Imaging appearances of types of pneumonia
What are the common causes and imaging appearances of: Bacterial pneumonia? Viral pneumonias? Fungal pneumonias? Mycobacterial pneumonias? Pneumocystis pneumonia?
Bacterial pneumonias such as those caused by Streptococcus pneumoniae usually present with a pattern of bronchiolitis, bronchopneumonia, or lobar pneumonia. Thus, one will see varying degrees of tree-in-bud opacities, ground glass opacity, and pulmonary consolidation. Bronchopneumonia tends to be multifocal, patchy, and peribronchial, reflecting endobronchial spread of infection. Lobar pneumonia is largely confined to one lobe of the lung and tends to start in the lung periphery with consolidation and associated air bronchograms. It should be noted that some viral, fungal, and mycobacterial pneumonias can appear identical on imaging to bacterial pneumonias.
Many individuals with viral infection and lower respiratory tract symptoms (cough, dyspnea, wheezing) will have normal imaging exams. When abnormal, imaging will most often appear as a bronchiolitis (tree-in-bud opacities on CT or subtle perivascular interstitial opacities on chest radiographs), a bronchopneumonia (tree-in-bud opacities and patchy regions of consolidation on CT or multifocal areas of consolidation on chest radiographs), or as multifocal pneumonia (multifocal areas of consolidation on CT and chest radiographs).
Fungal pneumonias, with the exception of PCP, typically result in mass-forming areas of consolidation with or without cavitation. Miliary nodules (i.e., profuse ≤3 mm discrete uniformly sized nodules) may also be seen with fungal pneumonias.
Mycobacterial pneumonias, in particular those caused by Mycobacterium tuberculosis (TB), classically result in upper lobe predominant nodules, bronchiolitis, and cavitary lung disease. Nontuberculous mycobacterial infections in the lung will most often appear as scattered regions of bronchiectasis, bronchial wall thickening, and tree-in-bud opacities on CT and as irregular linear and nodular opacities on chest radiographs.
PCP characteristically appears as uniform diffuse ground glass opacity on CT and may appear as a very faint haze on chest radiographs. Some opportunistic viral infections, including those caused by cytomegalovirus (CMV) and herpes simplex virus (HSV), can also appear as diffuse ground glass opacities.
It should be noted that imaging features of pneumonia demonstrate significant overlap, and oftentimes one cannot definitively determine the category of causative pathogen based on imaging features alone.