What does the TI on the sidebar of the ultrasound display stand for?

Thermal Index (TI) is a biosafety metric used to describe the potential of the ultrasound beam to raise temperature in the path of the beam. It is the ratio of the power used by the machine to the power required to raise tissue temperature by one degree Celsius.  It does not reflect an actual temperature change, and does not correlate with absolute numbers. A TI of 2 is double the output power but does NOT mean a 2-degree Celcius temperature rise.

 

How much temperature rise is acceptable? According to the AIUM:

For exposure durations up to 50 hours, there have been no significant, adverse biological effects observed due to temperature increases less than or equal to 2°C above normal.

The British Medical Ultrasound Society has great guidelines for the safe use of diagnostic ultrasound equipment which include this graphic: