Final answer:
In an AP, bilateral projection of the posterior ribs below the diaphragm, the central ray is directed at the T10 vertebra level, which corresponds with the bottom edge of the scapula or about 3 to 4 inches below the xiphoid process.
Step-by-step explanation:
For an AP (anteroposterior), bilateral projection of the posterior ribs below the diaphragm, the central ray should be directed to a midpoint at the level of the T10 vertebra. This is effectively in line with the bottom edge of the scapula or approximately 3 to 4 inches below the xiphoid process. This positioning ensures that the diaphragm is at its lowest point to avoid superimposition over the lower ribs during a deep expiration.
Remember, the AP projection allows for visualization of the ribs in relation to the thoracic vertebrae, where the head of the rib articulates with the costal facets of the thoracic vertebrae, and the rib tubercle articulates with the transverse processes. For the ribs below the diaphragm (typically ribs 8-12), positioning is critical to provide clear visualization and avoid overlap with other anatomical structures.