Final answer:
In a PA projection, the posterior (dorsal) surface of the hand should be in contact with the IR. The wrist flexors originating from the medial epicondyle of the humerus facilitate this positioning.
Step-by-step explanation:
When performing a PA (posteroanterior) projection of the hand, the surface of the hand that should be in contact with the IR (image receptor) is the posterior (dorsal) surface. This means that the back of the hand is placed against the IR. During this position, the hand is moved from the palm backward position to the palm forward position, ensuring that the dorsal aspect of the hand lies flat against the IR for proper imaging. It is crucial to correctly position the hand so that the bones and soft tissues can be properly visualized without distortion.
The muscles involved in positioning the hand in this manner are, for example, the wrist flexors which originate from the medial epicondyle of the humerus, as opposed to the lateral epicondyle. These muscles allow the palm of the hand to be pointed towards the body while flexing, facilitating the PA projection required.