Pulmonary alveolar proteinosis (PAP) is a rare, diffuse lung disease that is characterized by the alveolar and interstitial accumulation of a periodic acid-Schiff (PAS) stain-positive phospholipoprotein that is derived from surfactant. [1, 2, 3] The lung architecture is otherwise normal, and any associated inflammation or fibrosis is limited in extent. A radiograph and a computed tomography (CT) scan of PAP appear below.
Although CT scan (in particular, high-resolution CT [HRCT] scan) findings in PAP are often characteristic, fiberoptic bronchoscopy with bronchoalveolar BAL and transbronchial biopsy are sufficient for confirming the clinical diagnosis in most patients. Chest radiographs alone, while occasionally suggestive, are rarely diagnostic in patients with PAP. CT scanning, especially HRCT scanning, often displays findings that are characteristic of, although not pathognomonic for, PAP. [4, 5, 6, 7, 8, 9] Indeed, Johkoh et al include at least 15 different entities in the differential diagnosis for the characteristic HRCT scan findings of PAP. 
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The classic chest radiographic finding of pulmonary alveolar proteinosis (PAP) is bilateral, symmetrical air-space opacity, which appears either as ground-glass or frank consolidation, with a perihilar or basilar predominance (see the images below). Air bronchograms are uncommon.
Radiographic opacities are often vaguely nodular and may be accompanied by fine, linear opacities or reticulation. Serial radiographs may demonstrate persistence of this pattern over time, resulting in a more limited differential diagnosis of chronic air-space opacity.
Unlike in hydrostatic pulmonary edema, the mediastinum is not widened, the heart is not enlarged, and pleural effusions are not common. Adenopathy is not a feature of the disease.
Atypical manifestations include a linear or reticular pattern that is unaccompanied by air-space opacity, lower lobe predominant consolidation, or poorly defined nodules as the primary radiographic manifestation of disease.
The differential diagnosis of PAP on chest radiographs includes all entities that result in diffuse air-space disease, such as hydrostatic and noncardiogenic pulmonary edema, diffuse infections (particularly Pneumocystis pneumonia), pulmonary hemorrhage, interstitial pneumonia, the diffuse form of bronchioloalveolar carcinoma, toxic lung injuries and hypersensitivity pneumonitis, eosinophilic pneumonia, drug or radiation-induced pulmonary disease, and lipoid pneumonia.
The initial radiographs after BAL reveal increased opacity in the lobes in which the BAL fluid was instilled; this opacity is transient, clearing in hours to a few days.
After BAL, the aeration of the washed lung gradually improves. By 6 weeks after the BAL procedure, chest radiographs usually demonstrate improved aeration over preprocedure radiographs.
Rarely, in the course of resolution, new opacities may develop in either the washed lung or the contralateral lung. Occasionally, these opacities are related to endobronchial obstruction, usually representing atelectasis. Areas of decreased lung opacity, which represent hyperinflation, may also occur by the same mechanism.
Opacity in the contralateral lung may result from spillage of BAL fluid from the washed lung or atelectasis after the BAL procedure.
Complications of the BAL procedure, including pneumomediastinum and pneumothorax, may be evident on immediate postprocedure radiographs.
Degree of confidence
Radiographic findings of PAP are not specific, and as stated above, a large differential diagnosis must be considered. Similar to the case with other diffuse lung diseases, HRCT scanning provides a more specific morphologic evaluation of the disease pattern and may provide a more limited differential diagnosis.
Routine CT scans (7-10 mm collimation) may reveal bilateral areas of consolidation and reticulation in the lungs of patients with pulmonary alveolar proteinosis (PAP) (see the images below). [14, 15, 16, 4, 7, 8] The disease distribution of PAP is usually bilateral and patchy.
Occasionally, an underlying linear abnormality that represents thickened interlobular septa may be appreciated.
Crazy paving is the characteristic finding in PAP on HRCT scanning; it consists of patchy, bilateral geographic areas of ground-glass opacity, which are associated with interlobular septal thickening. The HRCT scan features of PAP are seen in the images below. [10, 5, 9]
PAP is often distributed uniformly from the lung apex to the base. Interlobular septal thickening may be encountered more frequently in the lower lung zones. Although the ground-glass opacity in PAP is usually patchy, centrilobular nodules have been described in pediatric patients with PAP, as well as in adult patients with acute silicoproteinosis. 
Although findings of the interlobular septa are commonly abnormal for pathologic specimens from patients with PAP, septa findings may occasionally be pathologically normal, despite a thickened appearance on HRCT scans. This apparent septal thickening on HRCT scans presumably reflects the aggregation of PAS-positive lipoproteinaceous material immediately adjacent to the interlobular septa.
Classically, the abnormal pulmonary parenchyma is demarcated sharply from normal lung areas without a discernible anatomic boundary. (The images below show no such sharp demarcation and, therefore, suggest a diagnosis other than PAP.)
Similar to the case with other diffuse lung diseases, HRCT scanning provides a more specific morphologic evaluation of the disease pattern and may provide a more limited differential diagnosis. HRCT scanning is also useful for targeting the optimal site for tissue sampling.
Generally, PAP in children is often more acute and severe than PAP in adults. Chest radiographic and HRCT scan findings in childhood forms of PAP are few but are likely similar to their adult counterparts.
McCook et al suggested that a miliarylike pattern is encountered more frequently in the pediatric entity than in the adult form and that basilar predominant linear and reticular abnormalities may be observed more often on CT scans in children. 
HRCT scan descriptions of childhood PAP by Zontsich et al suggest that the crazy-paving pattern may be seen in children as it is in adults. 
Pleural effusions and adenopathy are uncommon in PAP, and the clinician should suspect a superimposed infection or malignancy if these are present. Occasionally, pleural effusions may be encountered in uncomplicated cases of PAP shortly after BAL.
Degree of confidence
Although the appearance of crazy paving is highly suggestive of PAP, this finding is not pathognomonic for PAP. Many other entities may present with a pattern similar to crazy paving on HRCT scans, including the following:
Hydrostatic pulmonary edema
Diffuse alveolar damage from any number of causes
Diffuse pulmonary infections (including Mycobacterium tuberculosis, M pneumoniae, other bacterial pneumonias)
Diffuse form of bronchioloalveolar carcinoma
Acute respiratory distress syndrome (ARDS)
Bronchiolitis obliterans organizing pneumonia
Chronic eosinophilic pneumonia
Acute interstitial pneumonia
Atypical manifestations of PAP also occur. Do not expect all instances of PAP to present as crazy paving on HRCT scans.
Magnetic Resonance Imaging
Few data are available regarding the MRI appearance of pulmonary alveolar proteinosis (PAP). Because of magnetic susceptibility effects and the short echo times of lung parenchyma, lung tissue is challenging to image with MRI.
A study by Moore et al addressed the appearances of diffuse air-space disease on MRI and found that air-space opacity in PAP has a short T1 value. This opacity demonstrated relatively little increased signal intensity with T2-weighted imaging.  Presumably, the short T1 signal of PAP reflects the relative lack of water within the lipoproteinaceous material that fills the air spaces in this disease.