In Ultrasound controls part I, I described the time-gain compensation controls and how they can optimize your image. Once those are set, it’s time to look at the other three controls on the machine; B-mode gain, depth and focal zones. Each of them has a different and important effect on your ultrasound image.
B mode Gain
The standard scanning mode we use in the abdomen is B mode, or brightness mode. The ultrasound waves are reflected and absorbed as they pass through the abdominal tissues, so they are weaker when they return to the transducer. Similarly to the TGC, we need to amplify the signal to a brightness that we can see. The B mode gain adjusts the overall brightness of the image. You’d like to see good contrast between different organs, but not so bright that you start to see echoes in the bladder or gallbladder. In the first image, the gain is set too low. The image is dark, and it’s hard to see the three anechoic areas in the far field. You can see the small hyperechoic spot in the near field because of the inherent high contrast. In the second image, the gain is set too high. The bright echoes in the near field are just noise, or random echoes that have been amplified to high brightness. They have obscured the small hyperechoic spot in the near field. The increased gain has increased contrast in the far field so that the anechoic areas are more visible. Set your gain keeping the contrast between hyperechoic and hypoechoic areas optimal.
Every ultrasound machine has a scale on one side of the screen that marks your depth of image in cm. The depth control allows you to increase or decrease the depth of the image. For the liver, you need quite a bit of depth, depending on the size of the dog. Here you need to be able to see the entire curve of the diaphragm. When looking at smaller organs, adjust the depth until the organ fills your screen. That way you’ll get the most detailed image of what you are looking at.
Most machines have an adjustable focal zone, which looks like a triangle or other symbol along the depth scale. When the ultrasound beam exits the transducer, it’s a fairly thin slice, but is not a uniform width along the depth of the entire image. It converges at the point of the focal zone, then spreads out wider as it travels distally. The focal zone is the thinnest part of the slice, so you will get the best detail here. Adjust the focal zone to the depth of the area you are looking at.
You can usually add more than one focal zone to your image to increase the resolution. The trade-off here is that the ultrasound machine is now sending out multiple sets of echoes, one with the focus at each of the focal zones. This takes more time, and you will probably start to see “flickering” or slow motion effect on the screen. The frame rate of the image slows enough that your eye can detect the change from frame to frame. Try it on your machine to see the positive and negative effects.
Once you get used to adjusting these controls on a regular basis, it becomes second nature. Optimized controls make a big difference to the quality of your images. If the depth is not adequate, or the focal zone is not at the right level, you could miss a liver or splenic lesion. Besides, we all like to admire a nice ultrasound image!