"CT and MR angiography are most useful in defining the level and the degree of stenosis of arteries supplying the posterior circulation. The gold standard is intra-arterial angiography but this is not employed as first-line as it is invasive and carries a 1-2% risk of stroke. Contrast-enhanced MR angiography (MRA) is more sensitive than CT angiography (CTA) in the detection of >50% stenosis of the vertebral arteries; however, CTA is similar to MRA in the detection of >70% stenosis 4."