When does the "heel effect" occur in radiography?

The "heel effect" occurs due to variation in x-ray exposure across the beam and is predominantly influenced by the geometry of the x-ray tube, specifically its design and the positioning of the anode and cathode. In radiography, this effect results in a difference in radiation intensity between the cathode end and the anode end of the x-ray beam. Typically, the exposure is greater on the cathode side, which is where the x-ray beam has a higher intensity, while the anode side has less intensity.

This phenomenon can significantly affect image quality, particularly in cases where a uniform exposure is necessary, such as imaging larger anatomical structures. Knowledge of the heel effect allows radiographers to position the denser parts of the anatomy towards the cathode side to optimize image quality and ensure that the most diagnostic information is captured.

The other options provided relate to factors that influence x-ray production and image quality but do not correctly describe the heel effect itself. High kVp settings, tube angling, and short source-to-image distances affect overall exposure and image characteristics but do not represent the principle of the heel effect, which specifically addresses how exposure varies across the x-ray beam's profile.