In the case of a conventional coronary angiogram, a catheter has to be inserted in to the body, which is then manipulated into the coronary arteries. A dye is injected through the catheter in to the coronary arteries while a movie is obtained using X-rays.
A Cardiac CT on the other hand, is done just like a normal CT scan. If your heart rate is high, a medicine will be given 30 minutes to one hour before the scan to reduce the heart rate. During the examination, you will be lying on a comfortable table. A small needle will be inserted into a vein in your arm as in a blood test, and an injection of dye will be given through this needle during the scanning. You may experience a feeling of warmth during the dye injection, which will dissipate in a few seconds. Also, you will be asked to hold your breath for just 5 - 8 seconds during scan.