Heating of soft tissue is proportional to which index?

• A. Thermal Index
• B. Mechanical Index
• C. Attenuation
• D. Duty Cycle

Answer: (A)

Heating of tissue from ultrasound depends on the energy delivered and how that energy is absorbed by the tissue. The Thermal Index is the parameter that ultrasound systems use to estimate how much the tissue temperature might rise at a given depth, based on the output power, frequency, and tissue attenuation. As the Thermal Index increases, the predicted temperature rise also increases, making it the direct indicator of heating potential. Mechanical Index relates to nonthermal bioeffects like cavitation, not heating. Attenuation describes energy loss as sound travels through tissue and indirectly affects heating, but it isn’t an index that predicts temperature rise. Duty cycle affects the total energy delivered over time but isn’t the metric used to gauge heating potential. Therefore, heating of soft tissue is proportional to the Thermal Index.