Final answer:
For the PA oblique projection (scapular Y) of the shoulder, the central ray should be directed perpendicular to the Image Receptor (IR). This ensures a clear and accurate representation of the scapula and its articulation with the humerus.
Step-by-step explanation:
The proper direction for the central ray in the PA oblique projection (scapular Y) of the shoulder is d. Perpendicular to the IR.
This perpendicular alignment ensures that the anatomy of interest which includes the scapula and its relationship to the humerus is accurately represented in the image. It's important for the scapula to be correctly positioned in an upward direction and to avoid angles unless specifically instructed for different projections or purposes.