Thermal heating of ultrasound decreases the viscosity of

• A. Fluid elements
• B. Tissues
• C. Bones
• D. Nerves

Answer: (A)

Viscosity is a property that describes how a fluid resists flow. When temperature rises, molecular motion increases and intermolecular forces weaken, so the fluid flows more easily and its viscosity decreases. Thermal heating from ultrasound raises the temperature mainly of the fluid components within tissues, so the viscosity of those fluid elements drops.

Tissues, bones, and nerves aren’t simple fluids; bones are rigid minerals, and nerves and many tissues are complex solids with fluids embedded in them. Because viscosity in the strict sense applies to liquids, the most accurate choice is the fluid elements—the parts of tissue that behave like fluids.