Final answer:
The humeral epicondyles should be parallel to the image receptor in the AP shoulder projection with external rotation, allowing clear visualization of the shoulder's anatomy.
Step-by-step explanation:
The relationship of the humeral epicondyles with the image receptor (IR) during an AP projection of the shoulder in external rotation involves positioning to ensure that the humerus is properly aligned with the IR. This allows for accurate visualization of the shoulder anatomy, including the humeral head, rotator cuff, and the attached musculature.
For an AP projection in external rotation, the humeral epicondyles should be parallel with the imaging plane. This means that if you were looking at a cross-section of the humerus, the medial and lateral epicondyles would form a straight line that is perpendicular to the central ray of the X-ray beam. Precise positioning is integral for orthopedic assessments and diagnoses, as it ensures that the epicondyles are visible without overlapping or distortion.