The photoelectric effect is primarily responsible for the contrast seen in radiographic images due to its unique interaction between X-rays and matter. When X-ray photons encounter dense material, such as bone, they can be completely absorbed by that material, resulting in a high degree of attenuation of the X-ray beam. This absorption is particularly pronounced when the energy of the X-ray photon is just above the binding energy of the inner-shell electrons of the tissue being imaged.
As a result, areas of high-density tissues like bone will appear lighter on a radiographic film, while less dense tissues, such as muscle or fat, will allow more X-rays to pass through, rendering them darker. This differential absorption creates the visual contrast that allows radiologists to distinguish between various structures in the body.
In comparison, while Compton scattering contributes to image formation by scattering X-rays off electrons, it does not provide the same level of contrast as the photoelectric effect. Pair production occurs at much higher photon energies and is significant mainly in radiation therapy contexts but contributes little to the contrast in standard diagnostic imaging. Elastic scattering, on the other hand, generally does not lead to significant absorption and is not a major factor in creating the contrast necessary for detailed imaging.
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