Clutter from multiple scattering and aberration in a nonlinear medium
Aberration, clutter, and reverberation degrade the quality of ultrasonic images. When an acoustic pulse propagates through tissue these effects occur simultaneously and it is difficult to obtain independent estimates for the precise source of the point spread function broadening. The purpose of this paper is to characterize the sources of clutter and reverberation with a simulation of ultrasonic propagation through the abdomen. A full-wave equation that describes nonlinear propagation in a heterogeneous attenuating medium is solved numerically with finite differences in the time domain (FDTD). Three dimensional solutions of the equation are verified with water tank measurements of a commercial diagnostic ultrasound transducer and are shown to be in excellent agreement in terms of the fundamental and harmonic acoustic fields, and the power spectrum at the focus. The linear and nonlinear components of the algorithm are also verified independently. In the linear non-attenuating regime solutions match results from Field II, a well established software package used in transducer modeling, to within 0.3 dB. In addition to thermoviscous attenuation we present a numerical solution of the relaxation attenuation laws that allows modeling of arbitrary frequency dependent attenuation, such as that observed in tissue. A perfectly matched layer (PML) is implemented at the boundaries with a novel numerical implementation that allows the PML to be used with high order discretizations. A -78 dB reduction in the reflected amplitude is demonstrated. The numerical algorithm is used to simulate a focused ultrasonic pulse propagating through a histologically determined representation of the human abdomen. An ultrasound image is created in silico using the same physical and algorithmic process used in an ultrasound scanner: a series of pulses are transmitted through heterogeneous scattering tissue and the received echoes are used in a delay-and-sum beamforming algorithm to generate a images. The resulting harmonic image exhibits characteristic improvement in lesion boundary definition and contrast when compared to the fundamental image. We demonstrate a mechanism of harmonic image quality improvement by showing that the harmonic point spread function is less sensitive to reverberation clutter. © 2007 IEEE.
