If the thermal index (TI) is greater than 1, in which tissue would the temperature rise be greatest?

• A. Liver
• B. Lung
• C. Kidney
• D. Bone

Answer: (D)

The main idea is that warming from ultrasound depends on how much energy the tissue absorbs and how quickly that heat can be carried away. The thermal index predicts how much the temperature could rise at the focus, and when TI is above 1, there is potential for noticeable heating. Bone absorbs ultrasound energy much more than soft tissues because of its high acoustic attenuation, and heat is dissipated more slowly there due to lower perfusion in bone compared to soft tissues. In contrast, lung contains air, which reduces energy transmission and absorption, so heating is minimal; liver and kidney are soft tissues with good blood flow that helps dissipate heat, resulting in smaller rises than bone. Therefore, the greatest temperature rise would occur in bone.