TECHNIQUES AND NORMAL ANATOMY
A variety of techniques have been developed to evaluate the heart and great vessels (Table 3-1). In this section, we briefly describe the major tests used in imaging this system.
The most common imaging test for evaluating the heart and great vessels is the chest radiograph, which consists of an up-right posterior-to-anterior (PA) and left lateral (LAT) pro-jections. The terms PA and left lateral refer to the direction the x-ray beam takes through the body before it reaches the radiographic cassette. Chest radiographs are usually obtained with high kilovoltage and milliamperage to minimize expo-sure time and cardiac motion. When possible, the distance between the x-ray tube source and the film is at least 6 feet to minimize magnification and distortion.
The examination is ideally performed with the patient at maximal inspiration. A good rule of thumb for estimating adequate inspiration is to be able to count 9 to 10 posterior ribs or 5 to 6 anterior ribs from the lung apices to the hemidi-aphragms through the aerated lungs (Figure 3-1). When a chest radiograph is taken in the expiratory phase of respira-tion, the patient may appear to have cardiomegaly, vascular congestion, and even pulmonary edema. This appearance, however, is merely artifactual and caused by the lack of inspi-ration (Figure 3-2).
Severely ill, debilitated patients or patients who cannot be transported to the radiology department can have their chest radiographs obtained with a portable x-ray machine. Patients in the ICU who have intravascular catheters or who are undergoing mechanical ventilation frequently have chest radiographs performed as a survey for compli-cations that may not be revealed by physical examination or laboratory data. These examinations are done with the cassette placed behind the patient in bed and are therefore anterior-to-posterior (AP) projections. The technical fac-tors, which are controlled by the technologist at the time of the examination, vary with the size of the patient and the distance of the radiographic plate from the x-ray source (or machine). An attempt is still made to obtain the examina-tion during maximum inspiration, but this objective may be difficult to achieve in some patients, especially those who have dyspnea.
With the patient in the supine position, there is normally a redistribution of blood flow to the upper lobe pulmonary veins (cephalization), and the heart may appear enlarged rela-tive to its appearance on the upright PA radiograph, because of magnification (Figure 3-3). Some patients are able to sit for their examinations, whereas others are radiographed in a semiupright position. Ideally, the technologist should mark the exact position of the patient when the radiograph is ob-tained, and the date and time of the examination should be recorded in all cases. Changes in patient positioning and ven-tilator settings can have substantial effects on the radiographic appearance and must be taken into account when evaluating any change in the radiograph from a previous study.
The chest radiograph, whether it is obtained in the up-right, semiupright, sitting, or supine position, should almost always be the initial screening examination in the evaluation of the cardiovascular system. Because it is essentially a screening study, the chest x-ray must be correlated with the clinical symptoms and physical examination to determine the overall significance of the radiographic findings. This in-formation is also used to decide if other imaging tests are ap-propriate and which ones will potentially result in the highest diagnostic yield. Decisions regarding further imaging also depend on the impact on the clinical management of the pa-tient, the potential for treatment of any abnormality that may be discovered, the cost and availability of the technique, and the expertise of the interpreting radiologist.
The conventional radiograph is an excellent screening test for the patient suspected of having disease involving the heart and great vessels, because the overall anatomy of these areas is demonstrated well. Whenever possible, all radiographs should be reviewed with all prior relevant imaging studies. Even when a prior chest radiograph is not available, additional in-formation may be ascertained by reviewing other prior im-ages such as thoracic spine or rib-detail image when available. Advanced imaging studies such as computed tomography (CT) and magnetic resonance (MR) imaging can also be used to help clarify complex findings on chest radiographs.
The normal cardiac silhouette size may be determined by the cardiothoracic ratio, a measurement obtained from the PA view. This ratio is calculated by dividing the transverse cardiac diameter (measured from each side) by the widest di-ameter of the chest (measured from the inner aspect of the right and left lungs near the diaphragm). The average normal value for this ratio in adults is 0.50, although up to 60% may be normal (Figure 3-4). A measurement over 50% is gener-ally considered abnormal in an upright inspiratory-phase PA film, although this may not always be clinically significant. The cardiothoracic ratio cannot be reliably used for the AP projection of the chest, because the heart is magnified (see Figure 3-3). The size of the patient and the degree of lung ex-pansion also should be considered. For instance, in a small person with a petite frame and a small thoracic cage, the heart size may be normal, but the cardiothoracic ratio may measure over 50%. Similarly, if the patient has pulmonary disease such as emphysema, the heart may be enlarged, but because of the overinflation of the lungs, the cardiothoracic ratio may still be normal. In clinical practice, most radiolo-gists do not perform this measurement and rely on experi-ence and “gestalt” to evaluate heart size.
The contours of the heart, mediastinum, and great ves-sels on the PA view should be evaluated on each chest film (see Figure 3-1A). A reasonable approach is to begin in the upper right side of the mediastinum just lateral to the spine and below the right clavicle. The curved soft-tissue shadow represents the right border of the superior vena cava (SVC). The border of the SVC forms an interface with the lung and should not be confused with the right paratracheal stripe. Below the SVC is the right cardiac border formed by the right atrium. The inferior heart border, or base of the heart, is the area just above the diaphragm and is composed pri-marily of the right ventricle, although there is some contri-bution from the left ventricular shadow. The left ventricle makes up the majority of the apex of the heart, which points to the left of the spine. The origins of the right and left pulmonary arteries are generally well demarcated on the normal PA film as they emerge from the mediastinum. The most prominent and recognizable component of the right pulmonary artery, the right descending pulmonary artery (RDPA), is seen just to the right of the superior cardiac bor-der and descends inferiorly. It can usually be easily followed until it branches. The left main pulmonary artery is less well defined, but its origin can usually be seen above and lateral to the left atrial appendage just before it branches. When en-larged, the main pulmonary artery may be seen superimposed over the left pulmonary artery and filling in the normal spacebetween the left pulmonary artery and transverse aortic arch (the AP window). The aorta originates posterior and to the right of the main pulmonary artery, and the border of the as-cending portion of the aorta can usually be seen superim-posed on the inferior portion of the SVC. The majority of the transverse arch is not outlined by air and therefore cannot be seen as it crosses the mediastinum. However, the distal trans-verse and descending aorta can be seen to the left of the mediastinum as it turns inferiorly. The left border of the descending thoracic aorta should be followed down to the aor-tic hiatus. Any loss of this contour or any contour abnormality may indicate pathology and should be investigated. Dilation or ectasia, localized bulges, and calcification may occur within the aorta as a normal part of the aging process, but should be viewed as abnormal in younger individuals. Of course, the spine, ribs, adjacent soft tissues, and upper ab-dominal contents should all be scrutinized. The left atrium lies just inferior to the tracheal carina, but it is usually not visualized as a discrete structure on the normal PA view.
The lateral view of the chest also reveals important infor-mation regarding the cardiac contour (see Figure 3-1B). Just behind the sternum there is normally a radiolucent area called the retrosternal clear space (RSS). This region repre-sents lung interposed between the chest wall and the anterior margin of the ascending aorta. Any density present within the RSS may be due to an anterior mediastinal mass or post-surgical changes. The anterior border of the cardiac shadow is composed primarily of the anterior wall of the right ventri-cle. Right ventricular enlargement may also encroach into the retrosternal clear space. The posterior margin of the cardiac silhouette is formed by the left atrium and left ventricle. Just posterior and inferior to the left ventricle is a linear soft-tissue shadow leading into the heart formed by the inferior vena cava (IVC). The left ventricular shadow should not project more than 2 cm posterior to the posterior border of the IVC. The transverse aortic arch can usually be discerned on the normal lateral chest film as a smooth curving shadow origi-nating anteriorly, crossing the mediastinum in a semilunar fashion, and then descending posteriorly as a linear shadow superimposed over the vertebral bodies. The left pulmonary artery (LPA) produces a similar curvilinear shadow just below the aortic arch before it branches. Just below the LPA, the left main/left upper lobe bronchus can be seen (projected end-on) as a round lucency. The right pulmonary artery (RPA) is seen en face down its lumen as an oval soft-tissue structure anterior to the bronchus intermedius and below and anterior to the left pulmonary artery.