Calcium phosphate (hydroxyapatite) in coronary arteries, an indicator of coronary artery atherosclerosis. CAC is found in diseased but not healthy coronary arteries. During ultrafast CT scanning of the heart, tissue densities that exceed 130 Hounsfield units typically contain significant amounts of deposited calcium. The total amount of calcium present in a person’s coronary arteries can be measured by assessing the length of calcified artery as well as the density of the calcium identified. These factors together are used to generate a “calcium score.” A calcium score > 100 is often cited as a measurement indicative of a moderately high risk of future myocardial infarction or ischemia. People with scores > 100 should begin taking daily aspirin and should actively modify atherosclerotic risk factors such as smoking, high blood pressure, hyperlipidemia, and diabetes mellitus. A score greater than 400 is often cited as indicating an urgent need for stress testing with radionuclide imaging (e.g., thallium or sestamibi).