What does the term 'radiopacity' refer to in medical imaging?

Radiopacity refers specifically to the ability of a substance to block x-rays, which results in varying degrees of visibility in an x-ray image. In medical imaging, different tissues and materials have differing levels of radiopacity, leading to the visual contrast we observe in x-ray films. Denser structures like bones are radiopaque, meaning they absorb more x-ray radiation and appear white on the image, while less dense substances allow more radiation to pass through and appear darker. This property is crucial for identifying structures and diagnosing conditions in medical imaging, as it provides valuable information about the composition and density of tissues. The other options describe different phenomena unrelated to the blocking ability of materials when exposed to x-ray radiation, thus clarifying why they do not accurately define radiopacity.