Depth? Gain? Frequency? Understanding key terms when using ultrasound machines with animals: A beginner’s guide

Hello!

I’m Mike, and I recently joined the Portable Ultrasound Machines (PUM) team. When I started work with PUM, I didn’t have a background in ultrasound. So to help me get up to speed, I kept a note of all the new terms I was coming across along with a short definition.

This is the second in a series of blog posts sharing some of these with you. I hope you’ll find them a useful introduction to the – sometimes confusing! – language used in this sector and that they will help you if you are just beginning your journey with ultrasound and the machines we use.

Last time, we introduced the ‘transducer’ (also known as a probe). In that article we touched on how the choice of transducer can influence image quality. In this post we will think about that a bit more, with four interlinked terms all of which will influence the quality of the picture you will see: frequency, depth gain and resolution.

Image quality

Most of the ultrasound machines available to purchase from PUM have a screen – colour or black and white – built into them. Those that don’t will transmit the images being received by the transducer from the machine to an App on a tablet or smartphone. Regardless of how you are viewing the image, you’re going to want to make sure its quality is as good as it can be. This will help ensure an accurate interpretation for your own use, or for your client.

There are many operator-related factors that can influence image quality. These include the position of the scanner on the animal, the pressure applied, the presence or absence of gel and many more. These factors you can learn as you progress and PUM offers extensive training to support you. Our focus here however, is on four key and interlinked terms you will hear again and again in relation to the equipment (or hardware) you use, and form the foundation of high image quality: frequency, depth, gain and resolution.

Frequency

Frequency simply refers to the number of ultrasound waves emitted by the transducer per second. The previous article in this series highlighted that different transducers have different ranges of frequency, which in turn makes them appropriate for different tasks. We will return in more detail to the qualities of different transducers (for example convex, micro-convex, linear) in a future article.

There is a trade-off to be made between quality of image and how far away from the transducer we are able to see. At lower frequencies whilst we are able to penetrate further into the animal to see what we need, the image quality reduces. Whereas at higher frequencies we are not able to penetrate as far into the animal, but due to the higher frequency the quality of the image is much higher.

A note on safety: all ultrasound probes emit thermal energy. This thermal energy increases as the frequency of the ultrasonic waves rises and if not within safe limits can harm the animal you are scanning. Reputable manufacturers and suppliers will ensure that their machines have been tested and fall well within safe limits (sadly holding a CE mark is not sufficient). All machines supplied by PUM meet this criteria, and you should also be sure to make yourself aware of the safe limits for the animals you are scanning.

Depth

Depth refers to how far ‘into’ the animal we are scanning we are able to see, from the head of the transducer.

Depth – along with gain – is such a frequently adjusted setting it tends to have its own dedicated knob or button on your device and are quite easily identifiable. For example, our video here shows you where to find them on the Elite KX5600V.

Gain

Altering the gain will improve definition between lighter and darker areas on your screen. We do this by adjusting (either increasing or reducing) the amplification of the ultrasonic waves that have been returned to the machine via the transducer. Similarly to depth, you will tend to find the nob/button to adjust this setting falls easily to hand on your device.

This does mean, however, it boosts (or reduces) both signal and noise – so does not tend to help with contrast enhancement. Machines tend to have a separate contrast enhancement adjuster, similar to that on any monitor, which can help with this.

Resolution

Think about when you take a photo with the camera on your phone. The image is composed of a large number of dots per inch (DPI). When you first see it, the image looks clear. However the more you zoom in, the more grainy it becomes. The higher your resolution (the more dots per inch) the more zooming the image can withstand before it becomes grainy. The resolution is a function of both the lens of the camera (how many ‘dots’ can it receive?) and the resolution of the screen (how many ‘dots’ can it display?)

It is similar with ultrasound machines. Resolution is heavily influenced by how many crystals there are in the transducer to send/receive the ultrasonic waves (take a look at the previous article which outlined how this works). The more crystals there are to transmit/receive, the higher the resolution and so the clearer the image on your screen. For example, one transducer may contain 80 crystals and deliver average image quality. Another transducer may contain a mainboard which can deliver 128-element image quality, with much better signal processing components.

What have we learnt?

• Factors influencing image quality include those of the operator (technique) and those of the equipment you are using (hardware). Here we have focused mainly on the hardware;
• ‘Frequency’ relates to the number of ultrasound waves emitted by the transducer per second. Higher frequencies mean a clearer image, but sacrifice depth;
• ‘Depth’ relates to how far ‘into’ the animal we are able to see, and can be altered by increasing or decreasing the frequency;
• Altering ‘gain’ means we are altering the definition between lighter and darker areas on the screen by adjusting the amplification of the ultrasonic waves.
• Resolution refers to both the detail your transducer is able to receive (a higher crystal count means more detail) and the detail your screen is able to display (more dots per inch mean a higher resolution).

What next?

We hope you have found this second article interesting and a helpful ‘starting point’ in thinking about image quality. If you would like to discuss any questions you have we would be delighted to speak with you (with no obligation to purchase a scanner from us). We are proud to offer the experience of professional sonographers with a combined experience of over 25 years and should you purchase from PUM will always be available to answer your questions and solve your problems. Free of charge, whenever you need us, we are here to help.