On a lateral decubitus, amounts as small as 50ml of fluid are possible. Pleural effusions typically have a meniscus visible on an erect chest radiograph, but loculated effusions (as occur with an empyema) may have a lenticular shape (the fluid making an obtuse angle with the chest wall). Pleural thickening may cause blunting of the costophrenic angle, but is distinguished from pleural fluid by the fact that it occurs as a linear shadow ascending vertically and clinging to the ribs. The differential for diffuse shadowing is very broad and can defeat even the most experienced radiologist. It is seldom possible to reach a diagnosis on the basis of the chest radiograph alone: high-resolution CT of the chest is usually required and sometimes a lung biopsy. The following features should be noted:  Pleural effusions may occur with cancer, sarcoid, connective tissue diseases and lymphangioleiomyomatosis. The presence of a pleural effusion argues against pneumocystis pneumonia. Disease mimics are visual artifacts, normal anatomic structures or harmless variants that may simulate diseases and abnormalities. While chest radiographs are a relatively cheap and safe method of investigating diseases of the chest, there are a number of serious chest conditions that may be associated with a normal chest radiograph and other means of assessment may be necessary to make the diagnosis. For example, a patient with an acute myocardial infarction may have a completely normal chest radiograph.