Ultrasound technology is a non-invasive diagnostic imaging tool that allows clinicians to visualize internal body structures without using ionizing radiation. It works on the pulse-echo principle: a transducer sends high-frequency sound waves into the body. These waves reflect off tissues and boundaries, returning to the transducer as echoes. The machine processes these echoes to construct a real-time image. To optimize image quality for diagnosis, “gain” is one of the most frequently adjusted settings.
Understanding Ultrasound Gain
Gain is the electronic amplification of the electrical signals received by the ultrasound system. After the transducer converts the returning echoes into electrical energy, the gain control uniformly increases the strength of this incoming signal. This is a post-processing adjustment, acting on information already gathered, similar to turning up a radio amplifier.
Gain is necessary because sound waves weaken as they travel deeper into the body, a process called attenuation. Echoes returning from deep structures are much weaker than those from surface tissues. Increasing the gain boosts these faint signals, making deep structures visible. Adjusting the gain only changes how the machine interprets the returning information; it does not affect the sound waves sent into the patient.
The Visual Impact of Gain Adjustment
Adjusting the gain setting changes the overall brightness of the displayed image. If the gain is set too low (under-gained), the image appears excessively dark because weak echoes are not sufficiently amplified. Important soft tissue boundaries or subtle abnormalities may be missed due to the lack of visual information.
Conversely, setting the gain too high (over-gaining) results in an image that is too bright and washed out. Excessive amplification also boosts electrical noise and random scatter, leading to a grainy appearance known as speckle artifact. This high background noise decreases visual contrast, making it difficult to distinguish between normal structures and pathology. A skilled operator seeks a precise balance to clearly delineate necessary structures while minimizing background noise.
Gain Versus Acoustic Power
It is important to distinguish gain from acoustic power, as they affect the image differently. Acoustic power controls the intensity of the sound waves the transducer transmits into the patient. Increasing acoustic power sends out a stronger sound wave, resulting in stronger echoes from all depths, but this directly involves patient exposure to sound energy.
Gain, conversely, is a receiver control that processes the data by amplifying the electrical signal after reception, involving no change to the energy delivered to the patient. Because acoustic power introduces energy into the body, it is subject to safety regulations limiting potential thermal or mechanical effects. Technicians prioritize adjusting gain first to optimize brightness before changing acoustic power. Maximizing electronic amplification often achieves an optimal image without increasing sound intensity directed into the patient, adhering to the principle of keeping patient exposure as low as reasonably achievable.
Specialized Gain Controls
While the main gain control applies uniform amplification across the entire image depth, specialized adjustments are necessary due to the physics of sound wave travel. Echoes from structures located deeper in the body are inherently much weaker than those from shallow structures. To counteract this depth-related signal loss, ultrasound systems include Time Gain Compensation (TGC), sometimes called Depth Gain Compensation (DGC).
TGC allows the operator to apply varying amounts of amplification at different image depths. For example, the operator applies minimal gain to strong echoes from superficial tissues and progressively increases amplification for echoes returning from deeper structures. This selective adjustment corrects for signal loss, ensuring that structures of similar composition appear with uniform brightness across the display.