New developments in MRA: time-resolved MRA
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For many clinical questions, less invasive angiography methods have at least in part already replaced digital subtraction angiography (DSA) as a routine diagnostic procedure. However, temporal resolution achievable with DSA is still indispensable in some patients. Recent advances in MR hardware performance and imaging techniques permit sub-second frame rate MR angiographies to be performed using a rapidly repeated fast T1 weighted gradient echo sequence during administration of a contrast bolus, and subsequent complex subtraction in k-space to emphasise the difference in phase between stationary tissue and contrast-enhanced blood. This technique allows two-dimensional projection angiograms to be obtained at a temporal frame rate of three images per second and with a spatial in-plane resolution of about one square millimetre. This time-resolved information is important for (a) the detection and follow-up of arteriovenous malformations, including their arterial feeders, the size of the nidus and their venous drainage patterns, (b) the demonstration of dural arteriovenous fistulas, recognised as an early filling of a dural sinus during the early arterial phase, and (c) the characterization of the vascularisation of brain tumours. With regards to vascular malformations, the direction of venous drainage and therefore the most pressing clinical questions concerning the need for therapeutic intervention can be ascertained. This technique is the only MR imaging method able to provide dynamic information on the cerebral vasculature. It therefore constitutes a helpful adjunct to the imaging armentarium in many clinical situations.