The Mechanical Index (MI) estimates the likelihood of cavitation — the formation, oscillation, or collapse of gas microbubbles in tissue — from the ultrasound beam. MI is calculated as peak negative pressure divided by the square root of frequency. Values below 0.3 are considered low risk; values above 1.9 approach the regulatory limit for diagnostic ultrasound. MI is highest in B-Mode imaging. It is particularly important when using ultrasound contrast agents, which contain stabilized microbubbles that can rupture at higher MI values, and in first-trimester scanning.