Last month, we addressed the commonly asked question: is ultrasound safe? One of the recommendations for following the ALARA (As Low As Reasonably Achievable) principle was to keep the machine's power setting as low as possible - but what is power, and what is it doing to the body?

Early A-mode and B-mode display modes were borrowed from metallurgical testing and radar technologies, and have since given way to high-performance, real-time imaging (Martin, 2010). As technologies continue to advance, so too do the number of advanced functions and associated power output of machines in particular applications.

 

Some basic physics

As students of our level two canine ultrasound training course will know, the sound waves used for veterinary and medical imaging are longitudinal waves. This means that, unlike waves on the surface of a pond which are displaced vertically, ultrasound waves displace matter through regions of compression and rarefaction. There is no resulting net (or permanent) movement of the body’s fluids or tissues – only an oscillation as energy travels through it.

The maximum distance moved by a particle from its normal rest position is a measure of the amplitude or strength of the wave, which can by a large extent be controlled by the operator in terms of the power output of the machine. It is important to note, however, that power is not distributed evenly across the beam, and may be more concentrated nearer the centre and at focal points.

 

To summarise

By increasing the power output on your ultrasound machine, you are increasing the amount of disturbance experienced by the tissues of the body. Whilst there is no net (permanent) movement of any tissues of the body, ultrasound waves do have the ability to oscillate these tissues. The degree of oscillation or disturbance is directly related to the power output. Whilst no adverse effects of ultrasound use have ever been reported in the clinical setting, always follow best practice by keeping your power output - and thus the level of tissue disturbance in your bitch - as low as reasonably achievable (i.e. as low as possible whilst still generating a clear image).