Radiation Risk of Medical Imaging for Adults and Children
What are the generally accepted indications for a medical imaging procedure? An appropriate imaging procedure is justified if it changes…Read more
Angiography is the X-ray imaging of blood vessels using X-ray contrast agents injected into the bloodstream.
CTCA uses computed tomography (CT) scanning to take images (angiograms) of the coronary arteries. It requires the use of rapid CT scanning techniques, and can only be carried out in centers where the equipment is suitable and the medical/technology staff are trained appropriately.
CTCA has its largest indication in symptomatic patients with suspected coronary artery disease. It has a very good negative predictive value for the presence of coronary atherosclerosis.
Main indications: CTCA is mainly used for patients with suspected coronary artery disease who have a low or intermediate risk of the disease based on the following standard risk assessment criteria:
CTCA has a limited role in asymptomatic patients, although it might have a role in a subset of patients with strong family history of premature coronary artery disease. It has been used in higher-risk patients in whom invasive angiography is not desirable or in whom it is contraindicated for some reason.
This area is evolving rapidly, and it is probable that new uses for this test will appear in the future. Potential indications have included: lesion assessment before coronary artery angioplasty; the evaluation of chest pain after coronary artery bypass surgery; the assessment of stent restenosis; and the anatomical assessment of congenital anomalies in the origin and subsequent course of the main coronary arteries.
See Indications for CTCA above.
Chest pain suggestive of coronary artery disease with:
Patients should be assessed for renal impairment with estimation of glomerular filtration rate (eGFR) before the procedure. The timing of the test in relation to the procedure should be determined based on the stability of the patient’s medical condition and kidney function. If it is likely that the kidney function is acutely deteriorating, eGFR should be carried out on the day of the procedure, whereas if the patient is a clinically stable outpatient, an eGFR within the past 4–6 weeks is probably satisfactory. If there is any doubt, this should be discussed with the radiologist or cardiologist carrying out the procedure.
The procedure is contraindicated in patients with severe renal impairment because of the high risk of contrast-induced nephropathy (CIN) and some permanent decrement in kidney function.
Pregnancy is a contraindication due to radiation exposure.
Known coronary artery disease: Most patients with known coronary artery disease and symptoms will be evaluated with catheter angiography rather than CT scanning. (see: imaging pathways1 listed in useful websites below). Patients with symptoms consistent with obstructive coronary artery disease, and who have associated abnormal ECG and/or elevated cardiac enzymes are considered high risk and are not suitable for cardiac CT.
Renal impairment: Contrast-induced nephropathy (CIN) is more frequent in patients with pre-existing kidney function reduction, cardiac failure and possibly also hypertension (see Iodine-containing contrast medium (ICCM) for a full list of factors increasing the risk of contrast induced nephropathy). Some diuretics also increase the likelihood of CIN.
Asthmatic patients: Patients with asthma might not be able to tolerate beta-blockers (often used in the procedure). The respiriatory effort involved in dyspnoeic patients could degrade image quality, making the study non-diagnostic.
Morbid obesity: Image quality will be impaired in very large patients. There are weight limits for CT scanner tables. If your patient is obese, it is prudent to check the weight limit for the scanning facility to which you are referring them.
Contrast medium hypersensitivity/allergy: A history of a severe allergic reaction to contrast medium is a relative contraindication to CTCA. A history of minor hypersensitivy reactions might not necessarily be a contraindication.
Other relative contraindications include: the presence of arrhythmias, high coronary calcification scores (see Coronary Artery Calcium scores) and an inability of the patient to concentrate and follow instructions in English. If the need for an interpreter is anticipated, this should be advised to the CT scanning facility to allow for an interpreter to be pre-booked.
The adverse effects include contrast-induced nephropathy, contrast allergy, side-effects from beta-blockers and nitroglycerin (Anginine) and the possible adverse effects of incorrect treatment as a result of over-estimation of coronary artery disease due to the imperfect specificity of the test. Contrast allergy can be life threatening, and the procedure should only be carried out where there is adequate medical supervision and the ability to treat severe contrast reactions (see InsideRadiology: Iodine-containing contrast medium (ICCM)).
In the event that the patient is given heart rate reduction medications, usually beta-blockers before the scan, there is often a relatively short period (approx. 30 minutes) of post-CTCA observation and monitoring to confirm that there is no significant effect on heart function or blood pressure. Most untoward effects develop before the CT scan rather than after completion of the study.
Many alternative tests are available to identify cardiac ischaemia.
Invasive coronary angiography is regarded as the gold standard for the assessment of coronary artery disease. This test can be made more precise by the inclusion of intravascular ultrasound or fractional flow reserve (FFR) pressure measurements with a radiwire at the time of the procedure.
Exercise stress testing is a commonly used test for the assessment of coronary artery disease, but has significant limitations.
Dobutamine stress echo or MRI (see Cardiac MRI (Stress Perfusion)), nuclear medicine myocardial perfusion imaging, MRI myocardial perfusion imaging, and PET perfusion and viability assessments are also available. These tests assess the functional significance of coronary artery disease, as opposed to the anatomical abnormalities shown by CTCA and coronary catheter angiography. All these tests provide useful information for the assessment of patients with coronary artery disease, and the appropriate test for a particular patient can only be assessed by the cardiologist and radiologist involved.
The exact role of all of these tests in the diagnosis and understanding of coronary artery disease is evolving as more information becomes available and as technology improves.
(For further information see: imaging pathways1 listed in useful websites below).
Referrers need to be aware that the value of cardiac CT lies in its potential to exclude the presence of coronary artery atherosclerosis in patients with no biochemical or electrocardiographic markers of ischaemia. A normal study gives a 99% probability that no disease exists. It needs to be emphasized that there is significant discrepancy in published material with regard to the clinical significance of any visible moderate severity disease on cardiac CT. CTCA has been shown to overestimate the severity of coronary artery disease with positive predictive values as low as 53%.
The Conjoint Committee for the Recognition of Training in CT Coronary Angiography website
Page last modified on 18/8/2017.
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