A therapist is examining an AP chest radiograph of a neonate and notices a structure projecting away from the mediastinum toward the right upper lung. This structure looks like a sail with a sharp inferior margin and lateral margins with wavy contours. Which of the following structures is the therapist observing?

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The mediastinum is composed of the heart, aorta, main pulmonary artery and proximal branches, origins of the great vessels from the aorta, the superior vena cava, and thymus. Thymic tissue is usually prominent in the neonate and becomes less apparent with age because of regression of the thymus and growth of surrounding structures. Because it is an anterior mediastinal structure, the thymus in the small child fills the anterior clear space normally seen on the lateral projection of a teenager or adult. On the AP or PA projection it may only cause widening of the superior mediastinum. When the thymus projects away from the mediastinum, typically into the right upper lung, it appears as a "sail" with a sharp inferior margin. The lateral margins often have a characteristic wavy contour (see Figure 6-4 in the textbook). Unlike a pathologic mass such as lymphoma, the normal thymus does not exert mass effect on the trachea.

A Therapist Is Examining an AP Chest Radiograph of a Neonate