• brain infarction results from a localized decrease in cerebral blood flow (CBF)
  • CT perfusion (CTP) can be used to assess cerebral blood flow and differentiate the penumbra (salvageable brain tissue) from the core (area of damaged brain tissue)
  • CTP does not detect necrotic tissue; it only demonstrates the decrease in blood flow that is very often associated with necrosis
    • however, necrosis may not necessarily be present in the core area – if the altered CBF corresponding to the core is detected shortly after stroke onset and rapid recanalization occurs, then part of this zone may be saved
    • conversely, in necrotic tissue,  flow improvement may be achieved after late recanalization, and the core and penumbra may disappear or be reduced 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, even though it is a zone of complete ischemia 
  • the CTP mismatch evaluated by the RAPID system became the basis of breakthrough studies with recanalization therapy beyond the 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 brain regions (detected by CTP) resulted in a higher percentage of patients with no or minor neurological deficit compared to placebo

Technical comments

  • after intravenous administration of iodinated contrast agent, there is a transient increase in parenchymal density that is proportional to the amount of contrast agent in the area
    • administration rate: 5-6 mL/s; the green cannula is required (18G, 1.3 mm)
  • CTP parameters are calculated from time-attenuation curves based on the differences between arterial inflow and venous outflow
  • software processing of the measured brain tissue density during the passage of a contrast agent yields 4 parameters:
    • Cerebral Blood Volume (CBV) – amount of blood in a given volume of tissue (mL per 100 mg of tissue)
    • Cerebral Blood Flow (CBF) – blood flow (mL/100g of tissue/minute)
    • Mean Transit Time (MTT) – 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
  • based on the measured and calculated values, colored perfusion maps are created for each parameter, allowing areas with different blood flow to be distinguished and their extent to be compared

Evaluation of CTP maps

Mean Transit Time (MTT)

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

Cerebral Blood Flow and Volume (CBF and 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

  • a 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 intensity ratio (HIR)

  • Hypoperfusion Intensity Ratio (HIR) or hypoperfusion index : Tmax > 10s / Tmax > 6s
  • index ≥ 50% correlates with poor collaterals on angiography; these patients had an 83% probability of significant core growth [Guenego, 2018]
  • poorer (higher) HIR is associated with the occurrence of PH after EVT (HIR might reflect tissue vulnerability for reperfusion hemorrhage) (Winkelmeier, 2022)
  • high HIR on baseline imaging may warrant repeat 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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CT perfusion (CTP)
link: https://www.stroke-manual.com/ct-perfusion-ctp-stroke/