Thermal Index (TI)
Definition
The Thermal Index (TI) is an on-screen safety metric displayed on ultrasound systems that estimates the potential for tissue heating due to acoustic energy absorption. It represents the ratio of the acoustic power used to the power required to raise tissue temperature by 1°C.
Tissue Heating
For the same beam, bone will heat up more than soft tissue.
Dwell Time
The amount of time a tissue is exposed to ultrasound is the dwell time. For longer TI, keep the dwell time shorter. Longer TI requires a shorter dwell time) to keep the patient safe. If the TI < 1, heating risk is negligible.
Calculating the TI
There are three forms of the Thermal Index, and which one is shown in the spot labeled TI depends on the exam preset and the tissue characteristics near the ultrasound beam’s focus.
Variant |
Full Name |
Use Case / Anatomic Context |
Gestational Age Cutoff |
|---|---|---|---|
TIS |
Soft Tissue Thermal Index |
1st trimester, adult abdomen |
≤10 weeks gestation |
TIB |
Bone Thermal Index |
2nd and 3rd trimester due to ossified bone, adult MSK |
>10 weeks gestation |
TIC |
Cranial Bone Thermal Index |
transcranial studies |
All ages |
Display Behavior on Clinical Systems
Most clinical ultrasound machines (e.g., GE, Siemens):
- Display a single TI value without labeling the subtype (TIS/TIB/TIC).
- Automatically select the appropriate TI variant based on the exam preset and transducer used.
- Do not allow manual switching of TI variant on the user interface.