The student will be able to describe basic chest radiographic patterns and identify normal contours created by the boundary of intrathoracic structures with aerated lung tissue.
The student will be able to recognize common abnormal radiographic patterns and be able to localize lung pathology and gather clinical information from the radiographs.
The student will be able to identify the basic anatomical unit of lung structure and how different diseases variably affect these structures on chest CT scans.
The student will be able to apply knowledge of chest x-rays and CT scans to assist in the diagnostic interpretation of clinical cases.
Imaging the chest with radiographs (x-rays) and computed tomography (CT) is an integral part of the diagnostic work-up of patients with symptoms of respiratory disease. The natural contrast of aerated pulmonary tissue provides a window into the body allowing for x-ray and CT evaluation of diseases involving the lungs, tracheobronchial tree, and pleura, as well as the thoracic lymph nodes, thoracic skeleton and chest wall, heart, esophagus, and upper abdomen. For chest x-rays in particular, it is important to understand fundamental aspects of image acquisition and the technical factors that can limit the utility of the resulting studies and interpretation of their findings. Simplistically, radiography uses ionizing x-ray radiation to generate a film image. Interactions of the x-rays with the targeted matter and the subsequent fate of x-ray photons collectively determine the generation of an image, whose characteristics will vary according to whether the radiation is completely absorbed, transmitted unchanged through the patient, or scattered within the body (Fig. 15.1).
Schematic of the relative visual densities of biological structures visualized by standard x-ray imaging intensities.
Normal anatomical features of the respiratory system, as well as pathologies of the lungs and cardiothoracic system are visualized by the interplay among seven different radiographic densities (Fig 15.1): air, fat, soft tissue (eg, muscle vs blood or fluid), calcium, x-ray contrast media, and metal (≅ dense bone). Discrimination among these features is accomplished because of differential absorption of radiation by various normal or diseased tissues that ultimately results in the creation of radiographic images. Of note, an intra-thoracic structure is best rendered visible by the juxtaposition of two different radiographic densities. Moreover, it is necessary for the x-ray beam to tangentially strike the interface between tissues of different density in order to appear as a well-defined boundary line on chest radiographs. Because diseases of the respiratory system often result in differential x-ray absorption, both the absence of a normal radiographic interface and the presence of an unexpected interface are valuable clues to underlying pathological changes.
By convention, the routine frontal chest x-ray view is taken in the radiological suite with the patient upright and during full inspiration. In this view the x-ray beam is horizontal to the patient, and the x-ray tube is ~2 m from the film detector as the beam traverses the patient from a posterior to anterior (PA) direction or view. This distance used in acquiring the PA view reduces unnecessary magnification and enhances sharpness as does the close apposition of the chest and the film. In contrast, it is frequently necessary in hospitalized and critically ill patients to obtain portable chest x-ray images at the bedside, which are performed using an anteroposterior (AP) direction or view. Such AP films involve less powerful radiation energy and a shorter x-ray tube-to-film distance, resulting in greater magnification but reduced anatomical resolution.
Using only the PA view, it is often difficult to detect chest lesions located behind the heart, near the mediastinum, or near the hemidiaphragms because these are radiographically denser structures with poor interfaces. For this reason, the lateral chest x-ray view is also performed to pinpoint such lesions three-dimensionally and localize them within the lungs. The lateral view is routinely taken with the left side of the patient against the film cassette.