Time gain compensation

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A way to overcome ultrasound attenuation is time gain compensation (TGC), in which signal gain is increased as time passes from the emitted wave pulse. This correction makes equally echogenic tissues look the same even if they are located in different depths.

The basis of this is that of returned ultrasound echoes from tissues. It is known that early echoes represent wave reflections in superficial layers, while late echoes come from deeper layers. This is the base of axial localization in ultrasound.

It is also known that the emitted ultrasound wave amplitude gets smaller as it penetrates tissue, a phenomenon called attenuation. So, one might expect late echoes (from deep layers) to have smaller amplitudes than early (superficial) echoes even if those layers have the same echogenicity.

If the ultrasound image was formed directly by the raw returned echoes, image would appear lighter in superficial layers and darker in deep layers but TGC is used to overcome this artifact.

Imaging physics