Brain CT perfusion

Core concept

CT perfusion is an advanced form of multiphasic imaging. The method is based on the analysis of X-ray attenuation curves as a contrast agent passes through tissues and their afferent and efferent vessels.

The procedure requires rapid bolus administration of a contrast agent into a vein, after which the scanner repeatedly acquires data from the same anatomical region. The device records how the contrast fills the capillary bed of the brain and how it leaves. Meanwhile, the radiation dose at each individual phase is strictly minimized to prevent excessive radiation exposure to the patient while maintaining high physiological information quality.

Evaluated parameters (perfusion maps)

After scanning, the workstation automatically generates color maps, each reflecting a specific hemodynamic parameter:

• CBF (cerebral blood flow) — speed of cerebral blood flow in a given area.
• CBV (cerebral blood volume) — total volume of blood per unit of brain tissue.
• MTT (mean transit time) — average time it takes for blood to pass through the capillary network of the tissue.
• TTD (time to drain) — drainage time, an indicator of perfusion delay.

Comprehensive analysis of these maps allows the radiologist to precisely localize microcirculatory disorders.

Application in acute ischemic stroke

The value of CT perfusion is most evident in the diagnosis of acute cerebral infarction. To understand the process, it is useful to consider a classic clinical case.

A patient with an acute onset of hemiparesis is admitted to the clinic. On a non-contrast CT scan in a standard window (W:120 L:40) or a dedicated "stroke" window (W:40 L:40), only subtle signs of ischemia may be noticeable: effacement of the gray-white matter differentiation (e.g., in the insular cortex) or mild hypodensity of the basal ganglia.

CT perfusion is then performed, which dramatically changes the picture:

1. The CBF map reveals an extensive area of reduced blood flow in the territory of the affected artery (e.g., the middle cerebral artery, MCA).
2. On the CBV map, a significantly smaller area of severely reduced blood volume is visible. This is the established infarct core (infarct core) — tissue that has already died irreversibly.
3. The time to drain (TTD) map visualizes a vast area of perfusion delay.

Main diagnostic conclusion: the difference between the extensive delay zone (TTD) and the small infarct core (CBV) represents the penumbra (penumbra). This is the zone of potentially reversible ischemia, to save which resuscitation measures and surgical interventions (such as mechanical thrombectomy) are performed.

Comprehensive approach to vascular diagnostics

Although CT perfusion provides comprehensive information on microcirculation, it is combined with large-vessel imaging techniques for treatment planning:

• CT angiography (CTA): using volume rendering (VRT) makes it possible to locate the exact site of occlusion (e.g., in the M1 segment of the middle cerebral artery) or evaluate extracranial artery atherosclerosis.
• Digital subtraction angiography (DSA): the gold standard used both for confirming occlusion and for intraoperative guidance during clot removal.
• Magnetic resonance angiography (MRA): TOF (time-of-flight non-contrast angiography) and CE MRA (gadolinium-enhanced) techniques serve as excellent alternatives for assessing the neck vessels and the Circle of Willis in the subacute period or in chronic pathologies.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Principles of time-of-flight MRA (TOF MRA) and the gradient echo effect
• Diffusion-weighted MRI (DWI) and ADC maps in acute infarction
• Phase-contrast MR venography in the diagnosis of dural sinus thrombosis
• CSF flow studies and vascular pulsation
• Imaging of the brain's glymphatic system

Other topics in the Neuroimaging section

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