebral blood flow have been performed in baboons to assess the correlation between the acute and invasive nondiffusible microsphere technique and the noninvasive xenon-enhanced CT method. Blood flows in small tissue volumes (˜1cm3) were directly compared. The results of these studies demonstrate a statistically significant association between the two methods (P<.001). Similar correlations were obtained by both the Kendall tau (τ) and the Spearman (r) methods. The problems and limitations of such correlations are discussed.