What does mechanical index indicate on an ultrasound system?

• A. Minutes of exposure time before tissue heating is too great
• B. The maximum temperature increase in tissue
• C. The acoustic power in milliwatts
• D. The likelihood of cavitation during an ultrasound exam

Answer: (D)

Mechanical index measures how likely cavitation is to occur when an ultrasound beam passes through tissue. It combines how strong the negative pressure part of the wave is with the center frequency, roughly calculated as peak rarefactional pressure (in MPa) divided by the square root of the frequency (in MHz). A higher MI means the tissue or any gas pockets or microbubbles are more prone to cavitating—that is, to oscillating or collapsing under the acoustic field. This is a mechanical effect, distinct from heating, which is why MI is considered separately from index values that estimate temperature rise. So, the likelihood of cavitation during the exam is what the mechanical index indicates.