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 artery. 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 forward and backward 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 arteries show up as white lines on the x-ray. A narrowing or blockage in an artery will make that artery look like a thinner line on the x-ray compared to your other arteries. Your doctor can also inject some dye from the catheter into the left ventricle of your heart to see its outline. This way the x-ray can show how well your heart is squeezing with each heartbeat. The test usually requires between 30 minutes to one hour to perform start to finish.