Clutter and sources of image degradation in fundamental and harmonic ultrasound imaging

Although harmonic imaging is used extensively, the mechanisms for image quality improvement are still poorly understood. In the simplified case of a homogeneous medium the improvements in harmonic imaging can be linked to reductions in the main lobe width and the height of side lobes. It has been suggested, however, that the primary benefit of harmonic imaging is reduction of clutter noise and less sensitivity to phase aberration. This requires complex simulations or experiments that incorporate the effects of tissue heterogeneities, multiple scattering, and nonlinearity, which has imposed significant challenges in describing and quantifying the mechanisms of image quality improvement with harmonic imaging.