Harmonic Generation

In this module you will learn what harmonic imaging is, how harmonic frequencies are generated and how these frequencies differ from conventional ultrasound. What is Tissue Harmonic Imaging? THI is the reception and processing of harmonic frequencies generated by tissue. Where do harmonic frequencies come from? A sound wave traveling through the body is a pressure wave. Tissue is somewhat compressible when pressure waves pass through. The speed of sound is also affected by tissue compression. In sound waves high-pressure peaks travel faster than the low-pressure valleys. The higher the pressure, the greater the distortion of the wave. Any change from a perfect sine wave creates harmonics.

Top Waveform A typical sound wave begins as a symmetrical sine wave. A wave changes shape when it enters the body due to changes in the tissue through which it travels. More dense tissue allows the sound to travel faster. The portion of the wave above the baseline, which is called the high or positive pressure peak, compresses the tissue. Middle Waveform The positive pressure peak is traveling faster than the negative pressure valley and the wave becomes distorted. This becomes more pronounced the farther the wave travels into the body.

Bottom Waveform The more distorted the wave becomes, the more harmonic energy is generated. Harmonics are not produced in the very near field because the wave has not traveled the distance required to result in adequate distortion.