Arteriography involves the injection of dyes, which are radio opaque, into arteries. X-rays can then be taken, which outline the arterial tree (see Figures 5.4 and 5.5). Conventional arteriography involves inserting a needle into an artery and injecting the dye. The dye will flow down the artery and, therefore, an injection in the groin will only outline the vessels below the groin going towards the feet. If an outline of vessels nearer the heart is required, then, having inserted a needle into the artery, a fine guide wire is passed through the needle, the needle is withdrawn and a flexible catheter is inserted over the guide wire (Seldinger technique). This allows a fine catheter to be inserted higher up the arterial tree. The catheter can be advanced as far as the heart and indeed into the coronary arteries if necessary (selective arteriography). Once the injection has taken place, X-rays have to be taken in quick succession before the contrast material is washed out of the arterial system.