Final answer:
To achieve low contrast in an x-ray image, higher kVp values are used which allows more x-ray photons to penetrate the material, resulting in a radiographic image with a greater range of grays.
Step-by-step explanation:
When the question asks What kVp would be used to achieve low contrast it is referring to the kilovolt peak (kVp) settings on an x-ray machine to adjust image contrast. In radiography low contrast is achieved with higher kVp values. Higher kVp settings increase the energy of the x-ray photons, allowing more to penetrate through the material and reach the detector.
This results in a greater range of grays in the radiographic image which is synonymous with low contrast. For a simple example such as a chest x-ray a lower kVp around 50-60 kV might be used for higher contrast but for low contrast imaging which would be useful for distinguishing between tissues with similar densities (e.g., in the abdomen) a higher kVp, perhaps around 80-100 kV, could be used.\