American Registry of Radiologic Technologists (ARRT) Practice Exam

What effect does a low kVp have on X-ray image contrast?

1. Increases contrast

2. Decreases contrast

3. No effect on contrast

4. Increases patient exposure

The correct answer is: Increases contrast

A low kilovolt peak (kVp) setting in X-ray imaging increases image contrast. This is because lower kVp produces a narrow range of X-ray energies, which results in greater differences in the attenuation of X-rays by various tissues. Tissues with different densities absorb X-rays differently; lower energy X-rays are more likely to be absorbed by denser materials (like bone) and less likely to penetrate through them. This creates a sharper distinction between the various shades of gray on the image, enhancing the visibility of anatomical details. Additionally, the lower photon energies being used are less effective at penetrating certain materials, which can further contribute to the differential absorption and therefore the contrast. Consequently, structures that have significantly different densities appear more distinct from one another, contributing to a high-contrast image. This effect is particularly useful in diagnostic imaging when clear differentiation of tissues is necessary for accurate diagnosis. For higher kVp settings, the opposite would occur—there would be decreased contrast due to the increased penetration and resulting more uniform exposure of all tissues, which diminishes the differences in shades on the image.