What describes a mass or structure in sonography that has internal echoes and no posterior enhancement?

A mass or structure that has internal echoes and does not demonstrate posterior enhancement is best described as solid. In sonography, solid masses are characterized by their echogenicity, which is the ability to produce echoes within the mass itself. These echoes arise from the tissue composition, leading to varying degrees of brightness on the ultrasound image, thus confirming the solid nature of the mass.

Unlike cystic structures, which are filled with fluid and typically produce posterior enhancement due to the fluid's acoustic properties, solid masses do not show this enhancement. The lack of posterior enhancement indicates that the mass has a denser composition, which absorbs and scatters sound waves rather than allowing them to pass through and create a brighter appearance behind the structure.

In the case of complex masses, these can contain both solid and cystic components, and the presence of both types can result in varied echo patterns. Hypoechoic structures would typically indicate less echogenicity compared to surrounding tissues, but the defining feature in this scenario is the presence of internal echoes that signify a solid composition without posterior enhancement.