This case-insensitive code system http://hl7.org/fhir/us/breast-radiology/CodeSystem/DensityAbnormalityTypeCS defines the following codes:

Code	Display	Definition
Density	Density	The density types are referring to the amount of glandular tissue and fibrous connective tissue versus levels of fatty tissue.
DensityFocalAsymmetry	Density focal asymmetry	The density is focal asymmetry on two mammographic views.
DensityNodular	Density nodular	The nodule found on Mammogram is dense.
DensityTubular	Density tubular	The milk ducts have formed a tubular appearing dense structure.