Ischemic penumbra

The ischaemic penumbra denotes the part of an acute ischaemic stroke that is at risk of progressing to infarction, but is still salvageable if re-perfused. It is usually located around an infarct core which represents the tissue which has or is going to infarct regardless of reperfusion. 

The primary aim of current acute stroke intervention is to prevent the penumbra from going on to infarct. 

CT perfusion

On CT perfusion the ischaemic penumbra ishas been variably defined asusing a combination of parameters. Generally, it should be conceptually thought of a "the area of the brain with reduced perfusion" minus the "infarct core".  In practice, which parameters are used to define each component has varied. A fairly common definition for penumbra is the area of the brain with ^1-2: 

1. increasedprolonged (increased) T-max, typically >6 seconds (or other measures of delayed arrival of contrast such as mean transit time (MTT)
2. moderate decreased cerebral blood flow (CBF or time to peak (TTP)), and...
3. normal or increased cerebral blood volume (CBV): due to auto-regulation

Hence: penumbra = CBF - CBV

This region will have only a moderate decreased cerebral blood flow (CBF). This is in contrast to the infarct core which will have marked decrease in CBF and also a decrease in CBV. 

MRI

On MRI, the ischaemic penumbra is also determined by the area of the brain with reduced perfusion ¹.

• penumbra: shows perfusion changes same as that with CT
• infarcted core: shows restricted diffusion (established infarct) apart from decreased CBV and CBF