Harmonic imaging is a technique in ultrasonography that provides images of better quality as compared with conventional ultrasound technique.
Harmonic imaging exploits non-linear propagation of ultrasound through the body tissues. The high pressure portion of the wave travels faster than low pressure resulting in distortion of the shape of the wave. This change in waveform leads to generation of harmonics (multiples of the fundamental or transmitted frequency) from tissue. At present, 2nd harmonic is being used to produce the image as the subsequent harmonics are of decreasing amplitude and hence insufficient to generate a proper image.
These harmonic waves that are generated within the tissue, increase with depth to a point of maximum intensity and then decrease with further depth due to attenuation. Hence the maximum intensity is achieved at an optimum depth below the surface.