A solid mass on ultrasound contains tissue echoes and does not meet the criteria for a simple cyst. Solid masses may be hypoechoic, isoechoic, hyperechoic, or heterogeneous depending on their composition. Doppler is used to assess vascularity. Solid masses require further characterization based on their imaging features, clinical context, and often follow-up imaging or biopsy. The differential diagnosis varies widely by organ — liver, kidney, thyroid, testis, and breast all have organ-specific classification systems for solid masses.