It has long been known that both water and body tissue behave in a non-linear way when exposed to acoustic waves. For a long time, it was believed that the large attenuation in tissue compared with that of water meant that non-linear acoustics played a minor role in imaging. In connection with imaging with ultrasound contrast agents, which are non-linear scatterers, scanners that received at twice the transmitted frequency were made. This is a way of separating the echo of the contrast agent from the tissue, as the contrast agent 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. 
<p>
It was a big surprise that this appeared to give improved images even without injection of contrast agents. 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 distributed 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 in the subdermis and by reverberations in the same area. The fact box describes non-linearity in more detail.