Bruno Hans Haider

Pulse elongation and deconvolution filtering for medical ultrasonic imaging

Range sidelobe artifacts which are associated with pulse compression methods can be reduced with a new method composed of pulse elongation and deconvolution (PED). While pulse compression and PED yield similar signal-to-noise ratio (SNR) improvements, PED inherently minimizes the range sidelobe artifacts. The deconvolution is implemented as a stabilized inverse filter. With proper selection of the excitation waveform an exact inverse filter can be implemented. The excitation waveform is optimized in a minimum mean square error (MMSE) sense. An analytical expression for the power spectrum of the optimal pulse is presented and several techniques to numerically optimize the excitation pulse are shown. The effects of PED are demonstrated in computer simulations as well as ultrasonic images.

Implementation of vibro-acoustography on a clinical ultrasound system

Vibro-acoustography (VA) is an ultrasound-based imaging modality that uses two ultrasound beams of slightly different frequencies to produce images based on the acoustic response due to harmonic ultrasound radiation force excitation at the difference frequency between the two ultrasound frequencies. VA has demonstrated feasibility and usefulness in imaging of breast and prostate tissue. However, previous studies have been performed either in controlled water tank settings or using a prototype breast scanner equipped with a water tank. In order to make VA more accessible and relevant to clinical use, we report here on the implementation of VA on a General Electric Vivid 7 ultrasound scanner. In this paper, we will describe software and hardware modifications that were performed to make VA functional on this system. We will discuss aperture definition for the two ultrasound beams and beamforming using a linear array transducer. Experimental results from beam measurements and phantom imaging studies will be shown. The implementation of VA provides a step towards clinical translation of this imaging modality for applications in various organs including breast and prostate.

A beamforming study for implementation of vibro-acoustography with a 1.75-D array transducer

Vibro-acoustography (VA) is an ultrasoundbased imaging modality that uses radiation force produced by two cofocused ultrasound beams separated by a small frequency difference, Δf, to vibrate tissue at Δf. An acoustic field is created by the object vibration and measured with a nearby hydrophone. This method has recently been implemented on a clinical ultrasound system using 1-D linear-array transducers. In this article, we discuss VA beamforming and image formation using a 1.75-D array transducer. A 1.75-D array transducer has several rows of elements in the elevation direction which can be controlled independently for focusing. The advantage of the 1.75-D array over a 1-D linear-array transducer is that multiple rows of elements can be used for improving elevation focus for imaging formation. Six configurations for subaperture design for the two ultrasound beams necessary for VA imaging were analyzed. The point-spread functions for these different configurations were evaluated using a numerical simulation model. Four of these configurations were then chosen for experimental evaluation with a needle hydrophone as well as for scanning two phantoms. Images were formed by scanning a urethane breast phantom and an ex vivo human prostate. VA imaging using a 1.75-D array transducer offers several advantages over scanning with a linear-array transducer, including improved image resolution and contrast resulting from better elevation focusing of the imaging point-spread function.

Synthetic transmit focusing for ultrasonic imaging

Medical ultrasound imaging systems use large transmit apertures for improved azimuthal resolution. Uniform imaging characteristics are achieved by employing multiple transmit focal points. The separation between the focal points has to be small enough so that the beam degradation at the transition point is acceptable. The method presented here improves the beam characteristics away from the transmit foci, in particular at the transition points. The proposed method synthesizes transmit foci in between two real focal points. A synthesized signal is formed as the weighted sum of the echoes resulting from the transmissions to the real focal points. Since the first focal point is closer to the imaging aperture than the target point it creates a parabolic phase error. The other focal point is located beyond the target and creates a phase profile of opposite curvature. By properly combining the echo signals the opposing phase curvatures partially cancel each other. The reduced phase errors in the combined signal improve the shape of the synthesized beam. The weighting factor for the signal combination depends on the location of the target point with respect to the real focal points. Analytically, the weights are determined with a least squares method that minimizes the combined phase errors created by the pair of transmit focal points.