193 Physiological MRI Biomarkers & Cognition after SRS for Brain Metastases challenging when scanning patients at multiple time points due to the inability to reproduce exact head positions. Thereby, subtle changes in QSM, and consequently OEF and CMRO2, may be attributed to variations in head positioning. Additionally, the biophysical model used to estimate OEF is not optimized for regions with edema. Specifically, the prolonged T2 relaxation time (~200 ms) observed in these regions exceeds the maximum echo-time of the ME-GRE sequence (44.5 ms).73 However, as CVR is also affected within edema regions, it is likely other autoregulatory mechanisms, including OEF, play an important role. Furthermore, an inherent limitation of using BOLD-metrics is the dependency of the BOLD response on multiple factors, including changes in cerebral blood volume, cerebral metabolic rate of oxygen consumption, arterial partial pressure of oxygen, and baseline parameters such as hematocrit, OEF, CMRO2, and blood volume. 74,75 This complex interplay hinders the identification of precise underlying mechanisms, but also underscores the value of integrating multiple physiological MRI measures. Lastly, compared to 7T scanners, the BOLD and ASL signal sensitivity, especially in WM and edema, is notably lower. Combined with the heterogeneous and small patient in this preliminary study, a lack of significant findings could partly be attributed to these factors. CONCLUSIONS Our findings suggest that resolved edema regions show metabolic recuperation but ongoing vascular damage, highlighting the sensitivity of CVR as a marker for vascular changes. We also observed a global increase in CBF in healthy-appearing brain regions following SRS, possibly indicating a restorative mechanism against radiationinduced inflammation. While regions exposed to higher doses exhibited larger declines in CBF, CMRO2, and CVR, there was notable heterogeneity among patients and across dose regions. Case analysis demonstrated some of this heterogeneity may be attributed to the tumor response. Overall, our preliminary results suggest that within the 3-month follow-up window no radiotherapy effects on physiological parameters occurred in healthy-appearing brain tissue. Nevertheless, while SRS can lead to local control of the brain metastases it can also have long-term side-effects (i.e. months to years).76,77 Continued long-term research with larger patient samples allowing for meaningful grouping of patients will enhance our understanding of the intricate interplay between radiation dose, brain health, and physiological responses. 7
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