The diagnostic modalities available for assessing the patient with suspected or known respiratory system disease include imaging studies and techniques for acquiring biologic specimens, some of which involve direct visualization of part of the respiratory system. Methods to characterize the functional changes developing as a result of disease, including pulmonary function tests and measurements of gas exchange, are discussed in Chap. 252.
Routine chest radiography, generally including both posteroanterior (PA) and lateral views, is an integral part of the diagnostic evaluation of diseases involving the pulmonary parenchyma, the pleura, and, to a lesser extent, the airways and the mediastinum (see Chaps. 251 and e34). Lateral decubitus views are often useful for determining whether pleural abnormalities represent freely flowing fluid, whereas apical lordotic views can often visualize disease at the lung apices better than the standard PA view. Portable equipment is often used for acutely ill patients who either cannot be transported to a radiology suite or cannot stand for PA and lateral views. Portable films are more difficult to interpret owing to several limitations: (1) the single antero posterior (AP) projection obtained; (2) variability in over- and underexposure of film; (3) a shorter focal spot-film distance leading to lack of edge sharpness, and loss of fine detail; and (4) magnification of the cardiac silhouette and other anterior structures by the AP projection. Common radiographic patterns and their clinical correlates are reviewed in Chap. e34.
Advances in computer technology and the availability of reusable radiation detectors have allowed the development of digital or computed radiography. The images obtained in this format can be subjected to significant postprocessing analysis to improve diagnostic information. In addition, the benefit of immediate availability of the images, the ability to store images electronically, and the facility of transfer within or between health care systems have led many hospital systems to convert to digital systems.
Computed tomography (CT) offers several advantages over routine chest radiography (Figs. 253-1A, B and 253-2A, B; see also Figs. 261-3, 261-4, and 268-4). First, the use of cross-sectional images allows distinction between densities that would be superimposed on plain radiographs. Second, CT is far better than routine radiographic studies at characterizing tissue density, distinguishing subtle density differences between adjacent structures, and providing accurate size assessment of lesions.
Chest x-ray (A) and CT scan (B) from a patient with emphysema. The extent and distribution of emphysema are not well appreciated on plain film but clearly evident on CT scan obtained.