CT perfusion in ischemic stroke
Updates to Article Attributes
CT perfusion in ischaemic stroke has become established in most centres with stroke services as an important adjunct, along with CT angiography, to conventional imaging of the brain with a non-contrast CT scan.
Its advantage is that it is able to delineate ischeamic areas of the brain which may be salvaged by intervention (e.g. thrombolysis or clot retrieval), known as the penumbra, from the parts which are infarcted or irrevocably destined to go onto infarct regardless of therapy, known as the infarct core.
Although MRI is more sensitive to the early parenchymal changes of infarction (see DWI in acute stroke) its clinical application has been limited by difficulties in accessing MRI in a timely fashion in many institutions; this is especially important in this clinical setting as rapid imaging and treatment are crucial to successful intervention.
Radiographic features
The key to interpreting CT perfusion in the setting of acute ischaemic stroke is understanding and identifying the infarct core and the ischaemic penumbra, as a patient with a small core and a large penumbra is most likely to benefit from reperfusion therapies.
The three parameters typically used in determining these two areas are:
- mean transit time (MTT) or time to peak of the deconvolved tissue residue function (Tmax) 3
- cerebral blood flow (CBF)
- cerebral blood volume (CBV)
Normal perfusion parameters are:
- gray matter
- MTT: 4 s
- CBF: 60 ml/100 g/min
- CBV: 4 ml/100 g
- white matter
- MTT: 4.8 s
- CBF: 25 ml/100 g/min
- CBV: 2 ml/100 g
The infarct core is the part of the ischaemic brain which has already infarcted or is destined to infarct regardless of therapy. It is defined as an area with prolonged MTT or Tmax, markedly decreased CBF and markedly reduced CBV 1-3.
The ischaemic penumbra, which in most cases surrounds the infarct core, also has prolonged MTT or Tmax but in contrast has only moderately reduced CBF and importantly near normal or even increased CBV (due to autoregulatory vasodilatation) 1-3.
Practical points
In patients with poor cardiac output, atrial fibrillation, severe proximal arterial stenosis or poor placement of arterial and venous density regions of interest, the decreased blood flow can lead to inaccurate perfusion maps and specifically to overestimated MTT (i.e. erroneous diagnosis of extensive ischaemia or global hypoperfusion) and underestimated CBF.
Most CT perfusion protocols are centered upon the basal ganglia and supra-ganglionic level. This excludes a large volume of brain, e.g. the posterior fossa and superior cerebral hemispheres.
Small infarcts (e.g. lacunar infarcts) are poorly visualised on perfusion maps due to their low resolution.
In cases of seizures, the ictal region shows hyperperfusion, which may lead to interpretation of hypoperfusion in the contralateral hemisphere mimicking infarct.
Summary
The CBV, and to a lesser extent CBF, differentiates penumbra and core infarct:
-
Corecore- increased MTT
/ Tmax/Tmax - markedly decreased CBF
- markedly decreased CBV
- increased MTT
-
Penumbrapenumbra- increased MTT
/ Tmax/Tmax - moderately reduced CBF
- near normal or increased CBV
- increased MTT
-<li>Core<ul>-<li>increased MTT / Tmax</li>- +<li>core<ul>
- +<li>increased MTT/Tmax</li>
-<li>Penumbra<ul>-<li>increased MTT / Tmax</li>- +<li>penumbra<ul>
- +<li>increased MTT/Tmax</li>
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