What does a high mechanical index indicate in ultrasound?

A high mechanical index in ultrasound is indicative of a greater likelihood of cavitation, which is the formation of gas bubbles within a medium due to pressure changes. The mechanical index is a measure that combines the peak negative pressure and the frequency of the ultrasound wave, suggesting that with an increased mechanical index, the ultrasound has the potential to generate conditions that could lead to cavitation.

Cavitation can occur when the negative pressure of the ultrasound wave is sufficient to cause localized boiling and vaporization, leading to bubble formation, which can ultimately result in tissue damage. Hence, a higher mechanical index enhances this risk, making it crucial for sonographers to be aware of settings that could lead to this phenomenon, especially in sensitive tissues.

In contrast, lower mechanical indices are associated with a reduced risk of cavitation and, therefore, safer imaging for certain applications. Although thermal effects are also an important consideration in ultrasound imaging, they are not directly measured by the mechanical index; rather, they relate more to power output and exposure time, making those options less relevant when discussing mechanical index specifically.