Which parameter primarily affects the contrast of an X-ray image?

The parameter that primarily affects the contrast of an X-ray image is kVp, or kilovoltage peak. kVp controls the energy of the X-ray photons produced, which in turn influences the penetration ability of those photons through the body tissues. A higher kVp results in increased photon energy, allowing more X-rays to penetrate denser tissues, thereby reducing the difference in density between the various tissues in the image. This results in lower contrast, as similar tissues may appear more alike in density. Conversely, a lower kVp increases contrast because it produces a greater difference in X-ray absorption between different types of tissues, allowing for clearer differentiation of structures within the image.

While exposure time, mAs (milliamperes-seconds), and SID (source-to-image distance) all play important roles in image formation, they primarily affect image brightness and overall exposure rather than the contrast specifically. For instance, mAs primarily influences the quantity of X-ray photons produced, which can affect image density, while exposure time relates to how long the X-rays are allowed to expose the image receptor. SID affects the intensity of the X-ray beam reaching the image receptor, impacting the overall exposure and sharpness of the image but not directly altering the contrast as kVp does