CT coronary angiography-CTA (or multidetector computed tomography - MDCT )
Computed tomography (CT) is based on X-ray emission from a source that revolves around the body. The beam of X-rays is focused and sensitively detected, after it passes through the body. Each pixel of the resulting image has a specific value that corresponds to the X-ray attenuation in its passage through the tissues (Hounsfield units-HU). Because of heart movements, a high time resolution is required, combined with synchronization with the electrocardiogram (ECG gating).CT scan of coronary arteries
Currently, multislice computer tomographic machines (≥64 slices) are used for performing CT coronary angiography (multidetector computed tomography - MDCT ). To obtain satisfactory image quality a slow heart rate <65 / min is needed. This can be achieved by administering a beta-blocker (PO or intravenously). A non-dyhidropyridine calcium channel blocker (verapamil or diltiazem) may be given alternatively. Nitroglycerine is also administered sublingually to induce vasodilatation of the coronary arteries, that can facilitate imaging. The examination (CT coronary angiography) is performed with intravenous administration of a contrast agent. The patient is asked to hold his breath for 5-10 seconds while the CT images are obtained.
The test has a good sensitivity (about 90-95%), specificity (about 85-90%), positive predictive value (about 85 -90%) and negative predictive value (96%) for the diagnosis of significant coronary artery disease. These are values reported from studies evaluating the method for the diagnosis of significant coronary artery stenoses (> 50%). MDCT has a high negative predictive value. Thus, it is a good test to exclude the presence of significant coronary artery disease, particularly in patients with a low or intermediate likelihood of coronary artery disease. It can be used in patients with chest pain when the ECG does not provide diagnostic findings. Compared with MRI, MDCT is better for coronary artery imaging.
Indications of coronary CT angiography (MDCT)
In patients with symptoms (chest pain) who on the basis of risk factors and clinical picture have an intermediate probability of coronary artery disease. It is also helpful in patients with an abnormal baseline ECG or patients who have a limited ability to exercise.In patients with symptoms when other tests (e.g. exercise test, SPECT, stress-echocardiography) are non-diagnostic or show marginal or equivocal findings.
In patients with newly diagnosed left ventricular systolic dysfunction (usually with a dilated, diffusely hypokinetic left ventricle on echocardiography) to determine if coronary artery disease is the underlying cause.
In symptomatic patients, when a congenital abnormality of the coronary arteries is suspected (in this case, MDCT is a first-choice test).
Note that MDCT is not recommended for asymptomatic individuals who have an increased risk for coronary artery disease (due to the presence of risk factors).
Relative contraindications for MDCT include: pregnancy, severe allergy or anaphylaxis to contrast media, chronic kidney disease (there is a risk of the contrast medium to adversely affect renal function), inability of a patient to cooperate (e.g. to hold the breath), an unstable patient (e.g. acute myocardial infarction, acute heart failure, hypotension), presence of factors that can degrade the image quality such as morbid obesity, a high or irregular heart rate or severe coronary artery calcification (calcium score > 400).
A female 26 years old who suddenly collapsed during physical exercise (running).
After resuscitation the ECG, blood tests (complete blood count, usual biochemical tests and electrolytes) were normal and echocardiography showed normal left and right ventricular size, wall thickness and function and normal cardiac valves. She was evaluated with a coronary CT angiography. What is the diagnosis?
Anomalous origin of the right coronary artery -RCA from the left sinus of Valsalva of the aortic root, in proximity to the normal origin of the left coronary artery. It is a congenital coronary anomaly. Especially during exercise, due to the anomalous course of the RCA between the right ventricular outflow tract or the pulmonary artery and the aorta, the RCA can be compressed between these structures. Note: Transverse tomographic sections are presented as seen from a viewer looking from the direction of the patient's feet and so left-sided structures are displayed on the right of the image, right-sided structures are displayed on the left, anterior structures are displayed at the upper portion of the image and posterior structures at the bottom. 1 left sinus of Valsalva of the aortic root / 2 left main coronary artery (LMCA) 3 RVOT, 4 RCA 5 left atrium (LA) 6 right atrium (RA). The case is courtesy of Dr. Abdallah Almaghraby, cardiologist.
Advantages and limitations of CTA (MDCT)
An advantage of MDCT is that it also provides additional information (the dimensions and contractile function of the ventricles, diameter of the various segments of the thoracic aorta).In patients with severe obesity, tachycardia, atrial fibrillation or frequent extrasystoles, MDCT usually cannot provide images of the coronary arteries with a good quality.
In the assessment of the severity of coronary arterial stenoses, CT coronary angiography (multidetector computed tomography-MDCT) is not as accurate as an invasive (selective) coronary angiography. The latter provides a more accurate delineation of the coronary arterial lumen and a better resolution, whereas MDCT often displays a tendency of overestimating the severity of an atheromatous stenosis.
For treatment decisions to be made, functional tests that can reveal myocardial ischemia (e.g. exercise ECG testing, myocardial perfusion scintigraphy-SPECT, or stress-echocardiography) are usually required.
In the case of calcified atherosclerotic plaques or in the presence of a stent, coronary computed tomography angiography( CTA or MDCT) displays a limited accuracy in the assessment of the coronary arterial lumen for a stenosis. CTA tends to overestimate the thickness of a calcified atheromatous plaque (appearing white in the image) or of a coronary stent. This causes the arterial lumen to appear in some cases narrower than it actually is, resulting in an overestimation of the severity of stenosis in the region of a calcified atheromatous plaque. In the presence of heavily calcified plaques it can often be helpful to rotate the image with the aid of the software of the CT scanner. This can often clarify whether the plaque is on the surface and does not significantly protrude into the vessel lumen or whether it is a plaque with a significant degree of protrusion into the lumen, resulting in a significant stenosis (narrowing).
In the case of stents, assessment with MDCT for restenosis poses a challenge but generally, images are quite satisfactory for stents with a diameter ≥ 3 mm. However the test is not reliable for assessing whether there is a narrowing (from intravascular hyperplasia) in a stent with a diameter <2.75 mm.
In patients after bypass surgery,(CABG) coronary CT angiography has the advantage that it provides excellent images of the grafts, but also the disadvantage of less clear images of the native coronary arteries of these patients, distal to the connections of the grafts to the coronary arteries. The reason for this, is the commonly diffuse atherosclerosis and calcification present in the coronary arteries of such patients with advanced coronary artery disease.
A 60-year-old male, ex-smoker with a history of hypertension, experienced pain on the left parasternal region while climbimg stairs. The pain subsided after 2-3 minutes of rest. He denies having any symptoms of this kind in the past. His physical examination and ECG were normal. He has undergone a CTA. What do arrows 1 and 2 represent and what is the suggested treatment?
Coronary arterial calcium scoring (CAC scoring)
The coronary arterial calcium score (CAC score) is a powerful predictor of subclinical atherosclerosis. The calcium score is determined by the summation of the product of the calcified plaque area and a factor representing maximum calcium density in each lesion (1 for lesions with maximal density of 130-199 HU, 2 for 200-299 HU, 3 for 300-99 HU, and 4 for >400 HU) .
A generall classification of CAC score and hence the severity of coronary arterial calcification is the following:
Minimal 1-10,
Mild 11-100
Noderate 101- 400
Severe> 400
Studies have shown that asymptomatic people with CAC score >300 are at high risk for coronary events, whereas a low CAC score is associated with a low risk for coronary events. Coronary events at 5 years according to the calcium score:
Score 0 is associated with < 1% probability of a coronary event at 5 years
Score 1-100 < 5%
Score 101-300 6.5%
Score >300 11%
Measurement of the coronary calcium score for the assessment of cardiovascular risk is reasonable (although it has no absolute indication) in asymptomatic patients at intermediate cardiovascular risk (10%-20% ten-year risk as assessed with the Framingham risk score). On the contrary, CAC score measurement in asymptomatic patients at low risk (<6% 10-year risk) is not recommended.
Limitations of CAC score: Patients often have significant atheromatous lesions that are not calcified. Therefore although a low CAC score indicates a lower probability of coronary artery disease, it cannot exclude coronary artery disease.
The degree of calcification is not a sign of stability or instability of the atheromatous plaque, i.e. it does not reliably determine the risk of the atheromatous plaque to rupture resulting in thrombus formation, an event which results in an acute coronary syndrome. Moreover, the degree of plaque calcification often is not related to the severity of the stenosis of the arterial lumen.
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Bibliography and links
Eugene C Lin,. Coronary CT Angiography e-medicine/medscape
S Kyrzopoulos, L Neefjes P De Feyter, Cardiac Multidetector Computed Tomography: Where Do We Stand? Hellenic J Cardiol 2009; 50: 523-535
http://www.hellenicjcardiol.org/archive/full_text/2009/6/2009_6_523.pdf
Schroeder S, Achenbach S et al. Cardiac computed tomography: indications, applications, limitations, and training requirements: Report of a Writing Group deployed by the Working Group Nuclear Cardiology and Cardiac CT of the European Society of Cardiology and the European Council of Nuclear Cardiology
http://www.hellenicjcardiol.org/archive/full_text/2009/6/2009_6_523.pdf
Schroeder S, Achenbach S et al. Cardiac computed tomography: indications, applications, limitations, and training requirements: Report of a Writing Group deployed by the Working Group Nuclear Cardiology and Cardiac CT of the European Society of Cardiology and the European Council of Nuclear Cardiology
Eur Heart J (2008) 29 (4): 531-556.
Desjardins B, Kazerooni EA. ECG-Gated Cardiac CT.American Journal of Roentgenology. 2004;182: 993-1010
http://www.ajronline.org/doi/abs/10.2214/ajr.182.4.1820993
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