Coronary angiography overview

# Overview
Cardiac catheterization refers to the insertion of hollow tubes (catheters) into various structures of the heart.  When catheters are inserted specifically into the coronary arteries that supply blood to the heart muscle (the myocardium), this instrumentation is called coronary catheterization.  Coronary catheterization is to be distinguished form left heart catheterization in which catheters (hollow tubes) are inserted into the blood filled chambers of the heart.
Once the catheter is inserted into the coronary artery, coronary angiography is the process of injecting iodinated contrast material (i.e. dye) into the coronary arteries to assess the coronary circulation.  Coronary angiography is performed for both diagnostic and interventional (treatment) purposes.  Coronary angiography is a visually interpreted test performed to recognize occlusion or narrowing (referred to as a stenosis), restenosis (reblockage after successful opening of the artery), thrombosis (clot) or aneurysmal enlargement the coronary artery lumen.    
During coronary catheterization (often referred to as a cath by physicians), blood pressures are recorded and X-Ray motion picture shadow-grams of the blood inside the coronary arteries are recorded.  In order to create the X-ray pictures, a physician guides a small tube-like device called a catheter, typically ~2.0 mm (6-French) in diameter, through the large arteries of the body until the tip is just within the opening of one of the coronary arteries.  By design, the catheter is smaller than the lumen of the artery it is placed in; internal/intraarterial blood pressures are monitored through the catheter to verify that the catheter does not block blood flow.
The catheter is itself designed to be radiodense for visibility and it allows a clear, watery, blood compatible radiocontrast agent, commonly called an X-ray dye, to be selectively injected and mixed with the blood flowing within the artery.  Typically 3-8 cc of the radiocontrast agent is injected for each image to make the blood flow visible for about 3-5 seconds as the radiocontrast agent is rapidly washed away into the coronary capillaries and then coronary veins.  Without the X-ray dye injection, the blood and surrounding heart tissues appear, on X-ray, as only a mildly-shape-changing, otherwise uniform water density mass; no details of the blood and internal organ structure are discernible.  The radiocontrast within the blood allows visualization of the blood flow within the arteries or heart chambers, depending on where it is injected.
If atheroma, or clots, are protruding into the lumen, producing narrowing, the narrowing is seen as either a narrowing or increased haziness within the X-ray shadow images of the blood/dye column within that portion of the artery; this is as compared to adjacent, presumed healthier, less stenotic areas.
For guidance regarding catheter positions during the examination, the physician mostly relies on detailed knowledge of internal anatomy, guide wire and catheter behavior and intermittently, briefly uses fluoroscopy and a low X-Ray dose to visualize when needed.  This is done without saving recordings of these brief looks.  When the physician is ready to record diagnostic views, which are saved and can be more carefully scrutinized later, he activates the equipment to apply a significantly higher X-Ray dose, termed cine, in order to create better quality motion picture images, having sharper radiodensity contrast, typically at 30 frames per second. The physician controls both the contrast injection, fluoroscopy and cine application timing so as to minimize the total amount of radiocontrast injected and times the X-Ray to the injection so as to minimize the total amount of X-Ray used. Doses of radiocontrast agents and X-Ray exposure times are routinely recorded in an effort to maximize safety.
The patient being examined or treated is usually awake during coronary catheterization, ideally with only local anaesthesia such as lidocaine and minimal general sedation, throughout the procedure. Performing the procedure with the patient awake is safer as the patient can immediately report any discomfort or problems and thereby facilitate rapid correction of any undesirable events.