NEUROIMAGING / COMPUTED TOMOGRAPHY

CT perfusion (CTP)

Created 21/03/2021, last revision 14/02/2022

  • brain infarction results from a local decrease in cerebral blood flow (CBF)
  • CT perfusion (CTP) can be used to assess cerebral blood flow and to differentiate penumbra (salvageable brain tissue) from the core (the area of damaged brain tissue)
  • CTP, in fact, does not detect necrotic tissue; it only demonstrates the decrease of blood flow that is very often associated with necrosis
    • !! however, there may not necessarily be necrosis in the core area – if the CBF disorder corresponding to the core is detected shortly after its onset and rapid recanalization occurs, then part of this zone may be saved
    • on the contrary, in necrotic tissue, after late recanalization, an improvement of flow can be achieved, and the core and penumbra disappear CT perfusion - late reperfusion can lead to regression of core extent despite present necrosis
    • similarly, late recruitment of collaterals can lead to improved flow in the periphery of the “core” and thus  to its reduction on CTP, although it is a zone of complete ischemia 
  • CTP mismatch evaluated by RAPID system became the basis of breakthrough studies with recanalization therapy beyond standard time windows (4,5 h for intravenous thrombolysis, 6h for mechanical thrombectomy)
    • the DAWN study (2017) showed an effect in the 6-24h window in a strictly selected group (according to CTP)
    • in 1/2018, positive results of DEFUSE study (2018) were presented with a time window of 6-16h (OR 2.77 !!!)
    • EXTEND trial (2019) showed that the use of tPA between 4.5 – 9.0 hours after stroke onset in patients with hypoperfused but salvageable regions of the brain (detected on CTP) resulted in a higher percentage of patients with no or minor neurologic deficit compared to placebo

Technical comments

  • after intravenous administration of iodine contrast agent, there is a transient increase in parenchymal density in proportion to the amount of contrast agent in the vascular bed
    • administration speed: 5-6 ml/s, the green cannula is required (18G, 1.3 mm)
  • CTP parameters are calculated from time-attenuation curves that rely on differences between arterial inflow and venous outflow
  • software processing of the measured brain tissue density during the passage of a contrast agent gives 4 parameters:
    • Cerebral Blood Volume (CBV) – the amount of blood in a certain volume of tissue (ml per 100 mg of tissue)
    • Cerebral Blood Flow (CBF) – blood flow (ml / 100g tissue / minute)
    • Mean Transit Time (MTT) – the average time of arteriovenous blood passage by a given volume of tissue (in seconds)
    • Time To Peak (TTP) – average time to maximum density in the scanned area (in seconds)
  • the relationship between the above parameters is expressed by the equation: CBF = CBV / MTT
  • the numerical value of CBV is obtained as a calculation of the area under the perfusion curve, the relative value of MTT is obtained as half the time between the time from the increase of density from the basal level to its decrease back to the basal level
  • based on the measured and calculated values, color perfusion maps are created for individual parameters, where areas with different blood flow can be distinguished from each other, and their extent can be compared

Evaluation of CTP maps

Mean Transit Time (MTT)

  • start assessing the MTT (TTP) parameter, which is prolonged in ischemia (>145% of contralateral normal tissue) and which most markedly shows regional abnormalities in blood flow
    • normal MTT / TTP rules out artery occlusion
    • MTT is, however, not suitable for assessing viability; it only identifies areas with slower contrast filling
  • the MTT area includes:
    • benign oligemia (persistent occlusion usually doesn´t lead to infarction in this area)
    • infarction core (irreversibly damaged tissue)
    • penumbra (tissue that can be rescued by recanalization therapy)

Cerebral Blood Flow a Volume (CBF a CBV)

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Penumbra on CTP with normal or increased CBV
Core on CTP
CTP mismatch and final infarction after succesfull recanalisation of dominant M2 occlusion
CTP in patient with SCA occlusion
CT perfusion in patient with MCA stroke
Left MCA occlusion with CBF deficit (B) and areas with low CBV (C) showing core. IV thrombolysis led to rapid recanalisation, so NCCT and MRI control show infarction in regions of presumed core.
Stroke in PICA territory due to AV occlusion

Clinical-core mismatch

  • mismatch between age-adjusted NIHSS and CTP core (defined as stated above)   (Chen, 2021)

    • NIHSS ≥10 + core volume <31 ml (age <80)
    • NIHSS ≥20 + core volume 31–51 ml (age <80)
    • NIHSS ≥10 + core volume <21 ml (age ≥80)

Hypoperfusion index

  • hypoperfusion index (hypoperfusion intensity ratio) : Tmax > 10 / Tmax > 6 ratio
  • index ≥ 50% correlates with bad collaterals on angiography; these patients had an 83% probability of significant core growth [Guenego, 2018]
  • high index on initial CT performed in primary stroke center may warrant repeated brain imaging after transfer to a comprehensive stroke center to avoid futile endovascular treatment
Relatively favourable hypoperfusion index (40%)
Unfavourable hypoperfusion index (70%), with poor collaterals on multi-phase CTA (mCTA)
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