At the time of the initial vascular insult, perfusion pressure is reduced. This is followed by arteriolar dilatation and hypothetical capillary recruitment to route as much blood as possible to ischemic tissue, resulting in an increased capillary blood pool. (physiologic autoregulation)

Parameters of cerebral hemodynamics:

1. Cerebral Blood Flow (CBF): Represents instantaneous capillary flow in tissue.
2. Cerebral Blood Volume (CBV): Describes the blood volume of the cerebral capillaries and venules per cerebral tissue volume.
3. Mean Transit Time (MTT): Measures the length of time a certain volume of blood spends in the cerebral capillary circulation.

   MTT = CBV/CBF

4. Time To Peak (TTP): A parameter inversely related to CBF in which reduction of blood flow results in an increase in the time needed for the contrast to reach its peak in the perfused volume of brain tissue.

Following vascular insult, the first abnormality is hemodynamic derangement, which precedes and leads to the metabolic and histopathologic abnormalities.

CBF is low after the vascular insult unless reperfusion occurs either spontaneously or following intra-arterial thrombolysis.

Early reperfusion in the reversible stage of neuronal injury will salvage the brain tissue and stroke will not occur. Early MRI changes may revert to almost normal.

Late reperfusion will not prevent stoke from occurring. On the contrary, it may lead to hemorrhagic conversion because of the damage that the vascular endothelium has already suffered.

Hemodynamic zonal infarct characterization

(Infarction core)

• CBF and CBV are both low due to failure of the autoregulation and the end result is DEAD tissue.
• MTT will be variable depending on how low CBV and CBF go below the critical level.

(Penumbra)

• Low CBF: secondary to compromised main branch blood supply.
• Normal CBV or High: secondary to collateral blood flow from neighboring territories and active autoregulation that causes vasodilatation. This maintains the O₂ delivery in the range that keeps the cells viable, but not sufficient for normal function.
• High MTT: secondary to the slow collateral flow; maximizes O₂ unloading to the cells.
• Viable dysfunctional neuronal tissue.

(Normal perfused viable brain tissue)

• Normal CBF: no vascular insult in this region
• Normal CBV
• Normal MTT
• Normal functioning neuronal tissue

Perfusion Weighted MR Imaging (PWI)

The susceptibility contrast MR imaging method is used for evaluation of tissue perfusion.

• Gadolinium contrast agent decreases T1 and T2 in tissues in which it accumulates.
  1. T1 shortening (called the R1 effect) results in increased signal intensity on T1-weighted images.
  2. T2 or T2* shortening (called the R2 effect) represents loss of phase coherence of transverse magnetization, resulting in intravoxel signal loss due to magnetic susceptibility.
• Bolus contrast perfusion MR imaging is based on R2 effects resulting from the production of magnetic field gradients between the lumen of a vessel and surrounding tissue.
• After bolus injection of Gadolinium contrast agent, sequential images are obtained at 1-2-second intervals during the first pass through the cerebral circulation.
• Calculations and mapping are performed on a per-pixel basis by using the susceptibility change-versus-time curves to measure CBV, CBF, MTT and TTP.

Infarct zonal characterization on PWI

• In the first few hours of vascular insult there is only a penumbral pattern (low CBF and high CBV due to maximally functioning autoregulation mechanism and collateral circulation). This is viable tissue at risk that could be saved, but it may not yet show diffusion restriction.
• Subsequently, the autoregulation mechanism starts to fail and the infarction core spreads, gradually reaching its final extent, which is the entire penumbra (low CBF region).
• The mean lesion-to-contralateral region CBV ratio of 0.19 (range, 0.03–0.36) may suggest a viability threshold.

DWI and ADC map demonstrate an area of diffusion restriction in the right MCA territory consistent with acute infarction.
CBF and MTT map (PWI) demonstrate the infarct penumbra which is larger than the core, indicating the presence of salvageable tissue.
CBV map (PWI) demonstrates infarct core which is slightly smaller than the area of diffusion restriction.