Simulation and experimental analysis of ultrasonic clutter in fundamental and harmonic imaging

Harmonic imaging has been shown to yield significant improvements in image quality over conventional ultrasound imaging. It has been proposed that harmonic imaging generates these improvements by the reduction in clutter from reverberation in the tissue layers underlying the transducer, a reduction in beam distortion from aberration, and a reduction in clutter due to suppressed sidelobes. There is little research indicating the exact sources of clutter and how they may relate to the improvements observed with in vivo harmonic imaging. We describe simulation and experimental studies in human bladders describing the sources and characteristics of clutter and discuss their relationship to the above proposed mechanisms. The results indicate that a large source of clutter is the product of reverberation in the abdominal layers. Experimental and simulated harmonic images indicate a 3-5 and 3-8 dB reduction in clutter over fundamental images, respectively, in the upper bladder cavity, lending support for the first mechanism described above. Scattering was also observed from off-axis sources in both the fundamental and harmonic images. Simulations of the fundamental point-spread-function (PSF) showed clutter magnitudes of -43 dB in the isochronous volume. Harmonic imaging marginally improved clutter magnitude to -47 dB in this same region. When aberration was removed from the simulation while keeping the impedance constant, the isochronous volume in the fundamental PSF marginally improved to -47 dB, while harmonic imaging improved this region to -58 dB, a reduction of 11 dB. This indicates that the image quality improvements seen with harmonic imaging are more dependent on the reduction in clutter from near-field layers than with reductions in clutter due to aberration.