It has long been known that both water and body tissue are non-linear mediums for acoustic waves. For a long time, it was believed that the large attenuation in tissue compared with in water meant that non-linear acoustics played a small role in imaging. In connection with imaging with ultrasound contrast fluids, which are non-linear spreaders, scanners that received at twice the transmitted frequency where made. This is a way of separating the echo of the contrast fluid from the tissue, as contrast fluid has a much stronger non-linearity. Therefore, one can display where the blood flows in a clearer manner, for example in the small vessels in the kidneys and when examining an infarction, which is missing blood flow in a part of the heart muscle. 
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It was a big surprise that this appeared to give improved images even without contrast fluids in the blood. One of the reasons is that higher frequencies give better lateral resolution, i.e. a sharper beam. Moreover, the non-linearity results in less energy being sent in other directions than the intended (reduced sidelobes). The non-linear effect is also such that sound from the second harmonic appears to be generated from a split source that lies in the area between one cm from the probe to its focal point. This results in the beam being less affected by aberrations in fatty layers and such in the subdermis and by reverberations in the same area. The fact box describes non-linearity in more detail.