Ultrasound Signal Amplification, better known as Gain:
The term ‘Gain’ refers to the amplification (boosting/increasing) of the intensity of the returning echo and is necessary because the returning reflected echoes are otherwise too weak to be useful. There are two ways of strengthening the echoes, the first is by increasing the power of the transmitted signal and the second (which this section will focus on) is by increasing or amplifying the returning signal. Gain and power controls are in some way interchangeable, but it is good practice to keep the power as low as possible (to reduce the risk of damage caused by ultrasound).
There are two types of gain control on an ultrasound machine and Overall Gain (OG) and Time Gain Compensation (TGC)
Overall Gain (OG)
- The OG controls the amplification applied to all the returning echoes uniformly and from all depths in the image making the overall image appear brighter or darker (depending which way you turn the control).
- The OG compensates for the loss of energy which occurs as the beam passes through the tissue on its way from and back to the transducer
- The OG is usually a larger rotating control, and the overall gain value is usually displayed on the image screen
- Setting the gain too high will create an over-saturated, over bright or ‘hyperechoic’ image
- Setting the gain too low will result in a dark or ‘hypoechoic’ image
- Generally, the gain will need to be adjusted throughout an examination, increasing it for deeper structures and reducing it for superficial structures
- Incorrect gain settings can mask or mimic pathology so it is essential that you use the control as you scan
Time Gain Compensation (TGC or sometimes called near/far field gain)
- The TGC is (historically) a manual control which comprises a set of horizontal slider controls each of which corresponds to a depth of tissue displayed within the image field of view. In newer ultrasound systems, the TGC is often a ‘touch screen’ control
- The relative position of the controls/sliders determines which depths are affected when adjustments are made. The top corresponds to the near field and the bottom of the scale to the far field
- Generally, the sliders should all be positioned in the middle of the TGC when you start an examination and then adjusted as the scan progresses
- The TGC compensates for the fact that signals returning from deeper reflectors will be weaker than those from superficial/shallow reflectors. This is because those echoes returning from deeper structures have further to travel and so will be more affected by issues such as attenuation
- Increasing the TGC at a specific depth, amplifies signals from structures at that depth within the body. Signals returning from a greater depth within the body have a longer go-return time than those from superficial reflectors, hence the ‘time gain’ compensation description. Correctly adjusting the sliders corresponding to the full depth of the image leads to a more balanced (even) grey scale image (as long as there is no pathology to change this) throughout the depth of the image.
- Incorrect settings of the TGC causes artefacts / loss of information / misinformation, all of which can lead to misdiagnosis and patient harm
- TGC has no effect on overall gain or overall brightness of the image but adjusting the controls to the extreme can create a very dark or bright screen in a similar manner to the overall gain.
- It is important to adjust both the overall gain and TGC as an interplay – use both when optimising your image. Your aim is to produce an even grey scale throughout the depth of the whole image