Coronary angiography has been the standard test for coronary artery disease since the 1970s. This test involves injecting dye through a catheter into the coronary vessels and watching the dye as it passes through the arteries. In CAD narrowing of the arteries is seen. However it is the dye that is seen, not the vessel wall. If the dye stream is seen to narrow at a certain point in the artery it can be assumed that that part of the artery is narrowed. It cannot tell what is narrowing the artery, cholesterol or calcium.
Coronary angiography has risks attached. Bruising occurs at the point of insertion of the cateter into the artery in the arm or leg. A catheter has to be passed into the heart with its tip near the opening of the coronary arteries. The catheter itself travels along arteries to the heart and these arteries too may be damaged, as may be the valve near the outlet of the heart. The catheter may make the heart beat irregularly.
Can CT scanning make the diagnosis with the same accuracy?
The heart moves costantly, 60-80 times a minute. Until recently CT scanners were too slow to capture good images of the beating heart, just as a photo camera produces a blurred image on a long hand-held exposure. Recent advances in MSCT- multiple slice computed tomography - makes it possible to show images of the coronary vessels. If the cardiologist can slow the heart down enough the heart is in focus for the CTscanner. Beta blockers slow the heart to 65 beats per minute or less. The CT scanner is coordinated with an ECG machine so that pictures are taken at the time when the heart is least active. A dye is injected into a vein and the heart shows up on a scanner monitor.
CT of the heart is just as accurate as angiography.
Advantages of coronary CTThe CT scan only takes 10-15 seconds. Angiography takes much longer. Just the preparation of the catheter insertion site takes quite a long time. CT scan requires dye injected into a vein, a much easier procedure.
CT shows the function of the heart. The most accurate measurement of ventricular function is MRI: CT is within 10% of the accuracy of MRI.
CT shows the scars of old heart attacks: The CT shows thinning of the heart muscle where scar tissue has replaced dead or damaged heart muscle. Sometimes the thin area ballons out into an aneurism. The CT shows the aneurism.
CT shows calcium deposits: Calcium in the plaque is an important finding for the cardiologist.
The presence of calcium is a good predictor of future coronary events. This is called the
calcium score. People with a high calcium score are high risk for heart attack and sudden death. Patients who have no symtoms can be diagnosed and treated
before they have their first - and possibly last - heart attack.
CT shows all the structures of the heart: The CT gives really good pictures of all of the heart and surrounding structures including the aorta and pulmonary artery, and the lungs and mediastinum. The mediastinum is the area behing the heart where the arteries and veins run, and also the lymph glands. Lymph glands may be swollen by metastatic tumours or blood disorders.
Cardiac CT can rule out heart disease: This is called "negative predictive value". Just as it is important to diagnose heart disease in patients who do have heart disease, it is important to exclude heart disease in patients who have chest pain from other causes.
Cardiac CT is more accurate than a stress cardiogram test in dagnosing the source of chest pain suggestive of heart disease.
