Mechanical & Thermal Indices
We know that ultrasound deposits energy into tissue in two ways:
Due to the fact that tissue is being affected in these ways, the potential for damage exists, and it was for this reason that the Mechanical Index (MI) and Thermal Index (TI) were created.
One ‘quick fix’ image optimisation trick is to increase output power, and therefore the MI. This increases the signal-to-noise ratio, and can result in a clearer image. However, there are usually a number of other controls one can optimise first (such as gain, which amplifies only the returning signal), and power should always be modified with the ALARA (As Low As Reasonably Achievable) principle in mind.
Another control which affects the MI is frequency. Scanning at lower frequencies means that you are using longer wavelengths of ultrasound. This means that the duration of what physicists call “peak negative pressure” (the rarefaction phase of the compression wave) is longer, and so the potential for mechanical damage is greater.
Tips for minimising the risk of mechanical damage to your patient include keeping scanning times to a minimum, particularly in tiny patients (such as rodents); avoiding dwelling over gas-filled areas such as the lungs and intestines, as the excitation of microbubbles is most likely to cause harm; and, of course, using a EMDD (European Medical Device Directive) / FDA-compliant ultrasound machine, which will never exceed safe limits.
The thermal index is a lot more intuitive to understand, given that a TI of 1 means an expected 1 degree temperature rise in the body (a TI of 2 means 2 degrees, and so on). A study by Shaw et al. (1998) for the UK Department of Health found that it is highly unlikely that a temperature rise of more than 1.5 degrees C could occur in soft tissues. However, if an ultrasound beam were held over a particularly attenuating medium such as bone (which readily converts ultrasound energy to heat), temperatures could rise much higher, very quickly. This is very important to bear in mind when scanning pregnant animals late into their gestation, where foetal bone formation has occurred. Scanning time over bone should always be kept to a minimum.
It is your responsibility, as the operator, to understand MI and TI, and to always bear patient safety in mind. These indices exist for you; they’re your guide as to the likelihood of you causing damage to your patient. When scanning, always implement the ALARA principle. Could you obtain a diagnostic quality image without putting the patient at increased risk?
March 13, 2020
September 23, 2018