161 Hemodynamic Imaging in Brain Metastases: ASL vs. Hypercapnic BOLD different tissue types (GM, WM, edema and brain metastases), and also tissues exhibiting different hemodynamic characteristics (steal versus non-steal regions). When visually inspecting the MRI data, all vascularly compromised regions visible in the ASL data were also reflected in the CVR metrics. Results indicate a strong, positive relationship between regional baseline CBF and CVR. Additionally, there was a strong positive relationship between the temporal metrics AAT and hemodynamic lag. In both instances, however, these relationships do not hold in brain regions with exhausted cerebral autoregulation (i.e. steal regions). Thus, while ASL may be able to inform on some functional vascular aspects, CVR provides additional information regarding brain tissue at risk as indicated by either vascular steal or increased hemodynamic lag. Tissue-specific hemodynamic characteristics were observed in our results. WM areas were characterized by lower CBF and longer AAT, which has been previously reported in healthy subjects.36 Additionally, WM areas showed lower reactivity as well as longer hemodynamic lag values than GM. This is consistent with existing literature, and reflects lower WM blood volume, different venous draining architecture and/ or potential flow redistribution resulting from the strong GM CO2 response. 29,31,37 Our findings suggest that the hemodynamic characteristics of healthy tissue are preserved in the normal appearing brain tissue of patients with brain metastases . On the other hand, large variability in the hemodynamic vascular parameters was seen within tissue containing untreated brain metastases. This was reflected in both the ASL and the CVR parameters. High variability in CBF measurements has previously been shown both between patients with brain metastases as well as between multiple brain metastases within patients.38 The primary tumor (i.e. origin of metastatic cells), might cause some of this heterogeneity as it affects the brain metastases growth pattern.15,39,40 Our findings additionally show that edema regions are characterized by lower CBF, lower CVR and both longer AAT and hemdoynamic lag times. Previous literature has also found impaired CVR within perifocal edema surrounding diffuse gliomas.13 It could be speculated that the local pressure in edematous regions restricts the ability of vessels to dilate and thus cannot maintain adequate perfusion. Additionally, this could limit the vasodilatory reserve capacity, as reflected by the CVR parameters. GM CBF values varied between 30 and 50 ml/100 ml/min in our patient sample, which is similar to the GM CBF values as measured using PET.41 CBF and CVR values were strongly correlated within adequately functioning regions, both when performing ROI and voxelwise statistics. That is, regions with lower blood supply also showed lower vascular reactivity. This was to be expected since CVR measurements are 6
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