Depth 

Selecting proper depth is essential to be able to image deeper tissues or optimize the image of the superficial anatomical structures of interest.

Adjust the depth to optimize the images of the structure of interest.

Focus 

Focus adjustment is a common function in many ultrasound machines. When you engage the focus adjustment, the width of the ultrasound beam is narrowed at the depth of interest. 

Result: The operator achieves a focusing effect similar to that obtained using a convex lens. Selecting optimal focus improves the spatial resolution on the plane of interest.

A. Focussing effect: The principle, B. Annular focussing, C. Linear focussing.

Interesting facts

• Ultrasound machines have an indicator on the side of the screen to indicate the focusing depth.
• The reduction in beam width at the selected depth is achieved at the expense of degradation in beam width at other depths, resulting in a less optimal image deeper to the selected focal zone.

Gain/Time gain compensation

Ultrasound transmission exhibits a steady decline in amplitude with increasing depth. This is because: 

• Each successive encounter with the tissues and the consequent reflection removes a certain amount of energy from the pulse, decreasing the generation of later echoes. 
• Tissue absorbs ultrasound, so there is a steady loss of energy as the ultrasound pulse travels through the tissues.

This can be corrected by manipulating the gain. Gain is the ratio of output to input electric power; it controls the brightness of the image. The gain is usually measured in decibels (dB). Increasing the gain amplifies not only the returning signals, but also the background noise within the system in the same manner.

Time gain compensation

Time Gain Compensation (TGC) is a time-dependent amplification. The TGC function can be used to increase the amplitude of incoming signals from various tissue depths. The layout of the TGC controls varies from one machine to another. A popular design is a set of slider knobs. Each knob in the slider set controls the gain for a specific depth, resulting in a well-balanced gain scale across the image.

A, B, and C: The effect of the time-gain compensation settings. Time-gain compensation is a function that allows time- (depth) dependent amplification of signals returning from different depths.