What happens when we impair flow to the brain and decrease perfusion pressure?

What changes are we going to see on CBF, CBV and MTT?

MTT is used extensively but similar changes would be seen in Time To Peak and T-Max both of which are also reflections of the time that it takes for a contrast bolus to traverse through the microcirculation.

Now, one thing to remember is that the brain is able to auto-regulate blood flow at the level of individual arterioles, which is why we can perform functional MRI. This autoregulation aims to maintain CBF and it does this primarily by vasodilation, and thereby increasing CBV to compensate for a more spread out tardus-parvus perfusion curve, thus the prolongation of MTT. This response is normal and is seen when perfusion pressure is reduced by, for example, moderate stenosis. There are no symptoms, brain tissue functions perfectly well and can do so indefinitely. At some point however, these compensatory mechanisms are "Max-ed" out. MTT becomes increasingly prolonged, vasodilation is maximal, and therefore CBV is no higher increasing, but cerebral blood flow begins to drop. At this point, tissues can still function and there are no symptoms. Again this can last indefinitely, and this term is called benign oligaemia.

Severe stenosis, for example, in some patients with Moya-Moya, may have this. Some patients with stroke with good collaterals may have benign oligaemia at the margins of the penumbra.

Decrease perfusion pressure even further, and eventually there isn't enough blood getting through to fill these maximally dilated vasculature, and therefore CBV starts to drop, so does CBF. At some point, the remaining blood flow is insufficient to allow tissues to function normally, but not yet low enough to doom these tissues to die. This is penumbra.

Decrease it even further, and eventually tissues die, and this is infarct core.

CBF is usually reduced in both penumbra and core, and thus using the above demonstrated chart will lead to overestimating the size of the core. Instead, if assessing visually, you are best to use a combination of MTT and CBV, looking for roughly normal CBV and significantly prolonged MTT with only mild reduction in CBF; the penumbra. In contrast, the core is an area of significantly reduced CBF and CBV.

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