Final answer:
A 72" SID is used in PA chest x-rays to minimize distortion and magnification, producing a better quality image while optimizing patient safety by reducing radiation exposure.
Step-by-step explanation:
The necessity for a 72" Source-to-Image Distance (SID) in a PA chest x-ray is to ensure that the x-ray beams are as parallel as possible, which reduces image distortion and magnification of anatomical structures. This distance is chosen to optimize the quality of the x-ray image. With high energy x-ray photons, the x-ray tube may operate at different kilovolt peaks (kVp) depending on the type of tissue and the depth of penetration required. For instance, a chest x-ray might operate at around 50.0 kV. To capture finer details and reduce the amount of radiation exposure, the 72" SID, combined with appropriate kV settings and shielding practices like those described with the lead apron in FIGURE 32.7, are parts of essential x-ray safety and quality protocols.