☢ Conventional Radiographs
The most common imaging test for evaluating the heart and great vessels is the chest radiograph, which consists of an upright posterior-to-anterior (PA) and left lateral (LAT) projections. 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 exposure 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 hemidiaphragms through the aerated lungs (Figure 1). When a chest radiograph is taken in the expiratory phase of respiration, 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 inspiration (Figure 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 complications 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 factors, 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 examination 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 relative to its appearance on the upright PA radiograph, because of magnification (Figure 2). 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 obtained, and the date and time of the examination should be recorded in all cases. Changes in patient positioning and ventilator 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 upright, 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 information is also used to decide if other imaging tests are appropriate 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 patient, 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 information may be ascertained by reviewing other prior images 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 diameter 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 4). A measurement over 50% is generally 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). The size of the patient and the degree of lung expansion 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 radiologists do not perform this measurement and rely on experience and “gestalt” to evaluate heart size.
Lange – Basic Radiology