What abnormality is depicted in this CT image (Figure 4)?

Absent pericardium

Findings: Contrast-enhanced CT of the chest reveals lung tissue interposed between the aorta and the main pulmonary artery. Because this area is intrapericardial (superior pericardial recess) this finding can only be seen in the absence of the pericardium.A: Pulmonary agenesis refers to the absence of the lung and its bronchial and vascular supply. In the setting of pulmonary agenesis there is resultant shift of the mediastinum to the affected side. The right pulmonary artery is not visualized on this image because it is more superiorly located compared to the left pulmonary artery. The other findings of pulmonary agenesis are not present.B: Pulmonic stenosis results in dilatation of the left pulmonary artery which is thought to be secondary to the effect of the jet of blood coursing through the stenotic pulmonary valve. The right pulmonary artery is typically spared since it arises at an angle from the main pulmonary artery whereas the left pulmonary artery continues in line with the main pulmonary artery. The main pulmonary artery and left pulmonary artery are normal in caliber in this case.C: In patients with congenital absence of the pericardium there are indirect cross-sectional imaging findings that can lead to the diagnosis. If the sternopericardial ligaments are absent the heart is mobile and falls towards the left hemithorax. Lung tissue becomes interposed between the aorta and the main pulmonary artery as in this example replacing the superior aortic recess of the pericardial sac. If the defect is small the left atrial appendage may herniate through the defect causing it to extend beyond the mediastinal margin.D: The apparent filling defect in the superior vena cava is secondary to the mixing of opacified and unopacified blood. This is a common artifact and should not be mistaken for thrombus.