The Thermal Index (TI) is a dimensionless number displayed on ultrasound machines that estimates the ratio of acoustic power to the power required to raise tissue temperature by 1°C under specific conditions. Three variants exist: TIS (soft tissue), TIB (bone at focus), and TIC (cranial bone). TI values below 1.0 are considered safe for extended scanning; higher values warrant shorter exposure times. TI is highest in pulsed Doppler modes and lowest in B-Mode imaging. Monitoring TI is particularly important in first trimester and fetal Doppler examinations.