Final answer:
For an erect lateral sternum projection, arms should be elevated behind the head or positioned to the side, away from the thorax, to prevent obstruction and ensure a clear view of the sternum for imaging.
Step-by-step explanation:
For an erect lateral projection of the sternum, proper arm positioning is crucial to get a clear view of the sternum without obstruction from the arms or shoulders. The arms should be elevated and placed behind the patient's head, or they can be positioned to the patient's side, away from the thorax, to ensure that the clavicles are not superimposed over the sternum. It's important to remember that the sternum consists of the manubrium, body, and xiphoid process. The sternal end of the clavicle articulates with the manubrium at the sternoclavicular joint. This joint allows for considerable mobility, which is beneficial when repositioning the arms for the lateral view of the sternum.