Final answer:
Decreasing the kVp by 15 results in increased image contrast due to greater absorption of lower energy X-rays by the body, and a darker image since fewer photons reach the receptor.
Step-by-step explanation:
When kilovoltage peak (kVp) is decreased by 15, there are a few changes that occur to the resulting X-ray image. The kVp setting on an X-ray machine controls the quality and energy level of the X-ray beam. When kVp is decreased, the X-rays have less energy, and more of them are absorbed by the body rather than passing through to the image receptor. This results in increased contrast because there is a greater difference between the darks and lights representing different tissues. However, the overall image may also appear darker because fewer photons reach the image receptor. Therefore, the correct answer is that the image contrast increases while the image brightness decreases.