This past weekend we held another introductory ultrasound course, this time hosted at the UC Davis facility. There were about 25 enthusiastic participants willing to learn more about the theory and practice of abdominal ultrasound in dogs. Some had never tried ultrasound, others had taken classes at veterinary school, and a few were back for a second or third ultrasound course. Everyone agreed that it’s a skill that takes a lot of practice.
One of the first things we like to start out with is learning the important controls on the machine. While the right hand is doing the scanning, the left hand should be constantly adjusting the controls to get the best image of every organ. Today I’m going to start with time gain compensation, and will post more on the others later.
Time Gain Compensation
The first control to look at is the TGC which is the row of parallel sliders. Since ultrasound waves are attenuated, or absorbed, as they pass through tissue, the waves reflected from the more distant areas are weaker than the waves reflected from the areas near the transducer. Without TGC, the image would have a light to dark gradient from the near field to the far field. TGC lets us compensate for the loss of ultrasound waves in the far field by amplifying their signal. The result is an even brightness in the entire field of view. It’s a good idea to adjust this when looking at something homogeneous such as the liver.
TGC too low
Take a look at the examples of this ultrasound phantom. There should be 4 hyperechoic dots visible in the near field and 4 in the far field. The position of the TGC sliders is visible on the right side of each image. On the first image, the near field is very bright and the far field is dark. The near field dots are visible, but we are not getting any information from the deeper tissues. The near field TGC is set too high and the far field too low.
TGC too high
On the second image, we can see the dots in the far field. But the gain is so high in the near field that we can’t see the 4 dots any more. If this were a splenic lesion, you could certainly miss it. In this case the near field gain is set too high, or too far to the right.
In the third image, we’ve moved the near field sliders over to the left so that the TGC curve is slightly diagonal at the top. The entire image is a uniform brightness, and we can see the dots in the near field and the far field. This is the correct TGC setting.