What does the mechanical index (MI) indicate in ultrasound?

The mechanical index (MI) is a critical parameter in ultrasound that specifically indicates the potential for cavitation or bubble formation in tissues when exposed to ultrasound. It is derived from the peak negative pressure of the ultrasound wave and the frequency of the ultrasound. A higher MI suggests an increased likelihood of cavitation, which occurs when pressure variations from the ultrasound cause microscopic gas bubbles in tissues to undergo rapid changes—potentially leading to significant tissue effects.

Understanding the MI is essential for safe ultrasound practice, as it helps clinicians balance image quality with patient safety. The MI is particularly relevant when performing procedures involving high-intensity ultrasound, where the risk of cavitation must be carefully monitored. Overall, this measure helps ensure that the benefits of imaging do not come at the cost of tissue harm due to excessive mechanical effects.