The differential density sign of pericardial effusion.

Chest radiographs of 100 patients with suspected pericardial effusion who underwent echocardiography were reviewed. Fifty were echo-positive and 50 were negative. The differential density sign was present in 34 patients (68%) who were echo-positive and 2 patients (4%) who were negative; on frontal (62%) and lateral (41%) projections; and in patients with small, moderate, and large effusions. Computed tomographic findings in 1 patient correlated closely with those obtained radiographically. Experimental models suggest this sign is related to different tissue attenuation coefficients for blood and saline, different tissue thicknesses of the spherically shaped heart, the presence of epicardial fat around the heart, and chest film exposure factors. "Trapping" of the middle lobe or anterior lingula may also contribute. When pericardial effusion is not suspected, the presence of this sign should be an indication for echocardiography.