In the last 20 years, the number of suboptimal and inadequate ultrasound exams has increased. This trend
has been linked to the increasing population of overweight and obese individuals. The primary causes of image
degradation in these individuals are often attributed to phase aberration and clutter. Phase aberration degrades
image quality by distorting the transmitted and received pressure waves, while clutter degrades image quality by
introducing incoherent acoustical interference into the received pressure wavefront. Although significant research
efforts have pursued the correction of image degradation due to phase aberration, few efforts have characterized
or corrected image degradation due to clutter.
We have developed a novel imaging technique that is capable of differentiating ultrasonic signals corrupted
by acoustical interference. The technique, named short-lag spatial coherence (SLSC) imaging, is based on the
spatial coherence of the received ultrasonic wavefront at small spatial distances across the transducer aperture.
We demonstrate comparative B-mode and SLSC images using full-wave simulations that include the effects of
clutter and show that SLSC imaging generates contrast-to-noise ratios (CNR) and signal-to-noise ratios (SNR)
that are significantly better than B-mode imaging under noise-free conditions. In the presence of noise, SLSC
imaging significantly outperforms conventional B-mode imaging in all image quality metrics. We demonstrate
the use of SLSC imaging in vivo and compare B-mode and SLSC images of human thyroid and liver.
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.
Acoustic Radiation Force Impulse (ARFI) imaging uses short duration acoustic pulses to generate and subsequently determine localized displacements in tissue. Time delay estimators, such as normalized cross correlation and phase shift estimation, form the computational basis for ARFI imaging. This paper considers these algorithms and the effects of noise, interpolation, and quadrature demodulation on the accuracy of the time delay estimates. These results are used to implement a real-time ARFI imaging system and in an ex vivo liver ablation study.
Time domain algorithms that solve the Khokhlov--Zabolotzskaya--Kuznetsov (KZK) equation are described and implemented. This equation represents the propagation of finite amplitude sound beams in a homogenous thermoviscous fluid for axisymmetric and fully three dimensional geometries. In the numerical solution each of the terms is considered separately and the numerical methods are compared with known solutions. First and second order operator splitting are used to combine the separate terms in the KZK equation and their convergence is examined.