Mechanical index

The mechanical index (MI) is an attempt to measure part of an ultrasound beam's bioeffects. The mechanical index is found on most ultrasound display screens, along with the thermal index.

Mechanical index is proportional to an ultrasound beam's peak negative pressure (or peak rarefaction pressure) ⁴

It is also inversely proportional to the frequency of the beam. Therefore, higher frequencies have a lower mechanical index ⁴.

Mechanical index = peak negative pressure / √(center frequency of the US beam) where the pressure is in the units of megapascals while the frequency is in the units of megahertz ⁴.

Mechanical index is an indication of an ultrasound beam's ability to cause cavitation-related bioeffects, and this is currently thought a reasonable proxy for micromechanical damage. It is "strictly a cavitation index," but is meant to be interpreted more broadly as tissue mechanical stress/damage.

Theoretically, if one wished to decrease the mechanical index in a study, one should:

• set the focal zone further away from the transducer

• decrease the ultrasound beam output

Mechanical index is important in contrast-enhanced ultrasound, where cavitation of the bubbles is desired. Different bubble sizes are susceptible to cavitation at different frequencies. Smaller bubbles require higher acoustic pressure amplitude to overcome the stronger surface tension of the bubbles. Thus, small bubbles require higher frequency of ultrasound for cavitation ⁴. Two different ultrasound frequencies (e.g. 4 MHz and 9 MHz) cavitate bubbles of different sizes. 

Gas filled organs such as lungs and intestines are most sensitive to cavitation effects ⁴.

Current recommendations

In the U.S. the FDA mandates that the mechanical index (MI) be kept below 1.9 ⁴.

In 2009 the British Medical Ultrasound Society (BMUS) released guidelines for the safe use of diagnostic ultrasound equipment ³. The guidelines also detail recommended exposure time and index values for obstetric and neonatal ultrasound.

For obstetric and neonatal scanning there is no known reason to restrict scanning times with a MI value between 0-0.3. There is possibility of minor damage to the neonatal lung or intestine with MI values between 0.3-0.7 and minimizing exposure time at these values is suggested.

When MI values measure >0.7 there is a risk of cavitation if an ultrasound contrast agent containing gas microspheres is being used. The risk increases with MI values above this threshold.