KNOBOLOGY: What Is TGC?

So perhaps you have noticed the two knobs/buttons on the left side of the ultrasound keyboard above the gain button.  Well, what are they? 

These are the Time Gain Compensation knobs.  I know what you are thinking, the what?

Well, lets begin with a review of what the Gain button is first.  The Gain adjusts the amplitude of all the  sound waves returning to the machine from the soft tissue.  Think of the Gain as the overall volume control on your stereo system.

It makes the overall sound louder or quieter, but can't adjust the specifics of the music.  That's where the TGC comes in.

TGC is like the treble and bass adjustments on your stereo.  They adjust the sound in specific ways.

The TGC often appears as slides on larger US machines

Each of these slides controls the gain to a specific line on the screen.

So why would we need to adjust the gain in different areas of the screen?  The reason for this has to do with attentuation, the progressive weakening of sound over distance.  As our sound beam travels farther from the probe it becomes weaker and the amplitude of the returning sound waves becomes lower. 

To counteract this low amplitude sound, we need to increase the gain or amplitude of those soundwaves so that we can make an image visible.  The opposite is true to ojects that are close to the probe.  Since they are so close, the returning sound wave has a much higher amplitude, so the gain must be turned down a bit to create a uniform image.  On a larger US machine, the slides would then appear as a curve to adjust the gain at each level.

Our machine is not so fancy, after all, we are trying to make rapid decisions based on our scans at the bedside.  Our machine makes it simple by only giving us two choices.  We can adjust the gain in the near or the far field only.

Although this decreases the fine control of our image, it does aid us in visualization, especially of distal structures such as the uterus or the rectovesicular pouch or pouch of douglas on the FAST exam. 
Now lets adjust the distal gain and suddenly the uterus is visible!  This can make visualization of the uterus easier, but can also help to determine of there is scant free fluid in the pelvis on trauma patients.

So the next time you're using the machine, take a closer look at the TGC buttons, they can definitely make your images richer!