(TI) The definition of the thermal index is the ratio of the total acoustic power to that required raising a maximum temperature increase of 1°C under defined assumptions. A thermal index of 1 indicates the acoustic power achieving a temperature increase of 1°C. A thermal index of 2 has the doubled power but would not necessarily indicate a peak temperature rise of 2°C. The temperature rise is dependent on tissue type and is particularly dependent on the presence of bone.
Classifications of thermal indices:
TIS - thermal index soft tissue;
TIB - thermal index bone - bone at/near the focus;
TIC - thermal index cranial bone - bone at the surface.
(TIB) The bone thermal index is an exposure model for the case that the ultrasound beam passes through soft tissue and a focal region is in the immediate vicinity of bone.
The longitudinal waves of ultrasound are reflected and transformed into transverse waves, creating a heating effect. Muscle and bone absorb more energy at interfaces with other heterogeneous tissues.
See also Sheer Wave.