12 Chapter 1 performance in healthy individuals.46 Thereby, parallels have been drawn between the pathogenesis underlying vascular cognitive impairment (e.g. vascular dementia) and radiation-induced brain injury.47,48 Figure 3. Simplified illustration of brain metabolism. Oxygenated red blood cells travel from the heart through the arteries (red), to the arterioles in the brain where they deliver oxygen and nutrients to the neurons (green cells), before the deoxygenated blood returns to the heart through the veins (blue) for renewed circulation. The brain has an intricate network of blood vessels in order to meet its high oxygen and glucose demands (Figure 3). MRI is a non-invasive method that can be used to detect subtle vascular changes throughout the brain. Cerebral blood flow (CBF) indicates the amount of blood flowing to the brain through the arteries and can be measured using arterial spin labelling (ASL) MRI.49–51 To guarantee healthy functioning, CBF to the brain must remain relatively constant in response to changes in perfusion pressure or other hemodynamic events. Cerebrovascular reactivity (CVR) plays an important role in maintaining optimal CBF by regulating the diameter of cerebral vessels in response to changes in blood pressure and CO2 levels. 52–54 For example, increased CO2, a metabolic waste product, acts as a signal for heightened brain activity. In response, arterial blood vessels in these active regions dilate, facilitating increased blood flow to help remove excess CO2 and deliver oxygenrich blood to this tissue (Figure 4). This dynamic control, facilitated by smoothmuscle cells, also serves as a significant compensatory mechanism when cerebral hemodynamics are compromised by disease (e.g. narrowing of blood vessels). The CVR response can be assessed and spatially mapped using Blood Oxygenation Level-Dependent (BOLD) MRI in combination with controlled hypercapnic stimuli, like inhalation of CO2. When the brain’s metabolic demand is not (fully) met by blood flow compensation through CVR, the Oxygen Extraction Fraction (OEF) serves as an
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