Lowering kVp tends to have which effect on image contrast?

• A. Lowering kVp decreases image contrast
• B. Lowering kVp has no effect on image contrast
• C. Lowering kVp increases image contrast
• D. Lowering kVp first increases then decreases contrast

Answer: (C)

Lowering kVp increases image contrast because it lowers the average photon energy, making tissue differences in attenuation more pronounced. At lower energies, the photoelectric effect dominates and is highly sensitive to tissue density and atomic number, so bone absorbs much more than soft tissue. This stronger differential absorption translates to a crisper separation of structures on the image—more blacks and whites and thus higher contrast. If you raise kVp, photons are more penetrating and scatter increases, which smooths out these differences and yields a flatter, more gray image with less contrast. Note that lowering kVp often requires more exposure (higher mA or longer time) to maintain brightness, affecting patient dose.