13 General introduction and thesis outline additional regulatory mechanism. OEF reflects the amount of oxygen extracted from the brain’s arterial supply and is closely linked to oxygen usage and metabolism in the brain. Using innovative imaging techniques, OEF can be mapped by combining Quantitative Susceptibility Mapping (QSM) with quantitative BOLD (qBOLD). The cerebral metabolic rate of oxygen (CMRO2), is calculated by multiplying CBF and OEF, and thus reflects the balance between the two. As the brain heavily relies on oxygen-dependent glucose metabolism for energy production, CMRO2 serves as a key indicator for energy homeostasis and brain health.55 To illustrate, in a healthy brain CBF typically increases more than the oxygen metabolism, leading to a decrease in OEF and consequently a relatively steady CMRO2. 56 Figure 4. Illustration of vasodilation in arterial blood vessels, for example in response to increased CO2 levels in the blood. These different parameters thus reflect various aspects of the brain’s vascular reserve capacity, working together to ensure a continuous and sufficient supply of oxygen and nutrients.57,58 Disruptions to this hemodynamic balance, such as those caused by tumor growth or cranial irradiation, could result in altered blood vessel structure and function. While previous animal studies have shown radiation-induced vascular damage in surrounding healthy brain tissue, research on hemodynamic changes after cranial irradiation in humans is scarce. However, the field is continuously evolving and advances in imaging techniques now allow non-invasive MRI measurements of cerebral hemodynamics. Hypothetically, in humans vascular damage following cranial irradiation could impact the ability of blood vessels to constrict and dilate (i.e. CVR), reducing the capacity to modulate CBF to meet tissue needs. When blood flow compensation through CVR is insufficient, 1
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