History
    A 51-year-old man has suffered exertional dyspnea for 15 days.

Findings
    Chest radiograph shows diffuse ground glass opacities and patchy consolidations in entire lung fields. Some reticular densities are overlapped. HRCT scans show ground glass opacities and consolidations with air-bronchogram in both lungs, predominantly involving the peripheral lung zones. Interlobular septal thickenings are associated, showing the crazy paving appearance.

Discussion
    High-resolution CT findings of ground glass opacity with superimposed interlobular and intralobular septal thickening termed “crazy paving appearance” is thought to be characteristic of pulmonary alveolar proteinosis. However, this appearance has been encountered in other entities; pneumocystis carinii pneumonia, alveolar proteinosis, malignant lymphoma, leukemia, acute radiation pneumonitis, lymphangitic carcinomatosis, obstructive pneumonitis due to primary cancer, interstitial pneumonia associated with collagen disease and mucinous bronchioloalveolar carcinoma.
Tan and Kuzo reported a case of mucinous bronchioloalveolar carcinoma with crazy paving appearance. They explained the crazy paving appearance in bronchioloalveolar carcinoma as follows; the alveoli are filled with a low-density material (glycoprotein) and the alveolar septae and underlying parenchymal architecture typically remain normal. The low-density intraalveolar material likely accounts for the background ground-glass opacity. The superimposed network of linear opacities is thought to represent septal edema with thickening of the interlobular septa and intralobular interstitium or may result from infiltration of the interstitium by inflammatory cells. Whenever the parenchymal opacity is contiguous with a fissure, the adjacent fissure appears denser and thicker than normal on CT scans.

References
1. Tan RT , Kuzo RS. High-Resolution CT findings of mucinous bronchioloalveolar carcinoma: a case of pseudopulmonary alveolar proteinosis. AJR 1997; 168: 99-100
2. Myrayama S, Yabuuchi H, Murakami J, Masuda K. Crazy paving appearance on HRCT: clinical incidence and analysis. Radiology 1997; 205 (P); 677
3. Murch CR, Carr DH. Computed tomography appearances of pulmonary alveolar proteinosis. Clin Radiol 1989; 40: 240-243.