The quality and safety of our equipment is our top priority, and as a dedicated ultrasound company run by specialists, we take this responsibility seriously. Just as we source equipment through careful field testing by sonographers, feeding back necessary changes to manufacturers to ensure all of our machines are optimised for their intended use, we also make sure the safety readings on our scanners are true and accurate.
How is the mechanical index measured?
Ultrasound transducers send out pressure waves which oscillate between positive and negative pressures. It is this rapid pressure oscillation that has the potential to cause biological harm. Extreme negative pressures can bring gas out of solution in the blood and tissue, which can then explode as the pressure rapidly rises: similar in concept to the way that a diver gets ‘the bends’ from a rapid change in pressure.
The mechanical index is calculated from the peak negative pressure of the ultrasound beam (adjusted to account for the level of attenuation that would be expected in soft tissue), divided by the square of the frequency. It should be calculated and monitored by all ultrasound machines, should ideally be displayed on screen for the operator to monitor, and should never exceed 1.9 for normal ultrasound scans. For pregnancy scanning, users should be even more cautious.
An ultrasound transducer’s mechanical index (MI) is measured using a hydrophone in a water tank, which is lined up with the beam. An oscilloscope displays the waveform, pictured below. From this, you can see that it is true that an ultrasound transducer emits a pulse of ultrasound, consisting of more than one cycle.
The MI of the two scanners tested was well below 1, which makes them ideally suited for abdominal scanning in small animals.