Frandics P. Chan

Imaging Considerations

Catheter angiography is the gold standard for the determination of luminal coronary abnormalities, such as coronary stenosis.³ During catheterization, the arterial wall can be characterized with intravascular ultrasound,⁴ and the hemodynamic significance of a stenotic coronary lesion on myocardial perfusion can be measured using the fractional flow reserve technique.⁵ In infants, transthoracic echocardiography is the first-line imaging tool for the evaluation of structural anomalies of the proximal coronary arteries. Echocardiography is usually adequate to exclude the major types of lethal anomalous coronary arteries. Echocardiography is less useful in older children, and it cannot visualize the entire coronary vasculature.

Cardiac-gated multidetector row CT angiography (CCTA) and cardiac magnetic resonance angiography (CMRA) are important noninvasive alternatives to catheter coronary angiography.⁶ Both techniques record three-dimensional images of the heart, with the coronary vessels shown in the context of adjacent cardiac structures. Visualization is facilitated by three-dimensional postprocessing. These capabilities have made CCTA and CMRA the preferred method for the diagnosis and characterization of anomalous coronary arteries.

CCTA requires intravenous injection of an iodinated contrast agent. Cardiac synchronized images can be acquired in one of two modes: prospective or retrospective electrocardiographic (ECG) gating. Prospective ECG gating obtains a snapshot of the heart at a predetermined cardiac phase, whereas retrospective ECG gating scans throughout the cardiac cycle and can produce cine images of the heart. However, retrospective gating incurs a four to five times higher radiation dose compared with prospective gating. With 64-slice scanners, the typical scan time in either mode is less than 20 seconds. For young children who cannot hold their breath voluntarily, apnea is induced for a short interval with anesthesia. With an increasing number of detector rows, increasing gantry rotation speed, and multiple x-ray sources, it is conceivable that scan time will be shortened to a point where breath holding and anesthesia may not be necessary.

The greatest concern regarding CCTA is that it uses ionizing radiation (see Chapter 66). Every measure must be taken to reduce the dose of ionizing radiation delivered to a patient. First, CCTA should be undertaken only if the diagnostic goals are achievable and the results affect management of the patient’s condition. As much as possible, scanning should be limited to a single pass over the heart only. Tube voltage and tube current should be kept to a minimum for the patient’s size and acceptable image quality. If possible, the pitch factor should be adjusted to the heart rate. Prospective ECG gating and ECG dose modulation should be used when appropriate. With these measures, dose-equivalent radiation is less than 10 mSv for retrospectively gated CCTA and 3 mSv for prospective gated or nongated CCTA.⁷ Noise reduction with iterative reconstruction currently is being investigated as a method to achieve diagnostic images at a very low radiation dose.⁸ A dose equivalent to less than 1 mSv for prospectively gated or nongated CCTA likely is achievable.

The lack of ionizing radiation makes CMRA an attractive imaging choice. On most clinically available magnetic resonance (MR) scanners, whole heart CMRA is built on a three-dimensional, cardiac-gated, navigated echo, T2-prepared, steady-state, free-precession sequence.⁹ Depending on coverage, spatial resolution, heart rate, and breathing pattern, imaging time typically is 10 to 20 minutes. For this amount of time, breath holding is impractical. Respiratory motion is managed by restricting data acquisition at a predetermined range of diaphragm position monitored in real time with the navigator echo technique. Contrast for the coronary vessels is based on the intrinsic long-T2 value of blood, and no gadolinium contrast administration is required, which is an important advantage in patients who have renal failure and the risk of gadolinium contrast–related nephrogenic systemic fibrosis.¹⁰ However, the T2-contrast mechanism is not selective, and all vascular channels are equally bright. Furthermore, other materials with a high T2 value, such as pericardial effusion and pleural effusion, are as bright as blood. This lack of differentiation sometimes can confound diagnostic interpretation.

Compared with CCTA, CMRA lags in spatial resolution. In addition, CMRA is sensitive to metal artifacts from surgical clips and sutures, especially those made from ferromagnetic materials. Because CMRA has a long scan time, anesthesia is required for patients who cannot cooperate. The complexity of CMRA requires greater expertise to perform the procedure compared with other modalities, and high-quality results may not be achieved as consistently as with CCTA. Nonetheless, because it does not utlilize ionizing radiation and intravenous contrast material, CMRA is a safer procedure compared with other procedures for patients who do not require anesthesia. For adolescents and young adults who are cooperative and have larger coronary arteries, trying CMRA first is a reasonable strategy. If CMRA fails to answer the clinical question, then CCTA or catheter angiography may be used as a backup. For infants and small children who must be anesthetized while they are studied, CCTA may be a more reliable option.

Normal Coronary Artery Anatomy

Many anatomic variations of the coronary arteries exist, and the separation of normal variants from anomalous coronary arteries is arbitrary (Fig. 78-1). In persons with conventional coronary anatomy, two main coronary arteries originate from two of the three aortic sinuses of Valsalva closest to the pulmonary trunk. The three aortic sinuses are labeled according to their attached coronary arteries: the right coronary sinus giving rise to the right coronary artery (RCA), the left coronary sinus giving rise to the left main coronary artery (LMCA), and the noncoronary sinus giving rise to no coronary branch.

After its takeoff from the right coronary sinus, the RCA courses anteriorly into the right atrioventricular groove. It gives rise to a right conal artery as the first branch 50% of the time. Otherwise, the right conal artery arises directly from the right coronary sinus. This conal artery supplies the myocardium of the right ventricular outflow tract (RVOT). In the majority of cases, a sinoatrial nodal artery branches from the proximal RCA (Fig. 78-2, A). Otherwise, it arises from the proximal left circumflex (LCx) artery. The middle portion of the RCA gives rise to one or more right ventricular branches (Fig. 78-2, B). These branches are called acute marginal branches, analogous to the obtuse marginal branches from the LCx artery. The distal portion of the RCA wraps around the inferior surface of the heart. In most people, the RCA gives rise to a posterior descending artery (PDA) (Fig. 78-2, C). The RCA then enters the “crux,” the intersection between the interventricular septum and the atrioventricular groove. At the crux, the RCA bends and forms an inverted U and then continues into the left atrioventricular groove at the bottom of the heart, where it gives off multiple posterior left ventricular arteries that supply the basilar inferior wall of the left ventricle. This pattern of the distal RCA is called a right-dominant coronary system and occurs in 85% of the population. An atrioventricular nodal branch often arises from the RCA near the apex of the inverted U (Fig. 78-2, D). The PDA gives rise to many inferior septal perforator branches that supply the inferior septum, analogous to the anterior septal perforator branches that originate from the left anterior descending (LAD) artery.

The LMCA originates from the left coronary sinus and courses to the left for a short distance. In most people, it bifurcates into an LAD artery and an LCx artery. In other people, the LMCA trifurcates into an LAD artery, an LCx artery, and, in between, a ramus medianus branch (Fig. 78-3, A). This branch supplies the anterior left ventricular wall. The LAD artery gives rise to two sets of vessels: an epicardial set called the diagonal branches and an intramuscular set called the anterior septal perforator branches. The diagonal branches are responsible for perfusing the anterior left ventricular wall, whereas the septal perforator branches are responsible for the anterior septum. The distal LAD artery typically wraps around the cardiac apex and terminates at the inferior wall of the apex (Fig. 78-3, B). In a minority of cases, the LCx artery gives off a sinoatrial nodal branch (Fig. 78-3, C) before it enters the left atrioventricular groove. The LCx artery then gives rise to a number of obtuse marginal branches, which supply the lateral left ventricular wall (see Fig. 78-3, B). In 10% of the population, the LCx artery supplies the posterior left ventricular branches and the PDA instead of the RCA. This system is called a left-dominant coronary system. In 5% of the population, the RCA supplies the PDA while the LCx artery supplies the posterior left ventricular branches. This system is called a co-dominant coronary system.

Nomenclature of the coronary arteries can be confusing in the presence of ventricular malformations. In general, the coronary arteries are named relative to the morphology, not the position, of the underlying ventricles. For example, in l-looped ventricles where the morphologic left ventricle lies to the right, the coronary artery in the right-sided atrioventricular groove is named the left circumflex artery. When derangement of the ventricles is so severe that the ventricular morphology cannot be clearly identified, then the coronary arteries should be described relative to physical landmarks—for example, left-sided atrioventricular branch instead of LCx, right-sided atrioventricular branch instead of RCA, anterior interventricular septal branch instead of LAD, and posterior interventricular septal branch instead of PDA.

Congenital Coronary Artery Anomalies