Which index relates to the heating of tissue?

• A. Thermal Index
• B. Mechanical Index
• C. Pulse Repetition Frequency
• D. Acoustic Impedance

Answer: (A)

The heating of tissue is assessed by the Thermal Index, which estimates how much the ultrasound exposure could raise tissue temperature. It converts the acoustic power and exposure duration, along with the scan’s duty cycle, into an approximate temperature rise for a standard tissue model. This index provides a practical gauge of safety during an exam—higher values suggest greater potential for heating and therefore a reason to minimize exposure. In contrast, the Mechanical Index relates to nonthermal bioeffects, such as cavitation risk from acoustic pressure, and does not measure heating. Pulse Repetition Frequency affects how often ultrasound pulses are emitted and influences overall energy delivery, but it isn’t the specific indicator of tissue heating. Acoustic Impedance describes how readily sound waves propagate through and reflect at tissue interfaces, guiding reflections and refractions rather than indicating thermal risk.