Final answer:
To achieve a proper AP projection of the elbow, the coronal plane through the humeral epicondyles must be parallel to the image receptor (IR), ensuring accurate representation of the joint space and proper profiling of the medial and lateral epicondyles.
Step-by-step explanation:
The relationship between the coronal plane through the humeral epicondyles and the image receptor (IR) to achieve a proper anteroposterior (AP) projection of the elbow is that they should be parallel. When positioning for an AP elbow x-ray, it is crucial that the coronal plane of the humeral epicondyles is parallel to the plane of the IR. This ensures that the joint space is accurately represented and the medial and lateral epicondyles are in profile without rotation. For a well-defined AP projection, the patient’s arm should be in the correct position with the elbow extended, if possible, and the forearm in supination. This allows for the humeral epicondyles to be aligned with the IR, minimizing any potential distortion of the elbow structures.