Non-linear acoustics

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 relation to imaging with ultrasound contrast fluids, that are non-linear spreaders, scanners that received twice the frequency transmitted where made. This is a method to separate the echo of the contrast fluid from the tissue, as contrast fluid has a much stronger non-linearity. Therefore, one can depict where the blood flows in a clearer way, for example in the small vessels in the kidneys and when examining an infarction, which is missing blood flow in a part of the muscles of the heart. 

A big surprise was that it appeared to give improved images even without the contrast fluids in the blood. One of the reasons is that higher frequencies give better lateral resolution, 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 other 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.