How should the image receptor (IR) be positioned relative to the patient for an anteroposterior (AP) projection of the humerus?

For an anteroposterior (AP) projection of the humerus, positioning the image receptor (IR) correctly is crucial to obtaining a high-quality radiographic image that clearly demonstrates the entire humerus from shoulder to elbow. The correct positioning involves placing the top of the IR 1.5 inches (4 cm) above the shoulder level.

This positioning ensures that the full length of the humerus is captured on the radiograph, including both the proximal and distal portions of the bone. The top of the IR is elevated above the shoulder so that the shoulder joint is fully included in the imaging field, allowing for proper visualization of both the humeral head and the greater tubercle, which are necessary for accurate evaluation.

Furthermore, the alignment allows for the necessary collimation of the x-ray beam, helping to minimize radiation exposure to surrounding tissues and ensuring that the appropriate anatomical landmarks are well-defined on the resulting image.

The other positioning options do not accommodate the need for complete visualization of the humerus. For instance, positioning the bottom of the IR at the elbow joint would not effectively include the shoulder joint in the image. Similarly, aligning the center of the IR with the elbow does not account for the upper extremity's anatomy and