What do TI and MI indicate in ultrasound safety?

• A. TI measures tissue stiffness; MI measures tissue viscosity
• B. TI and MI measure depth penetration only
• C. TI estimates heating potential; MI estimates mechanical energy/cavitation potential; both indicate safety
• D. TI and MI are used to measure color Doppler shift

Answer: (C)

TI and MI are safety indicators used in ultrasound to gauge potential bioeffects rather than imaging performance. The Thermal Index estimates how much tissue heating could occur from the given exposure — it’s a way to predict potential temperature rise in tissue under worst‑case assumptions. The Mechanical Index estimates the potential for mechanical effects, such as cavitation, by relating the peak rarefactional pressure to the ultrasound frequency; a higher MI signals a greater risk of these mechanical phenomena. Together, they provide a safety context for scanning, rather than measuring tissue stiffness, viscosity, depth penetration, or Doppler shifts.