Why does the thermal index have a letter after it?
The thermal index has three variants depending on the tissue types that will be encountered: TIs for soft tissue, TIb for bone, and TIc for the cranium. The thermal index is the ratio of the power being produced by the machine to the power needed to raise tissue temperature by 1° centigrade. The mechanical index can be thought of as the likelihood of causing cavitation. Use of color flow Doppler and spectral Doppler US increases the output power. For documentation of embryonic and fetal cardiac activity, M-mode may be used because it does not require output power greater than regular gray scale B-mode imaging.