Final answer:
A barium enema and routine radiography are primarily used for visualizing the lower gastrointestinal tract to diagnose conditions like acid reflux disease, Crohn's disease, and ulcers.
Step-by-step explanation:
A barium enema and routine radiography should primarily be used for visualizing the lower gastrointestinal tract. These diagnostic tools enable healthcare providers to see the interior structure of the gastrointestinal organs by using barium sulfate, a contrast agent that coats the inside of these tissues. While barium sulfate is not absorbed into the body due to its insolubility, it provides a clear outline on X-ray images. This method is particularly useful for diagnosing conditions such as acid reflux disease, Crohn's disease, and ulcers.