Normal anatomy

In a frontal view of the heart and lungs, the right atrium constitutes the outline of the heart on the right side. Cranially, the right atrium continues into the superior vena cava. The opening of the superior vena cava into the right atrium is situated in the posterior part of the atrium. The atrial septum forms the posterior medial wall of the right atrium. In front of the atrial septum, the right atrium is situated next to the root of the aorta. The right ventricle, which is the most anterior part of the heart, is situated adjacent to the sternum. Anterior and to the left of the root of the aorta are situated the pulmonary valves and the right ventricular outlet region. The ventricular septum separates the right ventricle from the left ventricle. The cranial part of the posterior outline of the heart is occupied by the left atrium. The pulmonary veins from the right and left lungs connect to the posterior portion of the left atrium. The left ventricle lies anterior and slightly to the left of the atrium. In a frontal view most of the left border of the heart consists of the left ventricle (Fig. 17 a-d). A series of transaxial MR images (Fig. 18) displays the morphology and position of the cardiac chambers. Examination of these imaging facilitates understanding of the anatomy of the cardiac contours as depicted on plain radiographs.

Right atrium

The right atrium is best visualized by angiocardiography with injection of contrast medium either into the right atrium, superior vena cava, or inferior vena cava. The right atrium is almost globular in shape with an appendage (auricle), projecting anteriorly, cranially and leftward from the body of this chamber. The superior vena cava and inferior vena cava open into the right atrium at the upper and lower edge of the posterior wall. The tricuspid valve lies at the front and to the left of the centre of the right atrium. The coronary sinus opens into the posterior wall of the right atrium between the tricuspid orifice and the inferior vena cava.

Opacification of the right atrium with contrast medium demonstrates the thickness of the lateral wall against the air-filled right lung. The combined
thickness of the right atrial wall and adjacent pericardium is less than 4 mm.

Right ventricle

The internal anatomy of the right ventricle is demonstrated by angiography after injection of contrast medium into the right side of the heart or into the superior vena cava or inferior vena cava. In frontal views the right ventricle is triangular, with the apex pointing downwards to the left. The pulmonary orifice is seen cranially. Prominent trabeculations are characteristic for the right ventricle. The tricuspid and pulmonic valves are separated by a tunnel of muscle (crista supraventricularis or infundibulum). The right ventricle is not usually border-forming on a frontal view of the chest. In a lateral view, the anterior border of the right ventricle forms the anterior outline of the heart.

Left atrium

The left atrium is not globular like the right atrium, but more flattened anteroposteriorly. F our pulmonary veins empty in to the posterior aspect, two on each side. The mitral orifice lies in the caudal, anterior part of the left atrium, slightly to the left. Like the right atrium, the left atrium has an appendage (auricle), which, in a frontal view, constitutes the border of the left side of the heart between the left ventricle and the pulmonary artery .
In a lateral view, the posterior outline of the left atrium is clearly seen where it lies adjacent to the anterior wall of the contrast-filled esophagus. The left main bronchus usually runs along the upper posterior part of the left atrium.

Left ventricle
The left ventricle (Fig. 14) has an elliptical shape with the apex anteriorly, inferiorly and to the left. The aortic and mitral orifices lie near the base of this chamber. The mitral valve is bicuspid, the aortic valves, tricuspid. The ventricular septum runs obliquely from right posterior to left anterior, and consists of a muscular and a membranous part.

The internal anatomy of the left ventricle is demonstrated by angiography. Injection of contrast medium into the left ventricle is an important part of coronary angiography. The mitral valves are assessed by left ventricular angiography, which permits observation of the flow of unopacified blood from the left atrium into the left ventricle in order to evaluate the size of the mitral orifice.

Coronary arteries

The right and left coronary arteries (Fig. 14 c, d) arise from the aorta. The left coronary artery arises from the aortic bulb to the left, posteriorly. The right coronary artery arises from the right part of the aortic bulb with its opening in the front, and slightly to the right.

From its aortic origin, the left coronary artery courses to the left. The main trunk bifurcates into the left anterior descending artery (LAD), and the circumflex artery (CX).

The LAD proceeds along the front of the heart, where it provides a number of parallel branches to the interventricular septum (septal branches) and large branches coursing over the anterior surface of the left ventricle (diagonal branches) and supply the anterior and lateral walls of the left ventricle.

The CX courses on the posterior side of the heart towards the left, where it provides one of several large branches to the posterior lateral wall of the left ventricle (obtuse marginal branches).

The right coronary artery (RCA) courses from the aorta onto the right side of the heart, and then continues on the lower surface of the heart. During its course, the artery provides several branches, including one to the sinus node. The posterior descending branch and posterolateral branches arise from the distal RCA and run on the posterior surface of the heart, supplying the posterior portion of the ventricular septum and the posterior wall of the left ventricle.

Dominance of the coronary arteries is designated by the arterial supply to the posterior wall of the left ventricle. Right dominance is defined by origination of the posterior descending and posterolateral branches from the RCA. Left dominanceis defined by origination of these branches from the CX. Balanced dominance (co-dominance) results from origination of the posterior descending branch from the RCA and posterolateral branches from the CX. Roughly 80% of humans have a right dominant circulation and 10% have left dominant circulation. About 10% have balanced dominance.

Pericardium

The pericardium is a two-layered sac that surrounds the heart. The two layers of the pericardium are covered by a serous membrane. In the minimal space between the two membranes, there is normally about 20 ml clear fluid.

The pericardium completely surrounds the heart, and extends along the pulmonary veins, azygos vein, superior vena cava, and inferior vena cava. The pericardium also covers the pulmonary trunk up to the bifurcation into the right and left pulmonary arteries, and the caudal 2 cm of the ascending aorta.

The pericardium is not visible as a separate structure on the chest radiograph, but represents the true border of the heart shadow, together with epicardial fat, against the air in the lungs. The normal pericardium is easily identified on computed tomography and MRI. Pericardial fat pads are frequently seen, most commonly at the base of the heart.

Arnulf Skjennald and Charles B. Higgins