For the first 20-30 years of clinical ultrasound it was believed to be a linear technique and that non-linear behaviour was only found in very high power applications such as under water acoustics. The discovery that non-linear behaviour was part of clinical ultrasound arose in an interesting way: with the advent of microbubbles for contrast in echocardiography, a system was set up to detect non-linear behaviour (harmonics) and placed for evaluation in an echocardiography centre in the USA. However at that stage, microbubbles were not available. The application specialists subsequently contacted the ultrasound techs and they expressed great enthusiasm in the power of the new technique. Naturally the application specialist was confused – after all, they were not using contrast! It transpired that even without the use of contrast, harmonics were valuable in improving the quality of the images. This was the beginning of the perception that non-linear behaviour was occurring routinely in non-contrast diagnostic ultrasound.
Harmonics are additional frequencies based on multiples of the fundamental (transmitted frequency) and are commonly found in acoustics generally. For musical instruments the double and 4 times overtones give character to the different instruments and exactly the same thing occurs in ultrasound. We need to distinguish two different modes of harmonic generation: those produced by transmission through the tissue and those produced by interaction with microbubbles.
Tissue Harmonics are generated by propagation of the ultrasound beam through normal tissue. The principle is simple: the speed of propagation of an ultrasound beam depends on the density of the tissue, a well-known basic principle of ultrasound. The compression part of the ultrasound wave increases the density of the tissue fractionally and thus this part of the wave travels marginally faster than the rarefaction part of the wave where the density is lower. Thus, over distance the shape of the waveform becomes distorted and its angular components represent the overtones or harmonics. These can be selected out and used to for imaging. The advantage is that they depend on a high acoustical pressure for their formation and therefore are not produced as much by the weaker unwanted side and grating lobes or by reverberations than by the main lobe. In essence the harmonics are generated from within the tissue and only by the centre portion of the main ultrasound beam. Thus tissue harmonic images are cleaner with fewer side lobe and reverberation artefacts. In principle this results in an increase in tissue contrast. As always there is a down side: the harmonics are some 20dB less intense than the fundamental and this carries the risk of reducing the signal to noise ratio and making generally noisy images. This is why harmonics are better handled by systems with a good dynamic range than with less well-designed scanners. When it works well, harmonic imaging is spectacularly useful in sharpening up images since it increases contrast. Examples are given to illustrate this effect. In clinical practice, for most scanners the benefit in abdominal and general work is so great that most users now set the systems to default in a tissue harmonic mode. However it has to be said that the benefits are less marked for high frequency small parts scanning, though the reasons for this are not obvious.