Ultrasound Electronics

Ultrasound Time-Gain-Compensation (TGC)

Ultrasound imaging is based on pulse-echo method. The transmitted acoustic pulse will lose its energy due to two main factors: beam spread and attenuation. As shown in Fig. 1, along the axial axis through the center of a piston transducer, the beam will reach its maximum at certain depth and then decrease monotonously. How fast it decreases is determined by the geometry and bandwidth of the transducer, and frequency and bandwidth of the excitation pulse.

The echo signal will decrease in the same way even if the tissue structure is the same. The gain for the echo has to increase with depth to maintain a uniform brightness of the image from near field to far field. Soft tissue has an attenuation of about 0.3dB/MHz/cm to the acoustic pulse. The Time-Gain-Compensation (TGC) has to compensate the lose from both of beam spread and attenuation.

The TGC amplifier usually is a variable gain amplifier with gain controlled by a TGC voltage curve. The curve can be generated by an analog oscillation circuit or a digital curve through DAC, triggered by the pulse transmit signal.

The most simple TGC curve is a saw-tooth curve with adjustable slope. For most imaging system, TGC curve is adjustable independently for each depth segments such as 1cm at 5MHz.