What is the central ray entrance point for a PA projection of the hand?

The central ray entrance point for a PA (posteroanterior) projection of the hand is typically at the third metacarpophalangeal (MCP) joint. This point is chosen because it allows for the best visualization of the hand's anatomy, particularly the metacarpals and phalanges, while minimizing distortion and providing uniform exposure of the structures being imaged.

Positioning the central ray at the third MCP joint centers the hand in the image field and ensures that the bones of interest, as well as the soft tissues, are adequately captured. The hand should be properly positioned so that it is flat and extended, with the fingers slightly spread, which further assists in achieving a clear image.

Choosing the third MCP joint instead of the first, second, or fourth joint ensures that the critical mid-intersection of the hand is appropriately highlighted in the resulting radiograph. This central ray positioning is a standard practice in radiography to ensure clarity and consistency in imaging the hand.