Thermal index

The thermal index (TI) is intended as a measure of an ultrasound beam's thermal bioeffects. It is often displayed on ultrasound screens (along with the mechanical index).

Absorption of sound waves may cause heating in tissue.

The thermal index depends on:

• a measure of time-averaged acoustic power
• assumptions of the properties of the tissue being heated
• assumptions about the path of the ultrasound beam

If beam focusing is unchanged, then the thermal index is directly related to the output power of the system:

thermal index = W_p / W_deg

• W_p: relevant (attenuated) acoustic power at the depth of interest
• W_deg: estimated power necessary to raise the tissue equilibrium temperature by one degree Celsius

So, if the acoustic power of the ultrasound beam is enough to raise the tissue temperature by 1.0°C, then the thermal index = 1.0. It should be noted that this is a relative risk of heating, not an absolute scale of tissue heating.

The thermal index at the skin surface, and in other special circumstances, is calculated differently ².

The clinical relevance of the thermal index most often comes into play with imaging of the embryo. Color and spectral Doppler imaging calls for increased levels of ultrasound output power, and this in turn increases the thermal index. The use of color and spectral Doppler is discouraged when imaging an embryo to avoid crossing a theoretical thermal index threshold. Current guidelines recommend a thermal index <1.0 for first trimester screening (11 weeks to 13 weeks, 6 days) with exposure time to Doppler imaging for as little as possible, preferably less than 5-10 minutes.

The thermal index (and mechanical index) have been widely adopted in the radiology community as proxies for ultrasound bioeffects, but some recent discussion in ultrasound societies suggest that both indices may need revision to more accurately represent potential bioeffects ².

The thermal index may be displayed in some ultrasound systems as:

• TI_S: thermal index in soft tissue
• TI_B: thermal index for bone at the focus
• TI_C: thermal index for cranial bone

The conventional thermal index is not valid for 3D ultrasound or elastography.