Evaluating the Chest Radiograph
Step 8: The Lungs
Evaluation of the lung fields requires a systematic approach. Both a center-out, or periphery-in approach will work.
In general, the opacity (whiteness) of the lungs will increase from the top to the bottom of the lung fields. This is the result of both increasing thickness of the chest, as well as the interposition of breast tissue in women.
When moving from the hilum to the periphery, there will be a gradual diminution of the lung markings. Typically, clearly defined vessels and bronchial branches are noted in the proximal third of the chest. In the middle third, these vessels become much smaller and more difficult to individually evaluate. The peripheral third of the lung demonstrates a fine pattern of branching lines in a somewhat reticular pattern, without clearly defined vessels or airspace.
Lung markings should always extend to the very edge of the thoracic cavity. The presence of an area of chest without lung markings indicates the presence of a pneumothorax.
In certain circumstances, a tension pneumothorax can develop. In these cases in addition to the absence of lung markings in the periphery, there will also be a shift of the mediastinal structures away from the side of the tension pneumothorax.
The pleura and pleural space is evaluated for changes. Thickening of the pleura (increased soft tissue around the periphery of the lung) can be a sign of pleural disease either old or new. Blunting of the costophrenic angles can indicate the presence of pleural fluid. Decubitus imaging can be performed to determine if the fluid is mobile. Ultrasound sometimes can also be helpful in evaluating pleural effusion.
Evaluation of the lungs requires both an evaluation of airspace and vessels, as well as a comparison of the symmetric portions of the right and left lungs. Comparison helps us determine what the density of the lungs should be at any given point.
When air space disease is identified, analysis of the radiograph helps us determine what the problem might be. The lateral film helps us locate the area of airspace disease within upper and lower lobes.
Any process that causes filling of the alveoli will cause a fluffy 'alveolar' pattern to the lungs. This includes infection (pneumonia), fluid (pulmonary edema, congestive failure), or neoplasm.
Many disease processes can present with a 'thickening' of the interstitial components of the lungs, these processes can include, infection (atypical pneumonias), fluid (early pulmonary congestion with CHF), fibrosis, infiltrating diseases (sarcoid, amyloid), metastatic neoplasm.
The distribution of air space and interstitial disease can be described in several ways depending on the distribution. Segmental disease will follow the distribution of the individual segments. Lobar disease will involve an entire lobe. Diffuse disease may involve an entire lung or both lungs.
Another important observation when evaluating the lungs is the effect of a process on the lung. Is there a decrease in lung volume? An increase? Decreased volume can be seen when there has been blockage of the airway with a subsequent reabsorption of air from that segment(s) also known as atelectasis. Increased volume can be seen with process such as neoplasm or pneumonias that increase volume.
On occasion, a single nodule may be seen superimposing the lung fields. The solitary pulmonary nodule, described as a 1cm non-calcified nodule in the lung must raise suspicion for a neoplasm and indicates a need for additional evaluation such as CT scanning.
Many times, both infective processes and neoplasm can become necrotic in the central portions with the development of a cavity. These may be seen on the radiograph as a lucent area within a larger area of consolidation. An air fluid level might be present.
On a final note, emphysema can frequently be suggested by chest radiography evaluation. The presence of very lucent (black) lung fields with a paucity of lung markings suggests this diagnosis. This must be distinguished from a poorly exposed radiograph that causes the lung fields to be black because of over exposure.
Other things to keep in mind when evaluating the lung fields, are structures or changes outside the lungs that may change the apparent density of the lungs. These include increases in density caused by increased tissues as might be seen in breast implants, or decreased density as may be seen in the case of mastectomy.