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Background
Chest radiography is one of the most frequently performed imaging studies yet remains one of the most difficult to interpret, largely due to the superimposition of numerous structures of variable radiographic density. Mediastinal evaluation is particularly challenging due to the uniform radiographic density of its structures. Contact between the water density mediastinum and the air density lung produces visible lines, stripes, and interfaces. Knowledge of anatomy and of the radiographic appearance of these normal landmarks is crucial for accurate interpretation and identification of often subtle but important abnormalities. Correlation of these radiographic landmarks with corresponding cross-sectional imaging studies enhances understanding of normal imaging anatomy allowing for confident radiographic interpretation.

Educational Goals / Teaching Points
This exhibit will allow the learner to systematically review the mediastinal lines, stripes, and interfaces that characterize the normal frontal chest radiograph. Correlation with corresponding cross-sectional imaging will highlight characteristic normal radiographic anatomy. Normal findings will be compared and contrasted with a series of mediastinal abnormalities to emphasize the appearance of mediastinal abnormalities. The educational content will be particularly useful to radiologists and physicians in training who desire proficiency in radiographic interpretation, but will also serve as a review for radiologists in practice who routinely interpret chest radiography.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Lines are typically less than 1 mm thick and result from contact between the bilateral lungs anterior or posterior to the mediastinum to form the anterior and posterior junction lines, respectively. Stripes are thicker than lines, 2 - 5 mm thick, and result from contact between the lung and a specific mediastinal structure and intervening mediastinal fat. Examples include the right and left paratracheal stripes and the right and left paravertebral stripes. Mediastinal interfaces result from the contact between the air density lung and specific water density mediastinal structures. Examples include the cardiac borders, the pulmonary trunk, the left paraaortic interface, and the azygoesophageal recess. Recognition of alterations in the thickness and contours of these mediastinal landmarks allow the radiologist to confidently report radiographic abnormalities, formulate a focused differential diagnosis or suggest a specific diagnosis, and in some cases recommend cross-sectional imaging for further evaluation.

Conclusion
Understanding normal radiographic imaging anatomy is a vital skill for chest radiographic interpretation. Knowledge of the normal appearance of mediastinal lines, stripes, and interfaces allows recognition of often subtle but important thoracic disease processes and the formulation of an appropriate differential diagnosis.