CT Perfusion — Brain

A fixed slab through the brain, whose z-axis extent depends on detector width; wide-detector scanners cover most of the supratentorial brain, older scanners a limited slab that must be positioned deliberately.

Typical, not policy

Timings, volumes and delays here are representative values drawn from published guidance. Scanner generation, injector, cardiac output and local preference all move them. Confirm against your department's own protocol before you rely on a number.

When to use it

Selection for mechanical thrombectomy, particularly in the extended time window or when the time of onset is unknown.
Distinguishing established infarct from potentially salvageable tissue when that distinction will change management.
Occasionally as a problem-solving tool in suspected vasospasm after subarachnoid haemorrhage.

Technique

Repeated acquisitions through the same slab during first pass. Published guidance favours a total acquisition of at least 60-75 s so the venous outflow curve returns to baseline; sampling intervals of about 1-2 s are typical.
Sources differ on the acceptable sampling interval: intervals up to about 3 s have been shown not to distort the perfusion parameters materially in some studies, while others favour 1 s. Longer intervals degrade the arterial input function.
Head immobilisation is part of the protocol — the maps are computed across the whole time series.

Where it goes wrong

Truncating the acquisition before the venous outflow curve returns to baseline systematically distorts the maps and can exaggerate the mismatch volume.
Patient motion during the series corrupts every map, not one image, and often cannot be salvaged by re-processing.
Poor cardiac output or a proximal stenosis delays the bolus and can produce artefactually large hypoperfusion volumes.
The slab may simply miss the territory in question on narrow-detector systems; positioning is a clinical decision, not a default.

Clinical questions that reach this study

Suspected acute ischaemic stroke

Contrast

Iodinated, intravenous

A small tight bolus, typically 35-50 mL of non-ionic iodinated contrast at 4-6 mL/s with a saline chaser.

Perfusion is usually acquired alongside a CTA; count the combined contrast load rather than each study separately.

Acquisition

Reconstruction: Time-attenuation data are post-processed to cerebral blood flow, cerebral blood volume, mean transit time and time-to-maximum maps, with automated core and hypoperfusion volumes.
Preparation: Large-bore antecubital cannula; perfusion requires a tight bolus at high flow rate. Head immobilised — the study is a time series and motion invalidates the maps, not just one image.

Phases

Each phase is authored once and shared across every protocol that uses it, so the physiology below is the same wherever you meet it.

Dynamic perfusion acquisition (CT perfusion)A repeated cine acquisition over a fixed volume. Contemporary protocols run at least ~60 s and typically ~75–90 s, sampling around every 1 s early and every 2–3 s later. Confirm locally.
Instead of one snapshot at one delay, the same slab is sampled repeatedly so that a time–attenuation curve can be built for every voxel. The transferable principle is that this converts anatomy into physiology: blood flow, blood volume and transit-time maps are derived quantities, and their validity depends entirely on capturing the whole first-pass curve including its tail. Truncating the acquisition in a patient with poor cardiac output, atrial fibrillation or proximal arterial occlusion clips the curve and biases the derived maps in exactly the population the study was requested for. The cost is a comparatively high radiation dose over a limited coverage, so it is justified only where the treatment decision turns on tissue physiology rather than on structure.

Safety checks this protocol carries

Derived from the contrast agent and phases above, not authored here — which is why they cannot drift apart from what the protocol actually does.

Prior contrast reaction and elective premedication· nurse pre scan
Intravenous access adequate for the planned injection· radiographer at scan
Metformin and iodinated contrast· radiographer at scan
Child-sized technique and contrast dose· radiographer at scan
Kidney function and intravenous iodinated contrast· radiographer at scan