Findings
    Initial chest radiograph shows patchy areas of ground-glass opacity and reticular density in both lungs. Bilateral pleural effusions and accentuation of right minor fissure were associated. The heart is normal in size. CT scans obtained at the same day revealed patchy areas of ground-glass attenuation associated with smooth interlobular septal thickening at right upper lobe and both lower lobes. Small amount of bilateral pleural effusions and pericardial effusion was also seen. Follow-up chest radiograph (not shown here) obtained 4 days later showed complete disappearance of previously noted parenchymal abnormalities.  Bronchoscopic lung biopsy showed diffuse infiltration of eosinophils in the alveolar walls. Bronchoalveolar lavage fluid contained many eosinophils (70%), scattered lymphocytes (10%), and macrophages (20%). Eosinophil count in peripheral blood was within normal range in the first admission day, but was elevated (13%) two weeks later.

Discussion
    Idiopathic pulmonary eosinophilia syndrome includes simple pulmonary eosinophilia (known as Loeffler syndrome), acute eosinophilic pneumonia, chronic eosinophilic pneumonia, and idiopathic hypereosinophilic syndrome [1]. Simple pulmonary eosinophilia is characterized by migratory pulmonary abnormalities on chest radiograph, areas of ground-glass opacity (halo) around consolidation or nodule on high-resolution CT, increased peripheral blood eosinophils, and minimal or no pulmonary symptoms. The pulmonary symptoms and radiologic findings resolve spontaneously within one month.
Acute eosinophilic pneumonia (AEP) [2] is characterized pathologically by infiltration of eosinophils and mononuclear cells and by edema within the alveolar space, the bronchial walls, and, to a lesser degree, the interstitial space and pleura. The disease is characterized clinically by acute febrile illness lasting 1-5 days accompanied by myalgias, pleuritic chest pain, and hypoxemic respiratory failure (PaO2<60mmHg on room air), often requiring mechanical ventilation. The common radiologic finding in patients with AEP is a subtle interstitial infiltration, mixed alveolar and interstitial infiltration involving all lobes. The CT findings are diffuse or patchy areas of ground-glass attenuation with or without consolidation, pleural effusions, and pronounced septal thickening.
    On both radiographs and CT scans, findings of AEP should be differentiated from findings of overhydration pulmonary edema, adult respiratory distress syndrome (ARDS) or acute interstitial pneumonia, and atypical bacterial or viral pneumonia.  The heart, which is usually enlarged in patients with overhydration pulmonary edema, is of normal size in patients with AEP. Radiographic and CT findings in patients with ARDS or acute interstitial pneumonia included bilateral patchy areas of parenchymal consolidation, which are usually denser than parenchymal lesions in patients with AEP. Septal thickening is rare in patients with ARDS or acute interstitial pneumonia. Atypical  or viral pneumonia may manifest findings similar to those of acute eosinophilic pneumonia, but septal thickening and pleural effusion are also rare in patients with this disease.
    In summary, AEP is characterized clinically by rapid onset of fever and hypoxemia and rapid improvement without recurrence. On chest radiographs, the disease usually shows diffuse bilateral reticular lesions that, with or without steroid therapy, rapidly resolve. On CT scans, the disease shows bilateral patchy areas of ground-glass attenuation frequently accompanied by interlobular septal thickening and pleural effusion.

References
1. Kim YK, Lee KS, Choi DC, Primack SL, Im J-G. The spectrum of eosinophilic lung disease: radiologic findings. J Comput Assist Tomogr 1997;21:920-930
2. Cheon JE, Lee KS, Jung GS, Chung MH, Cho YD. Acute eosinophilic pneumonia: radiologic and CT findings in six patients. AJR 1996;167:1195-1199