NEUROIMAGING / COMPUTED TOMOGRAPHY

Created 21/03/2021, last revision 09/10/2023

• brain infarction is the consequence of 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 demonstrates the decrease in blood flow 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, a portion of this zone may be salvageable
    
  • 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 improve flow in the periphery of the “core,” potentially reducing its size 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 (based on CTP findings)
  • in January 2018, the DEFUSE study) presented positive results in the 6-16 hours window (Odds Ratio 2.77!!)
  • EXTEND trial (2019) showed that the use of tissue plasminogen activator (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

→ Ischemic penumbra

• after the intravenous administration of an iodinated contrast agent, a transient increase occurs in parenchymal density, which 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)
• the software calculates CTP parameters from time-attenuation curves, which are 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 generated for each parameter, thereby facilitating the distinction and comparison of areas with varying blood flow