156 Chapter 6 Mean group comparison For each hemodynamic parameter map, the group mean values were compared between steal versus non-steal regions and between different tissue types (GM, WM, edema and brain metastases) using Kruskal-Walis tests (Figure 3 and Supplementary Table 2). For the AAT, steal regions showed longer AAT than nonsteal regions, but only in WM regions (p = .015). Additionally, for both non-steal and steal regions AAT values in GM were lower than in WM (non-steal: p = 0.022; steal: p = .004), edema (non-steal, p = .006; steal, p = .001) or brain metastases (non-steal, p = .018; steal, p = .007). AAT values in WM, were lower than in edema in steal regions (p = .031), but did not differ from edema in non-steal regions (p = 0.37) or brain metastases (non-steal: p = .111; steal p = .143). Non-steal regions exhibited significantly higher CBF than steal regions in GM (p = .010), WM (p = .001) and edema (p = .034), but not within brain metastases (p = .129). Additionally, GM steal and non-steal regions showed higher CBF than WM (non-steal: p = .001; steal: p < .001) and edema (non-steal: p < .001; steal: p < .001). CBF was also higher in WM than edema, but only in the non-steal regions (non-steal: p = .004; steal: p = .136). While brain metastases showed high variability in CBF values, on average CBF values in brain metastases were higher than in WM (non-steal: p = .005; steal: p = .014) and edema (non-steal: p < .001; steal: p = .007), but did not differ from GM values (non-steal: p = .572; steal: p = .953). Hemodynamic lag values were shorter in non-steal than steal regions in GM (p < .001 ), WM (p < .001) and edema regions (p = .002), but not within brain metastases (p = .056). For non-steal regions both WM and edema had longer lag times than GM (p = .010 and p < .001, respectively). Additionally, edema exhibited longer lags than WM (p = .003). All of the steal regions exhibited similarly long lags across the tissue types. As steal regions were defined using the negative CVR values, all steal and non-steal CVR values significantly differed from each other (p < .001). In non-steal regions, GM showed a higher reactivity than WM (p = .002), edema (p < .001) and brain metastases (p = .009). WM also had a higher reactivity than edema (p < .001). Within steal regions, GM had a larger negative response than both WM (p = .003) and edema (p < .001), but not brain metastases (p = 0.61). Within these steal regions, WM had a more negative response than edema regions (p = .027). Next,Spearman’s Rho correlations were performed to test for an association between the different MRI metrics within the tissue types. For the non-steal regions,
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