NEUROIMAGING / COMPUTED TOMOGRAPHY
CT perfusion (CTP)
Created 21/03/2021, last revision 24/11/2022
- 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 does not detect necrotic tissue; it only demonstrates the decrease in 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
- 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 the 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, and 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
- normal MTT/TTP rules out artery occlusion
- 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 and Volume (CBF and CBV)
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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 bad collaterals on angiography; these patients had an 83% probability of significant core growth [Guenego, 2018]
- poorer (higher) HIR is associated with PH occurrence after EVT (HIR might reflect tissue vulnerability for reperfusion hemorrhages) (Winkelmeier, 2022)
- high HIR on baseline imaging may warrant repeated brain imaging after transfer to a comprehensive stroke center to avoid futile endovascular treatment