In the last few decades, research into the non-linearity of tissue and its ability to distort waves resulting in harmonics, has lead to better image quality. The figures below depict the same image from the same person, on the left with linear acoustics and on the right with non-linear acoustics. There is a larger contrast between tissue and blood and less reverberations in the near field. This type of imaging is also called harmonic imaging or octave imaging.
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<img src="/elaring/fag/ultralyd/teknologi/figur8.jpg" height="300">&nbsp;
<img src="/elaring/fag/ultralyd/teknologi/figur9.jpg" height="300">
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Apical image from the heart apex, image on the left is based on linear acoustics transmitting and receiving at 2.5 MHz, image on the right based on non-linear acoustics transmitting at 1.7 MHz and receiving at 3.4 MHz.