To perform an AP projection of the chest in the lateral decubitus position, which of the following orientations of CR and IR is necessary?

For an AP projection of the chest in the lateral decubitus position, the central ray (CR) must be horizontal to ensure that it aligns appropriately with the anatomical structures in the chest, particularly for visualizing pleural effusions or air-fluid levels. This horizontal CR is important because it allows for the demonstration of any fluid that may be present in the chest cavity to pool at the most dependent part due to gravity while the patient lies on their side.

The image receptor (IR) should be vertical in this situation. A vertical IR helps capture the full height of the thoracic cavity and ensures that the anatomical features are projected appropriately onto the receptor.

Thus, when the CR is positioned horizontally and the IR is vertical, it facilitates optimal visualization for diagnosis, allowing practitioners to effectively assess conditions affecting the chest cavity. This configuration is crucial for correctly obtaining the lateral decubitus view of the chest.