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Coronary CT Angiography

Practice Essentials

Coronary computed tomography angiography (CCTA) is a noninvasive method to image the coronary arteries. Applications include the following:

Diagnosis of coronary artery disease (CAD)

Diagnosis of in-stent restenosis

Evaluation of coronary bypass graft patency

Clinical application in CAD

Based on the combined efforts of 9 specialty societies,
the following indications were rated as appropriate for CCTA:

Detection of CAD in symptomatic patients without known heart disease, either nonacute or acute presentations

Detection of CAD in patients with new-onset or newly diagnosed clinical heart failure and no prior CAD

Preoperative coronary assessment prior to noncoronary cardiac surgery

Patients with prior electrocardiographic exercise testing – Normal test with continued symptoms or intermediate risk Duke treadmill score

Patients with prior stress imaging procedures – Discordant electrocardiographic exercise and imaging results or equivocal stress imaging results

Evaluation of new or worsening symptoms in the setting of a past normal stress imaging study

Risk assessment post-revascularization – Symptomatic if post-coronary artery bypass grafting or asymptomatic with prior left main coronary stent of 3 mm or greater

Evaluation of cardiac structure and function in adult congenital heart disease

Evaluation of cardiac structure and function – Ventricular morphology and systolic function

Evaluation of cardiac structure and function – Intracardiac and extracardiac structures

Recent technology

Dual-source CT-scanning

The primary advantage of dual-source CT scanning is greater temporal resolution, which allows CCTA to be performed at higher heart rates without the use of beta blockers.

256- and 320-slice CT scanning

The primary advantage of 256- and 320-slice CT is the increased craniocaudal coverage. In a comparison of prospectively gated 64- and 256-slice CT scanning, the 256-slice scan provided better and more stable image quality, at equivalent effective radiation dose.

Radiation dose

The reported effective radiation doses for retrospectively gated, single-source, 64-slice CT scanning have ranged from 9.5-21.4 mSv.
However, various technologies and techniques have made it possible to lower the dose to less than 5 mSv, and doses of less than 1 mSv are possible in some patients.

Patient preparation

At the author’s institution, patients are instructed to avoid caffeine and smoking 12 hours prior to the procedure to avoid cardiac stimulation. They are also instructed to avoid eating solid food 4 hours before the study and to increase fluid intake prior to the exam. Standard precautions with regard to contrast allergy history and renal function are taken.

Beta blockers

Beta-blocker administration is often helpful in cardiac CT scanning to lower the heart rate and decrease motion artifact. The level to which the heart rate should be lowered depends on the temporal resolution of the scan.

However, heart rate variability may be a more important determinant of image quality than absolute heart rate.
Beta blockers are also helpful in patients with irregular heart rates, supraventricular tachycardias, and arrhythmias.

Several contraindications to beta-blocker therapy exist, including a heart rate below 60 bpm, a systolic blood pressure below 100 mm Hg, and decompensated cardiac failure, among others.

Nitroglycerin

The administration of sublingual nitroglycerin dilates the coronary arteries and increases side branch visualization.
Nitroglycerin is contraindicated in patients who are allergic to it and in patients who are taking phosphodiesterase inhibitors for erectile dysfunction.
Patients should not have taken a phosphodiesterase inhibitor for at least 48 hours before the exam.

Nitroglycerin can cause orthostatic hypotension; it should be used with caution in patients who have low systolic blood pressure (eg, < 90 mm Hg) and who are volume depleted from diuretic therapy. Angina caused by hypertrophic cardiomyopathy can also be aggravated.

Artifacts

Artifacts include the following:

Stairstep artifacts: Associated with heart rate variability
(see the image below)

Stairstep artifact: Volume-rendered CT image demon

Stairstep artifact: Volume-rendered CT image demonstrates “stairstep” artifact. This is a phase-misregistration artifact that is typically secondary to an irregular heartbeat.

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Coronary artery motion artifacts: Result in image blurring (see the image below)

Coronary artery motion: Axial CT images reconstruc

Coronary artery motion: Axial CT images reconstructed at 90% (A) and 70% (B) of the R-R interval demonstrate the importance of imaging during the phase of least cardiac motion.

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Respiratory motion artifacts

Streak artifacts: Can result from beam hardening secondary to metal clips or can be caused by dense contrast

Blooming artifacts: Can cause small, high-contrast structures such as stents and calcium to appear larger than they are
(see the image below)

Blooming artifact: Multiplanar reconstruction (MPR

Blooming artifact: Multiplanar reconstruction (MPR) CT image demonstrates blooming artifact from a left anterior descending (LAD) artery stent. The stent lumen is poorly visualized secondary to this artifact.

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Image reconstruction

The coronary arteries are optimally imaged when there is the least cardiac motion. This occurs during so-called rest periods, which is typically in middiastole (diastasis). Coronary motion is also minimal during end-systole (isovolumic relaxation), but this is of shorter duration than diastolic diastasis at low heart rates.

A variety of postprocessing techniques are useful in CCTA.
Many interpreting physicians will start with the axial source images and then utilize multiplanar reconstructions in at least 2 planes.

Stenosis grading

There are many different methods to grade the degree of stenosis, including the following:

Visual assessment

Manually determined diameter or cross-sectional area on multiplanar reformats perpendicular to the median centerline of the vessel (“end-on” view)

Diameter on maximum intensity projection (MIP) images parallel to the long axis of the vessel

Software calculation of diameter or area

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