CT, which stands for "computed tomography," is a type of X-ray scan which creates a 3-dimensional image of the body or isolated organs as opposed to a single flat or 2-dimensional image with which most people are familiar, such as chest or dental X-rays. When images are needed of structures which contain blood, such as arteries or the heart's pumping chambers, a contrast dye is injected into a vein to allow all vascular structures to stand out brightly, a procedure called angiography. The combination of the two technologies is a CT angiogram.
Since most organs in the body do not move, the patient only needs to remain still and stop breathing while the image is being taken. To image the heart, which is constantly beating, additional measures are necessary. ECG leads are placed on the patient's skin to allow the computer to know exactly when each heart beat occurs. The final image is then reconstructed by the computer from multiple brief images which are taken only during the phase of the heart cycle when the pumping chamber has filled with blood and is relatively still.
Powerful computerized work stations are used by the interpreting physician to reconstruct the final image into various displays which can be further manipulated for diagnostic purposes. One type of display requires complicated mathematical algorithms to create a realistic 3-dimensional image in which the arteries can be seen on the surface of the heart. This is the same technology used by the movie industry to create images of monsters or dinosaurs.
This display can also used in a video movie:
Another display allows the arteries to be visualized with the surrounding structures removed:
Finally, high resolution 2-dimensional displays aligned along the course of specific arteries allows accurate assessment of the caliber of the vessels. Normal in this example:
And almost totally occluded in this example: