Clinical Neurology · Neuro-Radiology
CT Perfusion & RAPID
CT Perfusion Basics
CT perfusion (CTP) produces three core maps. RAPID and similar software post-process these to define the irreversibly injured core and the salvageable penumbra.
| Map | Stands for | Means | Normal values | Looks at |
|---|---|---|---|---|
| CBF | Cerebral blood flow | Volume of blood moving through a given volume of brain per unit time | 60 mL/100 g gray matter 25 mL/100 g white matter | Irreversibly injured tissue (CBF < 30%) |
| CBV | Cerebral blood volume | Volume of flowing blood within a given volume of brain | 4 mL/100 g gray matter 2 mL/100 g white matter | Irreversibly injured tissue (the core) |
| MTT | Mean transit time | Time it takes blood to transit a given volume of brain | ~4 s gray matter ~4.8 s white matter | Tissue at risk (prolonged in ischemia) |
TTP = time to peak perfusion.
Four States of Blood Flow & Tissue Vulnerability
- Normal flow: higher CBV and CBF in gray matter than in white matter.
- Core infarction: CBV < 40% of normal and CBF < 30% of normal, with increased MTT. CBV is more sensitive and specific for defining the unsalvageable core; CBF may overestimate the core.
- Penumbra: normal CBV, mildly decreased CBF, and markedly increased MTT > 6 s (the CBV/MTT mismatch). CBV may even be increased from autoregulatory vasodilatation.
- Post-ischemic hyper-perfusion: increased CBV and CBF with decreased MTT — all colors are reversed compared with acute ischemia (CBV and CBF appear red instead of blue). This means recanalization has started and can be misread as contralateral ischemia.
N.B. Chronic infarction looks exactly like acute infarction — decreased CBV and CBF with increased or normal MTT.
| Tissue | CBV | CBF | MTT / Tmax |
|---|---|---|---|
| Core infarction | Decreased < 40% | Decreased < 30% | Increased > 6 s |
| Penumbra | Normal | Mildly decreased | Increased > 6 s |
| Luxury hyper-perfusion | Increased | Increased | Decreased |
Technical Aspects
- AIF (arterial input function): gives the CTP program a sample of normal arterial inflow. Usually the A2 segment of the ACA is used, as it runs perpendicular to the axial plane and is easily seen on multiple slices. Any intracranial or extracranial artery can be used instead.
- VOF (venous output function): gives the program a sample of venous drainage flow. Usually one of the dural venous sinuses is used.
Assessing Cerebrovascular Reserve (CTP with Acetazolamide Challenge)
In arterial stenosis, the brain compensates by two mechanisms:
- Acute stenosis: autoregulatory vasodilatation, to bring more blood to the post-stenotic area.
- Chronic stenosis: building collateral circulation.
Acetazolamide causes vasodilatation. With CTP obtained before and after acetazolamide, areas of brain with a marked increase in MTT represent areas at risk.
Assessing Vasospasm
- MTT is increased in areas of vasospasm.
- Can be used to evaluate patients after intra-arterial vasodilatation or stenting.
Seizures
- Asymmetric hyper-perfusion of the ictal area occurs during seizures.
- It may give the false impression of contralateral hypoperfusion.
Ahmed Koriesh, MD