Thoracic Radiology
Core concepts
Master the silhouette sign and the air bronchogram: loss of a normal border localizes opacity to the adjacent lobe (right heart border = right middle lobe; left heart border = lingula; diaphragm = lower lobe). Know lobar and segmental anatomy, the fissures, and how collapse produces volume loss with fissure displacement and mediastinal shift toward the opacity. Understand the HRCT patterns of diffuse lung disease: usual interstitial pneumonia (peripheral, basal, honeycombing), nonspecific interstitial pneumonia, organizing pneumonia (perilobular/peribronchovascular), and hypersensitivity pneumonitis (centrilobular nodules, air trapping). Recognize the classic appearances of pulmonary embolism (Westermark sign, Hampton hump), pneumothorax (visceral pleural line, absent lung markings), and tension physiology (mediastinal shift away).
Traps
Questions love the false localizer: a lower-lobe opacity does not erase the heart border, whereas a middle-lobe or lingular opacity does. Do not confuse cardiogenic edema (smooth septal thickening, perihilar distribution, pleural effusions) with ARDS (patchy, peripheral, no cardiomegaly). A solitary pulmonary nodule question tests size, margin, and calcification pattern (popcorn = hamartoma; diffuse = benign; eccentric = suspicious). Cavitation with an air-fluid level suggests abscess; thin-walled cysts suggest bullae or pneumatocele.
Rule of thumb
When an opacity obscures a border, assign it to the lobe touching that border; when a fissure is bowed toward the opacity, think volume loss (collapse), and when bowed away, think volume expansion (effusion or mass).
Worked example
Right middle lobe collapse: opacity silhouettes the right heart border, the minor fissure moves inferiorly, and there is no mediastinal shift because the lobe is small.