A positive contrast agent is defined as one that:

A positive contrast agent is characterized by its ability to absorb x-ray photons. This property results from the high atomic number of the elements typically used in these agents, which absorb x-rays more effectively than surrounding tissues. As a result, when a positive contrast agent is introduced into the body during imaging procedures, it appears white or light on the radiograph, highlighting the areas where it is present. This increased radiopacity allows for better visualization of structures, aiding in the diagnosis of various medical conditions.

In contrast, negative contrast agents are those that do not absorb x-ray photons effectively; instead, they produce a darker area on the radiograph due to their lower density. Elements with low atomic numbers and compositions primarily consisting of water would also lead to lesser absorption and, subsequently, darker appearances on the radiograph, making them unsuitable for positive contrast applications. Thus, the defining characteristic of a positive contrast agent is its ability to absorb x-ray photons, enhancing the visibility of anatomical structures during imaging.