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.
Chest x-ray (A) and CT scan (B) demonstrating a right lower-lobe mass. The mass is not well appreciated on the plain film because of the hilar structures and known calcified adenopathy. CT is superior to plain radiography for the detection of abnormal mediastinal densities and the distinction of masses from adjacent vascular structures.
CT is particularly valuable in assessing hilar and mediastinal disease (which is often poorly characterized by plain radiography), in identifying and characterizing disease adjacent to the chest wall or spine (including pleural disease), and in identifying areas of fat density or calcification in pulmonary nodules (Fig. 253-2). Its utility in the assessment of mediastinal disease has made CT an important tool in the staging of lung cancer (Chap. 89), as an assessment of tumor involvement of mediastinal lymph nodes is critical to proper staging. With the additional use of contrast material, CT also makes it possible to distinguish vascular from nonvascular structures, which is particularly important in distinguishing lymph nodes and masses from vascular structures primarily in the mediastinum, and vascular disorders such as pulmonary embolism.
In high-resolution CT (HRCT), the thickness of individual cross-sectional images is ∼1–2 mm, rather than the usual 7–10 mm in conventional CT. The visible detail on HRCT scans allows better recognition of subtle parenchymal and airway disease, thickened interlobular septa, ground-glass opacification, small nodules, and the abnormally thickened or dilated airways seen in bronchiectasis. Using HRCT, characteristic patterns are recognized for many interstitial lung diseases such as lymphangitic carcinoma, idiopathic pulmonary fibrosis, sarcoidosis, and eosinophilic granuloma. However, there is debate about, the settings in which the presence of a characteristic pattern on HRCT eliminates the need for obtaining lung tissue to make a diagnosis.
Recent advances in computer processing have allowed the development of helical CT scanning. Helical CT technology results in faster scans with improved contrast enhancement and thinner collimation. The image is obtained during a single breath-holding maneuver that allows less motion artifact. In addition, helical CT scanning allows the collection of continuous data over a larger volume of lung than is possible with conventional CT. Data from the imaging procedure can be reconstructed as images in planes other than the traditional cross-sectional (axial) view, including coronal, or sagittal planes (Fig. 253-3A). Finally, sophisticated volumetric “3D” representations of structures can be produced (Fig. 253-3B) including the ability to perform a virtual bronchoscopy, mimicking direct visualization through a bronchoscope (Fig. 253-4).
Spiral CT with reconstruction of images in planes other than axial view. Spiral CT in a lung transplant patient with a dehiscence and subsequent aneurysm of the anastomosis. CT images were reconstructed in the sagittal view (A) and using digital ...
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