‘Time Gain Compensation’ is so called because it compensates for the attenuation of ultrasound energy with depth. Depth is synonymous with time in ultrasound, because the ultrasound machine calculates the depth of structures based on how long it takes for ultrasound to reflect off them and return to the transducer. Think of the echoes you hear when you shout down a tunnel or cave – in long passageways or deep caves, you will hear not just a single immediate echo but a successive series of echoes. If you listen carefully, you may continue to hear your echo for several seconds. The video below is a short clip from our sound waves series of videos, which will soon be available online for AUA members.
As the echoes return from deeper and deeper structures, the echoes become weaker. The ultrasound machine amplifies signals received from deeper targets more than those it receives from superficial ones, so that similar interfaces are displayed at a similar level of brightness on the screen, regardless of their depth. This is akin to Vincent, our photographer, gradually increasing the sound levels in his editing software to make it so that all the echoes you hear in the video come back at the same volume. By comparison, increasing the overall gain (via the gain knob) would be the same as Vincent simply turning up the volume of all returning echoes.
The ultrasound machine automatically uses the passage of time to amplify the returning signals, but when the operator is scanning areas of the body which do not attenuate ultrasound energy at the rate assumed by the machine, inaccuracies – termed “artefacts” – can occur.
When scanning a full canine bladder, for example, very little ultrasound energy is lost as it travels through the fluid-filled bladder. This is why the distal bladder wall and tissue immediately beyond the bladder is displayed on the monitor as very bright white. This is termed ‘acoustic enhancement’ and is one of the most common ultrasound artefacts.
Above: an example of acoustic enhancement distal to the bladder.
To a lesser extent, a similar phenomenon is observed distal to gestation sacs, particularly during early pregnancy when the majority of the sac is fluid-filled and the foetus itself is very small.
Above: Acoustic enhancement seen distal to a gestation sac in a Spanish Waterdog.
For this reason, all professional ultrasound machines will have a manual override in the form of TGC sliders. On the Scan Pad, these sliders are displayed on the monitor and adjusted by touch. Each slider corresponds to a specific depth and allows you to fine-tune your image.
TGC sliders on a touch-screen portable ultrasound machine (left), and hard-key TGC sliders (right).
Next time you are with an ultrasound machine, try bringing the top two or three TGC sliders up (all the way to the right). What do you notice happens to the image?
Now bring all of the remaining sliders up to the top as well. Besides returning all of the sliders back to their neutral position, can you think of another way to reduce the overall gain of your image?
Above: How the image should look, with correctly balanced TGCs