CT Coronary Angiography
CT coronary angiography (CTCA) images the coronary arteries, heart and proximal great vessels. It uses electrocardiogram gated acquisition on a multidetector scanner of at least 64 rows (and preferably wide detector or dual source), after an intravenous contrast bolus (50 to 80 mL of high concentration non-ionic contrast at 4 to 6 mL each second, followed by saline).
The heart rate must be low and regular, ideally under 60 beats each minute (up to 65 in modern scanners). Give oral or intravenous beta blockers (metoprolol 50 to 100 mg orally, or 5 to 20 mg intravenously) if no contraindication, with sublingual glyceryl trinitrate (400 to 800 micrograms) just before the scan to dilate the coronary arteries.
The scan takes 5 to 10 seconds, with a table time of 15 to 20 minutes. Images are reconstructed at 0.5 to 0.75 mm in multiple phases of the cardiac cycle and viewed with multiplanar, curved planar and volume rendered images.
Effective dose is about 1 to 5 mSv with prospective gating and iterative reconstruction (10 to 15 mSv with older retrospective techniques).
Coronary artery disease is the leading cause of death worldwide.
CTCA has a sensitivity of 95 to 99% and a negative predictive value of 97 to 99% for obstructive stenosis (over 50%) against invasive angiography, with a specificity of about 80 to 90% (lower with heavy calcification). It is therefore best used to rule out disease in patients with low to intermediate pretest probability.
It also shows non-obstructive plaque, which predicts events, and allows plaque characterisation. Functional data can be derived from computed fractional flow reserve in selected cases.
Indications. Stable chest pain with low to intermediate probability of coronary artery disease (first line test in many services). Acute chest pain with normal or non-diagnostic electrocardiogram and troponin, in the emergency department.
Exclusion of coronary disease in new heart failure or cardiomyopathy of unknown cause, before non-coronary cardiac surgery (valve surgery) in low risk patients, and in patients with equivocal functional tests.
Assessment of coronary anomalies (origin, course, relation to great vessels), coronary artery bypass graft patency, and stents of 3 mm or larger. Planning for transcatheter aortic valve implantation, left atrial appendage and ablation procedures, and structural interventions (with dedicated techniques for the root and access vessels).
Kawasaki disease follow up (aneurysms). Cardiac masses and pericardial disease. Not for patients with known severe calcification, an irregular rhythm, a fast heart rate that does not respond to treatment, or acute coronary syndrome with high risk needing urgent invasive angiography.
Contraindications and precautions.
- Previous severe contrast reaction.
- Renal impairment (estimated glomerular filtration rate under 30 mL each minute: avoid or weigh carefully).
- Pregnancy.
- Uncontrolled arrhythmia (atrial fibrillation gives poor gating).
- Inability to hold the breath for 10 to 15 seconds.
- Severe asthma or heart block (beta blocker contraindications), hypotension, severe aortic stenosis.
- Recent use of phosphodiesterase inhibitors (glyceryl trinitrate is withheld for 24 to 48 hours after sildenafil and similar drugs).
Metformin is handled as local renal function practice requires. Hyperthyroidism needs caution because of iodine load. Obesity may degrade the image.
The patient is told to avoid caffeine for 12 hours, fast for 4 hours, and take a few drops of water only.
Preparation and technique. A cannula of 18 G in the right antecubital vein, electrocardiogram electrodes, and breath hold practice. Heart rate is controlled with beta blocker (or ivabradine, or a calcium channel blocker if beta blockers are contraindicated), and glyceryl trinitrate given sublingually. A non-contrast scan for calcium may be made first.
Contrast timing uses bolus tracking in the ascending aorta (trigger at 100 to 150 HU) or a test bolus. Scan mode is prospective axial gating at 70% of the R-R interval (diastole) for slow heart rates, or with a wider window or retrospective gating for higher rates and irregular rhythm.
Reconstruction uses the phase with the least motion, with multiphase reconstruction when needed. A dual source scanner or wide detector scanner allows imaging in a single heartbeat. Findings are reported by segment (the 18-segment model), with stenosis grade.
