The right lung has three lobes, and is slightly larger compared to the two-lobed left lung. The right major fissure divides the right upper and middle lobes from the lower lobe, and the left major fissure divides the left upper lobe from the left lower lobe. Typically, the major fissures are only seen on lateral radiographs. The right lung has a minor fissure that separates the right middle lobe from the right upper lobe, and can be seen on a frontal radiograph as a horizontal linear opacity at the level of the fourth anterior rib. The tracheal air column is seen both on the frontal and lateral radiographs. The right- and left-sided bronchi vary by laterality and are visible centrally on a frontal chest radiograph.

The hila comprise a composite of pulmonary arteries, veins, central bronchi, and lymph nodes, with the radiographic appearance predominantly attributed to the pulmonary arteries and superior pulmonary veins. The right and left hila differ depending on pulmonary artery anatomy, which varies by laterality, and the relationship between the vasculature and bronchi. The left hilum is normally cephalad compared to the right hilum, and should never be inferior, relative to the right. On a lateral radiograph, the right pulmonary artery courses anterior to the bronchus, which is eparterial, whereas the left pulmonary artery arches over the hyparterial left main bronchus and courses posteriorly.

On the frontal view, the right mediastinal border is created by, from superior to inferior, the superior vena cava (SVC), paratracheal stripe, azygous vein, right atrium, and the inferior vena cava (IVC). The left mediastinal border is created by, from superior to inferior, the left subclavian vein, aortic arch and descending aorta, pulmonary artery, left atrial appendage (when enlarged), and the left ventricle. On the lateral radiograph, the mediastinal border is formed anteriorly, from superior to inferior, by the ascending aorta, pulmonary artery, and right ventricle; and posteriorly, aortic arch, descending aorta, left atrium, left ventricle, and IVC² (Figure 32.1).

The heart is normally positioned to the left of the sternum, with the cardiac apex pointing toward the left, termed levocardia. Malpositioning of the heart can be either dextrocardia (within the right hemithorax) or mesocardia (midline), and can be either primary or secondary. Primary malpositioning is a congenital cardiac anomaly affecting both the positioning and the axis, often associated with other congenital cardiac anomalies. Extracardiac causes, such as hypoplastic lung, or bony abnormalities, such as scoliosis or pectus excavatum, can secondarily impact cardiac position, termed dextroposition or mesoposition. In dextrocardia, the position of the heart and the direction of the cardiac apex do not correspond to the expected radiographic anatomy.³ The Van Praagh segmental approach, used in classifying congenital heart disease, takes into account the visceroatrial situs, ventricular looping, and great arteries’ position.³ The pulmonary artery and bronchial anatomy on chest radiographs provide information regarding visceroatrial situs, which, in combination with cardiac position, can suggest the possibility of congenital heart disease.⁴,⁵ Abdominal situs is often described by the position of the stomach bubble on the radiograph. The severity and incidence of congenital heart
disease is associated with situs and cardiac position. Situs solitus with levocardia, the normal configuration, has a congenital heart disease incidence of less than 1% (excluding bicuspid aortic valve), whereas situs inversus with dextrocardia has a 3% to 5% incidence. Situs solitus with dextrocardia and situs inversus with levocardia have a congenital heart disease incidence greater than 90%.⁴

The aortic knob corresponds to the distal most aspect of the aortic arch as it courses downward to become the descending aorta. At this point, the arch is tangential to the x-ray beam on the frontal view, projecting as a circle that is obscured on the right side by the mediastinum and delineated along its left border by the lucent lung.⁶

The normal arch courses to the left and displaces the trachea to the right, with the aortic knob positioned on the left side of the mediastinum. In a right aortic arch, the aortic knob is seen in the right paratracheal region, and there is displacement of the trachea to the left (Figure 32.2). There are two types of right aortic arch, which can be determined by the branching pattern of the great vessels.⁷,⁸ The right arch with mirror image branching is highly associated with congenital heart disease. In contrast, a right arch with an aberrant left subclavian artery is usually an incidental and benign finding, although the left-sided ligamentum arteriosum typically results in a loose vascular ring.

In a patient with limited clinical history, understanding cardiac implants and support devices and determining valvular position is extremely important. Using the lateral radiograph, a line is drawn from the hilum to the apex, parallel to the long axis of the heart. The aortic valve is above this line and the mitral valve below. On a frontal radiograph, an imaginary line will be drawn from the right cardiophrenic angle to the inferior of left hilum, perpendicular to the long axis of the heart. The aortic valve is above the line and the mitral valve below. On both views, the pulmonic valve is the most superior valve, and, on the lateral radiograph, the tricuspid is the most anterior and inferior on the frontal radiograph⁹ (Figure 32.3 A,B). Utilizing this technique on supine anteroposterior (AP) radiographs can be inaccurate, and other methods are recommended. The valve orifice method can be utilized, where an aortic valve will be in profile on the frontal radiograph (visualized as a line) while the mitral will be en face (visualize as the whole ring). The perceived direction of flow is useful, with aortic valves being toward the arch and mitral valves toward the apex.¹⁰ The radiographic appearance of prosthetic valves is beyond the scope of this chapter. However, there are multiple online resources with pictorial demonstrations of the appearance of mechanic, bioprosthetic, and transcatheter valves.¹¹

There are varied approaches to interpreting chest radiographs, and each individual should determine what suits his/her needs. The common denominator for accurate interpretation is a systematic approach. One should also be vigilant to avoid a common error in diagnostic radiology termed satisfaction of search, defined by prematurely ceasing to search for abnormalities once an abnormality has been identified.¹² Following a systematic interpretation scheme ensures that all aspects of the radiograph have been interpreted. The clinical value achieved from comparison with old studies cannot be overemphasized. When interpreting a chest radiograph from a cardiac standpoint, the findings have a dual purpose. One should not only attempt to identify findings associated with cardiac disease but, in addition, noncardiac thoracic pathology, if identified, may explain the patient’s symptoms.

The standard views in chest radiography are posteroanterior (PA), lateral, and AP radiographs. A PA radiograph is obtained with the patient standing up against the detector with the x-ray tube posterior to the patient, at a 6-foot distance, by convention; such that the patient is exposed to the x-ray beam, posterior to anterior. The scapulae are cleared from the lung field by the patient placing their hands on their iliac crests and shoulders rotated forward. The patient’s chin should be raised to ensure the apices are not obscured. The radiograph is obtained while the patient is in breath hold at full inspiration, permitting visualization of the 10th posterior rib above the diaphragm. The patient is positioned so that there is no rotation, with the medial ends of the clavicles equidistant from spinous processes. The entire lungs should be demonstrated including the costophrenic angles and apices. There should be adequate exposure so that the thoracic spine and intervertebral disc spaces are visualized in the retrocardiac region. For a lateral radiograph, the patient is erect, seating or standing, with the left side touching the detector. Careful attention is made to ensure the humeri and the chin are excluded from the field of view. Correct positioning is determined by the sternum being seen in profile, the costophrenic angles included, the intervertebral foramina open, and the bilateral anterior ribs and bilateral posterior ribs are nearly superimposed. Adequate exposure should demonstrate the hemidiaphragms clearly, and the ribs should be visualized through the heart and lung markings. When a patient is unable to perform a PA radiograph, a portable radiograph can be obtained. This is particularly useful for patients who are bed bound and/or immobile, because this can be performed supine or semi-recumbent. Typically, these are performed in an AP projection, with the detector positioned behind the patient’s back and the x-ray tube anterior to the patient. Therefore, the x-ray beam travels from anterior to posterior, usually at a distance of 3 feet.¹³,¹⁴

There are additional chest radiographic views, which were previously used more commonly, but still have a role in troubleshooting. The apical lordotic view is used to visualize the lung apices by projecting the clavicles above the lungs, by either having the patient lean back or angling the x-ray tube cranially if the patient is upright. The lateral decubitus view is useful for evaluation of layering pleural effusion and exclusion of loculation, as well as being useful for demonstration of a small pneumothorax in the nondependent lung. In this view, the patient lies on their side and the radiograph is taken using a horizontal beam.¹³