Final answer:
In a PA chest X-ray, the central ray should be directed to the midsagittal plane and 7-9 cm below the vertebral prominens for optimal imaging.
Step-by-step explanation:
For a PA (posteroanterior) chest X-ray, the central ray (CR) should be directed to the level of T7, which is approximately at the midsagittal plane and 7-9 cm below the vertebral prominens. This ensures that the heart and lungs are well-centered in the image. The vertebral prominens is the most prominent bony spike at the base of the neck, which can easily be palpated. Positioning the CR at this level allows for optimal visualization of the thoracic structures within the chest cavity.