08 Ultrasound
Mechanical index
MI = p_r / √f with p_r in MPa and f in MHz. FDA track-3 cap is 1.9.
Listen
Listen · English
Simulation
Mechanical index — Change the numbers; the scene follows.
Where it works
Ultrasound
Tissue path
Along the beam in tissue — impedance mismatch, Doppler shift, MI/TI at the focus.
Open this machineFormula
Variables
Results
MI
Mechanical index
0.866
Curve
Explanation
What it means
Mechanical index is the on-screen estimate of non-thermal bioeffects — principally cavitation. It scales as peak rarefactional pressure over the square root of frequency, because the cavitation threshold rises with f. Contrast-bubble studies deliberately use a low MI (<0.1–0.3) to avoid bursting microbubbles; lithotripsy and histotripsy live at the other end. The FDA 510(k) track-3 cap of 1.9 is a regulatory ceiling, not a biological cliff. This is a working relation in Ultrasound.
Where it is used
Clinically it sits on the Ultrasound — Tissue path. Along the beam in tissue — impedance mismatch, Doppler shift, MI/TI at the focus. Ultrasound equations sit on the probe face and along the beam: impedance, Snell, Doppler, MI and TI. They explain why gel matters, why aliasing appears, and why a mechanical index is on the screen.
Ultrasound · Open this machineHow to use it
Enter derated peak rarefactional pressure (MPa) and centre frequency (MHz). 1.5 MPa at 3 MHz → MI = 0.87. A value above 1.9 is flagged. Change one input and watch the curve and the simulation follow.
Symbols
- p_rPeak rarefactional pressure1.5 MPa
- fFrequency3 MHz
Worked example
A typical case from the default values: p_r = 1.5 MPa (Peak rarefactional pressure); f = 3 MHz (Frequency). Substituting into the relation gives MI = 0.866. These are teaching numbers — align them with your machine.
Typical values give
- MI = 0.866
Where it comes from
The displayed formula is the working relation. MI = p_r / √f with p_r in MPa and f in MHz. FDA track-3 cap is 1.9. Usual reference: AIUM / FDA / IEC 62359. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: AIUM / FDA / IEC 62359
Assumptions & limits
Derated (0.3 dB/cm/MHz) pressure at the point of the max MI, not the on-face pressure. Does not include gas-body (lung, bowel, contrast) modifiers that AIUM statements discuss separately. Not a thermal quantity.
Pitfalls
Soft-tissue 1540 m/s is an assumption — not a measurement in that patient. Doppler angle 90° gives no shift. MI and TI are on-screen estimates, not absorbed dose. Derated (0.3 dB/cm/MHz) pressure at the point of the max MI, not the on-face pressure. Does not include gas-body (lung, bowel, contrast) modifiers that AIUM statements discuss separately. Not a thermal quantity.
Keep this
Gel, angle, and assumed speed of sound — get those three right before you trust a centimetre. Derated (0.3 dB/cm/MHz) pressure at the point of the max MI, not the on-face pressure. Does not include gas-body (lung, bowel, contrast) modifiers that AIUM statements discuss separately. Not a thermal quantity.
In this specialty