Mechanical index (MI)

A dimensionless safety metric indexing the risk of inertial cavitation, defined as the peak rarefactional (negative) pressure in megapascals, derated for tissue attenuation, divided by the square root of the centre frequency in megahertz (MI = P_r / sqrt(f)). Higher MI means a greater chance that the negative-pressure phase will nucleate and violently collapse gas bubbles, which can mechanically damage tissue.

Diagnostic ultrasound is regulated to keep MI below roughly 1.9; neuromodulation and mote-powering protocols generally operate well under this and report MI explicitly. MI is central to any acoustic-neurotech safety case, though it is calibrated for diagnostic conditions and does not capture every risk — for example, the presence of injected microbubbles, which dramatically lowers the cavitation threshold.