All professional ultrasound scanners will use ultrasound gel at time of scanning. The primary purpose of this acoustic couplant is to allow the ultrasound waves to pass into the body without reflecting off intervening air. To further optimise their images, many scanners will also ask to shave the bitch underneath. This enables even better contact between the probe and the skin, as even a light covering of hair will trap air.
So what’s so bad about air?
Air is the enemy of ultrasound. This is down to a few fundamental physical principles of sound waves. To understand this, let’s first go back to basics.
The reason our ultrasound machine generates an image is because large reflections occur wherever there is an abrupt change in acoustic impedance. Boundaries between tissues are ‘seen’ as boundaries on our screens, only because of the acoustic mismatch between one interface and the next – for example, between the fluid in the bladder, and the bladder wall.
Two of the largest changes in acoustic impedance in the body is when ultrasound encounters air or bone. This is why reflections from bowel, or from foetal skeletal bone, show up bright white. The reflections may be so great as to generate an acoustic shadow behind it – i.e. reflection and absorption together account for 100% of the ultrasound energy, such that no ultrasound waves are able to pass through, thereby creating a dark area – or shadow – distal to the object. This is a particularly common sight when scanning a foetal rib cage.
Quite simply, the brighter the object is on your monitor, the bigger the reflection (and hence the greater the difference between the acoustic impedance of your sound waves and that of the object you are imaging); the darker it is, the fewer reflections you are receiving back (and in the case of fluid-filled structures, you are not receiving any echoes).
Clearly, if your ultrasound waves encounter air between the transducer and the animal, the vast majority of your ultrasound energy is going to come straight back to you. You’ve probably noticed when you first power up your machine that all you see on your screen are bright white reflections – this is in fact a single reflection of all of your ultrasound energy, and the subsequent apparent ‘reflections’ are reverberation artefact. If you were to hover your probe only a millimetre or so from your dog’s skin, or even place it probe gently on the skin without gel, you will observe just how much ultrasound energy is lost at this single probe head / air interface.
Acoustic impedance is determined by a medium’s density and stiffness. There is little need to give mathematical examples here; suffice to say, if the density and stiffness of one material is very different to that of another, there will be a large acoustic mismatch, and thus a large resulting echo. It is quite obvious that the properties of soft tissue in terms of its density and stiffness are very different to that of air.
Ultrasound physicists will often talk about the amplitude reflection coefficient, which is the ratio of reflected to incident pressure. Between most soft tissue interfaces, the amplitude reflection coefficient is less than 1% (Hoskins, Martin & Thrush, 2010), meaning that over 99% of the ultrasound energy continues to travel into the body – and hence generate more echoes and therefore images. Between soft tissue and air, however, it is 99.9%.
So now that we can see how important it is to have an acoustic couplant, does it matter what exactly is used?
Remembering that the key here is the acoustic impedance of the medium in question, the answer is, of course, a resounding “yes!” Tales of people scanning with KY Jelly or even moisturising cream (yes, really) are disturbing, because clearly, these substances have not been manufactured with the conduction of ultrasound waves in mind. Ultrasound gel from a reputable source is specifically formulated to be as close to the acoustic impedance of the transducer surface and/or soft tissue as possible (similar to the function of a matching layer in transducer design), minimising absorption or reflection of energy.
For the safety of your patient, it is important that you take pride in generating the clearest images possible, and using correctly formulated ultrasound gel is one cheap and easy way that you can get the most out of your machine.
Reputable brands or suppliers include:
AquaSonic – one of the world leaders in ultrasound gel. Made in New Jersey, so often a more expensive choice. Bright blue in colour; can be unpopular with some pet owners.
Vet Image Solutions – suppliers of CE approved and acoustically balanced ultrasound gel. Clear in colour; more appropriate for animal use.