CT Perfusion
CT perfusion (CTP) measures the passage of an intravenous contrast bolus through tissue over time, using repeated low dose scans of a fixed slab, to calculate quantitative maps of blood flow and volume.
After a small bolus of iodinated contrast (40 to 50 mL of high concentration non-ionic medium at 4 to 6 mL each second, followed by a saline flush), the same volume is scanned every 1 to 3 seconds for 40 to 60 seconds (and in some centres an extended 90 second acquisition). Coverage is 4 to 16 cm along the body axis (up to whole brain on wide detector scanners).
Time attenuation curves in an artery and a vein, and in each voxel, are analysed with deconvolution software to produce parametric maps: cerebral blood flow, cerebral blood volume, mean transit time and time to peak (or Tmax).
The most frequent application is acute ischaemic stroke, where CTP defines the infarct core (irreversibly damaged tissue) and the ischaemic penumbra (salvageable tissue at risk).
Other uses include vasospasm, brain tumour grading, and perfusion of the liver, pancreas, lung, prostate and tumours. Effective dose is about 2 to 5 mSv for brain CTP (higher in body perfusion, about 10 to 20 mSv), and care is taken with skin dose to the lens and scalp.
Acute stroke affects 12 million people a year, and a large vessel occlusion with a salvageable penumbra benefits from thrombectomy up to 24 hours after onset in selected patients.
CTP identifies patients with a mismatch (small core, large penumbra) with sensitivity for core of 70 to 85% and specificity of 90 to 95% when compared with diffusion MR.
It improves the sensitivity for acute ischaemic stroke detection over non-contrast CT (from about 40 to 60% to over 80%), especially for small and posterior circulation infarcts.
Indications. Acute ischaemic stroke in patients with large vessel occlusion beyond 6 hours or of unknown onset, and in those with unclear window, to select thrombectomy or thrombolysis by core and penumbra. Suspected stroke with a normal non-contrast CT, mimics, and uncertain diagnosis. Transient ischaemic attack with persisting deficit.
Delayed cerebral ischaemia and vasospasm after subarachnoid haemorrhage. Chronic cerebrovascular disease with haemodynamic compromise (carotid occlusion, moyamoya, before bypass surgery), sometimes with acetazolamide challenge. Brain tumour grading, biopsy planning, differentiating radiation necrosis from recurrence, and monitoring of response to anti-angiogenic therapy.
Brain death assessment. Body applications: tumour perfusion in liver, lung, pancreas, prostate, head and neck cancer, and response to treatment; perfusion of the liver in cirrhosis and transplant; and bowel ischaemia and inflammatory disease in research or selected use. Myocardial CT perfusion with stress for ischaemia (selected centres).
Not for established infarct in patients obviously unsuitable for intervention, and not as a replacement for non-contrast CT to exclude haemorrhage.
Contraindications and precautions. Previous severe contrast reaction, renal impairment (estimated glomerular filtration rate under 30 mL each minute; in acute stroke do not delay the scan), pregnancy (foetal dose is low for head scans; acceptable in an emergency), and inability to hold still (motion ruins the maps). Metformin and thyroid disease need the usual contrast precautions.
Severe heart failure or low cardiac output distorts the arterial input and the maps. Cannula of 18 G in the antecubital vein, flushed well. Radiation to the lens and skin is limited, with repeated studies avoided because of the skin dose (hair loss and erythema were reported when a scanner was misconfigured).
Preparation and technique. No preparation in emergencies. The patient lies supine with the head immobilised in a holder with straps. Non-contrast CT head is done first.
The perfusion slab is planned over the basal ganglia and over them (covering the middle cerebral artery territories, usually 4 to 10 cm), or the whole brain with a toggling table or wide detector. Contrast is injected with a pump, and cine acquisition begins 3 to 5 seconds later at 80 kV and 100 to 200 mAs.
Software performs motion correction, selects an arterial input function (anterior cerebral or middle cerebral artery) and venous output function (superior sagittal sinus), and generates maps.
Core is defined by relative cerebral blood flow under 30% of normal tissue (or a cerebral blood volume threshold), and hypoperfused tissue by a delay time (Tmax) over 6 seconds; mismatch volume and ratio are calculated.
CT angiography of the head and neck is acquired in the same visit.
