What is defined as the visible differences between two selected areas of density levels within an image?

The correct answer is contrast, which refers to the difference in density levels between various areas of an image. In radiography, contrast is crucial because it enhances the visibility of structures and tissues, allowing for better diagnosis and interpretation of images. High contrast means there is a significant difference between light and dark areas, highlighting boundaries and details, whereas low contrast results in a more uniform appearance, making it difficult to distinguish between different structures.

Sharpness relates to the clarity of the edges in an image, indicating how well-defined the boundaries of objects are. While sharpness can contribute to the overall quality of an image, it does not directly address the differences in density between selected areas.

Exposure refers to the amount of radiation that reaches the image receptor, which affects the overall darkness of the image but does not pertain to the differences between areas of density.

Brightness is related to the overall lightness or darkness of an image after it has been processed, but it does not specifically refer to the differences in density needed for diagnosing conditions. Therefore, contrast is the most pertinent term for describing the visible differences in density levels in radiographic images.