CT Brain Perfusion — Exam Card
Definition
CT Brain Perfusion (CTP) is a dynamic CT technique used to assess the delivery and passage of blood through brain tissue. It is particularly useful in selected patients with acute ischaemic stroke, where perfusion information can complement conventional CT and CT angiography. CTP involves the intravenous administration of iodinated contrast followed by repeated CT acquisitions of the same brain region over time. The changing contrast concentration within the brain is analysed to produce quantitative and visual perfusion maps.
Stroke and Perfusion
When a cerebral artery becomes occluded, blood flow to part of the brain may decrease. Two important regions are considered: the infarct core — severely affected tissue that is likely to have sustained irreversible injury; and the ischaemic penumbra — hypoperfused tissue that may remain viable and potentially recover if adequate blood flow is restored. Reduced blood flow leads to hypoperfused tissue, producing both a core and a potentially salvageable penumbra.
Acute Stroke CT Pathway
The acute stroke CT pathway follows NCCT Brain → CTA → CT Perfusion. NCCT assesses haemorrhage and early ischaemic changes. CTA assesses cerebral vessels and can identify large-vessel occlusion. CTP provides information about cerebral perfusion and tissue viability.
Perfusion Parameters
CBF (Cerebral Blood Flow) is the amount of blood delivered to a given mass of brain tissue per unit of time. CBV (Cerebral Blood Volume) is the volume of blood contained within a given mass of brain tissue. MTT (Mean Transit Time) is the average time taken for blood to pass through the cerebral circulation. TTP (Time to Peak) is the time taken for tissue enhancement to reach its maximum. Tmax is a measure of delayed contrast arrival compared with normally perfused tissue and is commonly used when assessing areas of hypoperfusion. Together, these parameters provide complementary information about cerebral blood flow and contrast transit.
Acquisition and Coverage
Unlike a routine CT brain, perfusion imaging repeatedly samples the same anatomical volume over time following contrast administration. The amount of brain covered depends on the CT detector configuration. Modern wide-detector systems may provide extensive or whole-brain coverage, while smaller detector systems may require targeted coverage. The acquisition must provide sufficient temporal sampling to capture the passage of contrast through the cerebral circulation.
Clinical Importance
The principal application of CTP is acute ischaemic stroke assessment. It can provide additional information about cerebral hypoperfusion, infarct core, potentially salvageable tissue, and the extent of perfusion abnormality. This information can contribute to treatment selection in appropriate patients.
Radiation Consideration
CTP involves repeated acquisitions and therefore generally delivers more radiation than a single conventional CT acquisition. Radiation exposure is influenced by the number of acquisitions, scan duration, tube current, and anatomical coverage. Appropriate protocol selection and dose optimisation are therefore important.
Conclusion
CT Brain Perfusion uses dynamic contrast-enhanced CT to evaluate cerebral perfusion and help distinguish infarct core from potentially salvageable hypoperfused tissue, particularly in acute ischaemic stroke.