Where does the central ray (CR) enter for a PA projection of a hand?

In a PA (posteroanterior) projection of the hand, the central ray (CR) is directed perpendicular to the image receptor and enters at the third metacarpophalangeal joint (MPJ). This specific entry point is crucial as it ensures that the structures of interest—the bones and joints of the hand—are optimally visualized without distortion. The third MPJ is typically selected because it is centrally located among the metacarpals and provides a balanced representation of both the proximal and distal aspects of the hand within the image. Proper positioning and CR placement are essential for achieving the best diagnostic quality in radiographic images.

In this case, the other choices do not represent the correct anatomical landmark for the CR entry in a standard PA projection, as each corresponds to different joints that would not provide the optimal visualization of the hand as a whole.