148 Chapter 6 with brain metastases, the macrometastases subsequently ensure adequate blood supply and thus alter the vascular microenvironment.16–18 The vasogenic edema often surrounding these brain metastases, caused by the local blood-brain barrier disruption19, can lead to additional physiological changes. All these brain metastases related physiological changes are likely to be reflected in hemodynamic MRI measurements. ASL and CVR measure associated responses, both describing the status of the vascular network. Thereby they can provide complementary information, however, it remains unclear how ASL and CVR parameters relate in different tissue types or under different hemodynamic circumstances. In the current exploratory study we test how ASL parameters measured during a physiological steady state relate to functional vascular parameters as measured using CVR in a population of patients with brain metastases. The expected tissue-heterogeneity caused by the brain metastases growth provides a testing ground to answer this question. CBF and AAT were compared to CVR magnitude and hemodynamic lag using two different approaches. Firstly, the inter-modality agreement between ASL and CVR was tested by comparing group values for different tissue types (grey matter (GM), white matter (WM), edema and brain metastases) for both ASL and CVR metrics. Secondly, we assessed the spatial correlation between ASL and CVR metrics throughout the brain using both a ROI and voxelwise approach. Hereby a distinction was made between brain areas showing adequate vascular responses and those exhibiting vascular steal phenomena. Understanding how baseline vascular physiology relates to dynamic vascular processes in cases with brain metastases will aid in interpreting these measures in future research. METHODS Study set-up and population For the current retrospective observational study, MRI datasets were included from the ongoing Assessing and Predicting Radiation Influence on Cognitive Outcome using the cerebrovascular stress Test (APRICOT) study. Participation consists of an elaborate neurocognitive exam and MRI scans, including a BOLD MRI scan during breathing challenges, before radiotherapy and approximately three months after radiotherapy. For the purpose of this study only the MRI data acquired before radiotherapy during the period between October 2020 and February 2022 was used. The study population consists of adult patients (≥18 years) with either radiographic and/or histologic proof of metastatic brain disease that were referred to the radiotherapy department of the UMC Utrecht for radiation therapy of the brain metastases. Specific in- and exclusion criteria for participation in the APRICOT
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