Physiology & Relevance of Cerebrovascular Reserve

Background: In addition to cerebral hemodynamics in a resting condition also the cerebral hemodynamics can be investigated after a challenge. This challenge can also be called stress-test and the parameter that is measured is often called cerebrovascular reserve or cerebrovascular reactivity. To investigate the Cerebrovascular reserve two things are needed: first a measurement method that can quantitatively or semi-quantitatively (for instance percentage change) measure cerebral hemodynamics. Second, a challenge that typically causes a vasodilatory change in the stress or challenge condition. The results are typically expressed as the (percentage) change between the rest and the challenge (vasodilatory) condition. In most of the MRI methods described below this change can be visually shown on the brain tissue level; to depict regions with for instance preserved and impaired reactivity. Generally speaking a smaller change in the ‘stress or challenge condition’ is often found in patients with cerebrovascular disease which is explained by vasculature that has a lower capacity to dilate. It is often hypothesized that in patients with cerebrovascular disease, such as a stenosis or occlusion, the vasculature distal to the stenosis or occlusion (at the level of the resistance arterioles) are chronically dilated to compensate for the lower perfusion pressure. The dilated arteries have a lower capacity to further dilate after a challenge. In a model of hemodynamic impairment this stage of dilation of resistance arterioles (with an increase in cerebral blood volume; CBV) has been doped stage I (with stage II hemodynamic impairment the oxygen extraction fraction is increased to maintain oxygen metabolism).[1] To investigate the health or disease status of the vasculature after a stress test is very common from the heart: in the last decade cerebrovascular reserve measurements have also gained popularity to investigate hemodynamic changes in the brain vasculature. It is hypothesized that hemodynamic changes (IE dilation of resistance arterioles) can be present even when the resting blood flow is within the normal range. In the past the most common method to measure cerebrovascular reactivity was blood flow velocity measurements changes of the brain vasculature in a rest condition and after a challenge. Typically, transcranial Doppler ultrasound was used for these flow velocity measurements after a carbondioxide or breathholding challenge. Also other measurement methods such as SPECT before and after a challenge have been used in many patient studies. In the current lecture we will describe MR Methods to measure the cerebrovascular reserve. To do so different measurements methods will be described and different challenging methods will be described with emphasize on the most commonly used methods. Finally the patients groups in which these cerebrovascular reserve measurements can be performed are indicated. For further reading, references can be checked below.[2-3]