Arterial transit artifact

The arterial transit artifact, sometimes known as the bright vessel appearance or trapped labelled spins, on noncontrast arterial spin labeling (ASL) MR perfusion of the brain refers to curvilinear high signal corresponding to labelled blood within cerebral arteries. Normally, in ASL imaging, labelled blood moves from the neck to the capillary bed of the brain by the time of image acquisition following the postlabel delay (around 2 seconds in most protocols). However, if there is a delay in the transit of blood to the capillaries, the labelled blood will appear as intravascular signal, termed arterial transit artifact, rather than as signal in the perfused parenchyma.

In general, the presence of arterial transit artifact indicates an abnormality of the upstream circulation, such as a stenosis causing slowed flow through it, an occlusion causing blood to have to arrive (late) via collateral pathways, or decreased cardiac output ^1,2. However, the artifact can also appear proximal to an occlusion ³. Additional significance varies by clinical situation:

• acute ischaemic stroke: arterial transit artifact adjacent to a hypoperfused region with an infarct suggests collateral flow and predicts better neurological functional outcome ^4,5
• transient ischaemic attack: arterial transit artifact adjacent to a hypoperfused region without an infarct is associated with lower likelihood of subsequent infarct ⁶
• carotid artery stenosis: arterial transit artifacts are only present with severe (<70%) carotid stenoses and are more likely in patients with recent transient ischaemic attack or stroke than in patients without ischaemic symptoms ⁷
• moyamoya disease: arterial transit artifacts identify the presence and intensity of collateral flow ⁸
• intracranial aneurysm: arterial transit artifacts identify the presence of active part of aneurysmal sac ⁹
• migraine aura ¹⁰