One end of a wire is threaded through the needle into the blood vessel, and the needle is then pulled out, leaving the wire temporarily in place. This wire is several feet long, but only a small part of it is inside your vein. The catheter can then be slipped over the outside end of the wire and moved forward along it like a long bead on a string until it is in place with one end inside the vein. The wire is pulled out of the catheter, leaving the catheter in place. Now the catheter easily can be moved forwards and backwards inside your vein by the doctor who holds the outside end.
Special controls allow the doctor to point the tip of the catheter in different directions.
The catheter shows up on x-rays and the doctor will watch it live on a video monitor to know exactly where the catheter is. X-ray pictures will be taken as the dye travels along the arteries and outlines all of the blood vessels in the lungs through which it can flow. The x-ray pictures look very much like a tree with branches getting smaller and smaller as they leave the trunk. A blood clot in the lungs will make the picture look as if one branch was sawed off of this tree, so that there are no small branches continuing off of it. The test usually requires somewhat longer than one hour to perform start to finish.