Which component of the x-ray tubehead shapes the x-ray beam?

The component of the x-ray tubehead that shapes the x-ray beam is the collimator. The collimator is a device that narrows the x-ray beam to reduce patient exposure and enhance the quality of the diagnostic images. By limiting the size and shape of the beam, the collimator helps focus the radiation on the area of interest, thus preventing unnecessary radiation exposure to surrounding tissues.

The metal housing serves primarily to protect users and patients from stray radiation but does not actively shape the beam. Aluminum disks are used to filter out non-penetrating wavelengths from the x-ray beam, but they also do not contribute to shaping it. The position-indicating device (PID) functions to direct the x-ray beam towards the receptor but does not alter the beam's shape itself. Therefore, the collimator is the correct choice for controlling the shape of the x-ray beam.