Objectives of Cardiac Imaging
Chest pain is the most frequent cause of emergency department visits in the US, and adverse cardiac events (ACEs) (cardiac death, myocardial infarction, unstable angina, surgical or percutaneous coronary revascularisation) are the most prevalent cause of morbidity and mortality in the Western world. 1 Therefore, establishing obstructive coronary artery disease (CAD) as the cause of chest pain and identifying subclinical coronary artery disease that is likely to progress to the symptomatic (angina) stage or to become the substrate of an ACE are important objectives of cardiac imaging.
Currently Available Imaging Modalities
Invasive, catheter-based coronary angiography using various modes of access to the vascular system is the standard for direct visualisation of obstructive CAD and has been in use since 1959.
All non-invasive or minimally invasive stress imaging modalities such as echocardiography, single photon emission computed tomography (SPECT) or positron emission tomography (PET) rely on indirect signs such as reduced perfusion reserve or stress-induced regional myocardial dysfunction to diagnose obstructive CAD from the ischaemia that blood-flowlimiting (significant) coronary stenoses cause. These imaging modalities generally cannot diagnose subclinical CAD that has not yet progressed to significant stenoses (usually defined as more than 70% to 75% of the diameter of a reference segment). Also, none of the invasive or non-invasive imaging techniques can predict the risk of future ACEs reliably.
Technical improvements of computed tomography (CT) and magnetic resonance imaging (MRI) have allowed their clinical use for structural or functional imaging of the heart since the early 1990s. MRI and CT have distinct strengths and weaknesses in imaging of the cardiovascular system, and the choice between the modalities currently depends on the clinical question to be answered. Their ability to directly but minimally invasively image the coronary arterial tree has generated wide interest.
Currently, two different CT modalities are in use for cardiac imaging. 2 Electron beam CT (EBCT) scanners use no mechanically rotating parts for image generation, allowing high temporal resolution. As the first CT technique suitable for cardiac imaging, EBCT is the standard, owing to validation studies beginning in 1984. However, EBCT is not widely available (approximately 200 scanners in the US3) because of its limited value in scanning organs other than the cardiovascular system. Multislice CT (MSCT) uses mechanically rotating gantries, acquiring multiple (currently up to 16) images of high spatial resolution and low image noise simultaneously with every rotation. MSCT scanners have been available since 1999 and are widely used as body scanners (approximately 2,000 scanners in the US3). The radiation dose received by the patient in cardiac MSCT studies is higher than that received in cardiac EBCT studies. 4
Cardiac Imaging with CT and MRI
Cardiac CT Imaging
Currently, the main uses of cardiac CT are for coronary artery calcium (CAC) scanning and CT coronary angiography (CTCA). CAC scanning does not require administration of iodinated contrast medium and is performed either to establish obstructive CAD as the cause of chest pain in symptomatic patients or to estimate the prognosis for having a future ACE in asymptomatic patients (screening). CAC represents coronary atherosclerotic plaque and is quantified as Agatston or volume scores. It is controversial whether CAC scores derived from EBCT and MSCT scanners are equivalent.
Diagnosis of Obstructive
Because of a biologic mechanism called ├óÔé¼´åİvascular remodelling™, not all coronary plaque results in significant stenoses. However, high CAC scores indicate high coronary plaque burden that is likely to have overwhelmed the vascular remodelling capacity and to have resulted in significant stenoses. This use of cardiac CT imaging for the diagnosis from CAC scores of obstructive CAD as the cause of chest pain is well validated and sensitive, but it requires use of age- and sex-specific threshold values of CAC scores to be acceptably specific. 5 An Expert Consensus Document issued in 2000 by the American Heart Association (AHA) and the American College of Cardiology (ACC) did not endorse the use of CAC scanning for the diagnosis of obstructive coronary artery disease because of low specificity. 6/>/>/>/>/>/>/>/>/>/>/>/>/>/>
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