Infographics / Ultrasound Physics

Mechanical index, cavitation and non-thermal bioeffects

An ultrasound pressure wave alternates between compression (positive) and rarefaction (negative) pressure. The peak negative (rarefactional) pressure, Pr, drives a gas nucleus to expand; it may oscillate stably without collapse, or grow and collapse violently (inertial or transient cavitation) with very high local temperature and pressure, causing tissue damage. The mechanical index MI equals Pr in megapascals divided by the square root of centre frequency in megahertz, so higher rarefactional pressure and lower frequency raise MI and cavitation risk. A scale shows that MI 0.25 is low risk, an onset of possible bioeffects in gas-containing tissue near 0.3, and increasing caution between 0.7 and 1.0.

Mechanical index and cavitation An ultrasound pressure wave alternates between compression (positive) and rarefaction (negative) pressure. The peak negative (rarefactional) pressure, Pr, drives a gas nucleus to expand; it may oscillate stably without collapse, or grow and collapse violently (inertial or transient cavitation) with very high local temperature and pressure, causing tissue damage. The mechanical index MI equals Pr in megapascals divided by the square root of centre frequency in megahertz, so higher rarefactional pressure and lower frequency raise MI and cavitation risk. A scale shows that MI 0.25 is low risk, an onset of possible bioeffects in gas-containing tissue near 0.3, and increasing caution between 0.7 and 1.0. Mechanical index (MI) and cavitation pressure + − time 0 compression (+) rarefaction (−) Pr = peak rarefactional (negative) pressure bubble response: stable: oscillates, no collapse gas nucleus / microbubble expands during rarefaction inertial (transient) collapse very high local T and P, microjet → tissue damage MI = Pr (MPa) ÷ √f (MHz) higher Pr, lower f → higher MI → inertial cavitation more likely increasing caution: inertial cavitation more likely 0.25 0 0.3 0.7 1.0 1.2 MI 0.25 = low risk (below the ≈0.3 threshold). ≈0.3 = onset of possible bioeffects in gas-containing tissue (neonatal lung / bowel).
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