What is the role of CT perfusion maps in stroke imaging?

Updated: Nov 30, 2018
  • Author: Andrew Danziger; Chief Editor: L Gill Naul, MD  more...
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Answer

Perfusion maps can usually be compared visually to qualitatively look for areas of gross or subtle asymmetry. Quantitative CBF values can also be examined with thresholds for ischemic and infarcted tissue in mind. A low CBV abnormality best correlates with the infarct core. [13, 69, 70]

In the early stages of stroke, the extent of regionally abnormal MTT has been found to exceed that of abnormal CBV and CBF, making it the most sensitive parameter for detecting decreased perfusion and ischemia. Prolonged MTT, however, has been found to overestimate final infarct size. [71, 72, 73]

CBV and CBF maps generally show smaller areas of abnormality than MTT and are therefore more specific for areas of ischemia and infarction.

The mismatch between abnormal CBV and abnormal CBF estimates the penumbra. [74, 75]

Another method to classify ischemic tissue involves measuring the CBF in the contralateral normal brain parenchyma and comparing it to the CBF in the area of perfusion abnormality to create a CBF ratio (see the images below). [76] Shaefer et al found the following threshold ratio values between the abnormal ischemic region and normal contralateral side relating to the ischemic core and penumbra and mean CBF ratios (p< 0.05) [76] :

  • Infarct core: 0.19 +/- 0.06

  • Penumbra likely to go on to infarction: 0.34 +/- 0.06

  • Salvageable penumbra: 0.46 +/- 0.06.

    Placement of ROI curves on arterial and venous pix Placement of ROI curves on arterial and venous pixels (image on right) are needed to generate time-concentration curves for perfusion imaging. These curves can then be used to generate perfusion maps.
    After selecting the appropriate arterial and venou After selecting the appropriate arterial and venous input functions, the computer software is able to generate perfusion maps of different parameters (CBF = cerebral blood flow, CBV = cerebral blood Volume, MTT = mean transit time, TTP = time to peak enhancement). Regions of interest can then be placed over these maps for quantitative information. In this patient with occlusion of the distal left MCA trunk, elevated MTT and diminished CBF exists in the left basal ganglia, insular and opercular regions. The CBV is mildly increased in this same region, which is believed to be due to autoregulatory vasodilation in response to ischemia.

The high sensitivity of MTT for decreased perfusion may make it less specific in detecting clinically significant ischemia. One study reported elevated MTT in an asymptomatic patient with proximal occlusion of the carotid artery but likely good collateral supply and in another patient who had reversible ischemic symptoms and a negative CTA. [71]

Although the regional abnormality in CBV has been said to best estimate infarct volume, CBV may also be normal or elevated in acute stroke. This may be secondary to autoregulatory changes and collateral flow to ischemic brain tissue. CBV had been reported in the same study above to be normal in 25% of patients with acute stroke. [71]


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