What term is used to refer to the mass density of a body part being imaged?

The term that refers to the mass density of a body part being imaged is tissue density. In medical imaging, particularly in modalities like X-ray and CT, tissue density is critical in differentiating between various types of tissues. It is a measure of how much mass is contained within a given volume of tissue, which influences how that tissue interacts with imaging modalities. For example, denser tissues, such as bone, will appear differently than less dense tissues, like fat or muscle, in imaging studies.

Tissue density encompasses all kinds of biological tissues, while other terms such as material density and particle density may not specifically pertain to biological contexts. Bone density, on the other hand, is a specific type of tissue density that refers solely to bone material. Thus, tissue density is the most appropriate and comprehensive term to use when discussing the mass density of body parts in medical imaging.