Christopher Robert Hazard

Integration of Crawling Waves in an Ultrasound Imaging System. Part 1: System and Design Considerations

An ultrasound system (GE Logiq 9) was modified to produce a synthetic crawling wave using shear wave displacements generated by the radiation force of focused beams formed at the left and the right edge of the region of interest (ROI). Two types of focusing, normal and axicon, were implemented. Baseband (IQ) data was collected to determine the left and right displacements, which were then used to calculate an interference pattern. By imposing a variable delay between the two pushes, the interference pattern moves across the ROI to produce crawling waves. Also temperature and pressure measurements were made to assess the safety issues. The temperature profiles measured in a veal liver along the focal line showed the maximum temperature rise less than 0.8°C, and the pressure measurements obtained in degassed water and derated by 0.3 dB/cm/MHz demonstrate that the system can operate within FDA safety guidelines.

Crawling Waves from Radiation Force Excitation

Crawling waves are generated by an interference of two oscillating waves traveling in opposite directions, with a progressive movement resulting from a frequency difference or a phase difference between the sources. While the idea has been applied to numerous applications, all the previous reports used mechanical sources to vibrate the medium. It is shown, through experiments and simulation, that crawling waves can be generated from focused beams that produce radiation force excitation within the tissue. Some examples are also shown.

Shear Wave Dispersion in Lean Versus Steatotic Rat Livers

The precise measurement of fat accumulation in the liver, or steatosis, is an important clinical goal. Our previous studies in phantoms and mouse livers support the hypothesis that, starting with a normal liver, increasing accumulations of microsteatosis and macrosteatosis will increase the lossy viscoelastic properties of shear waves in a medium. This increase results in an increased dispersion (or slope) of the shear wave speed in the steatotic livers.

Effect of Pulse Shaping on Subharmonic Aided Pressure Estimation In Vitro and In Vivo.

Subharmonic imaging (SHI) is a technique that uses the nonlinear oscillations of microbubbles when exposed to ultrasound at high pressures transmitting at the fundamental frequency ie, fo and receiving at half the transmit frequency (ie, fo/2). Subharmonic aided pressure estimation (SHAPE) is based on the inverse relationship between the subharmonic amplitude of the microbubbles and the ambient pressure change.

A complex elastographic hyperbolic solver (CEHS) to recover frequency dependent complex shear moduli in viscoelastic models utilizing one or more displacement data-sets

Abstract In this paper, we present a linear marching scheme to recover frequency-dependent complex shear moduli in viscoelastic models utilizing two sets of single component displacement data. The proposed method is designed to provide stable and accurate estimation of the tissue viscoelastic stiffness parameters by solving a first-order complex partial differential equation. To control the exponential growth of the numerical error resulting from one of the complex coefficients in the inverse equation, a modified upwind discretization is utilized on the first-order derivative terms of the target parameter. The algorithm is fully stablized when: (1) carefully chosen multiple data-sets are combined to eliminate the remaining complex coefficient that contributes to exponential error growth; and (2) a modified Tikhonov regularization is applied to the inversion method. We obtain the stability result in the norm so that the numerical scheme is convergent at fractional 1 / 2 order. Its performance is compared with the performance of the Algebraic Inversion Model previously investigated. We present shear modulus reconstructions from synthetic data, from laboratory phantom data and match frequency-dependent complex moduli from phantom data to several viscoelastic models. Since we have previously presented phase wave speed images from interference patterns, we exhibit those images here for comparison.