Selective magnetic resonance angiography of the head.

Selective magnetic resonance angiography (MRA) is a combination of two- or three-dimensional MRA with radiofrequency presaturation pulses. Presaturation pulses can be applied over user-defined regions. They reduce or eliminate signals within the presaturation slab. As a result, blood flowing within or through the presaturation slab turns dark whereas blood flowing outside and not through the presaturation slab still appears bright. Using this technique, flow direction and origin of flow can be determined and flowing blood can be differentiated from stationary bright tissues such as thrombus. Arteries and veins can be imaged selectively due to their usually opposite flow direction. The intracranial collateral circulation about the circle of Willis can be shown, and vessels feeding or draining arteriovenous malformations can be identified more accurately. In conclusion, selective MRA can provide functional information about the cerebrovascular system, which cannot be obtained from conventional MR imaging (MRI) or MRA alone.