NEUROIMAGING / COMPUTED TOMOGRAPHY

Created 21/03/2021, last revision 08/05/2023

• 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
    
  • 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

→ more about penumbra see here

• 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