High frame rate ultrasound imaging using parallel beamforming

The human heart contracts and relaxes approximately once each second. This isa complex process where different parts of the cardiac tissue contract and relax atdifferent times and at different rates. The accurate evaluation of this deformation withultrasound requires the use of a high frame rate. The frame rate of a conventionalultrasound image is limited by the round trip propagation time of the sound pulse alongeach of the scan lines covering the imaged object. A common technique to increase theframe rate is multiple line acquisition, MLA. Using this technique, several scan linesare acquired in parallel for each transmitted pulse. This technique is therefore alsocalled parallel beamforming. Although it increases the frame rate in proportion tothe number of parallel beams, this technique also introduces block-like artifacts in theB-mode image. These artifacts severely degrade the image quality, and are especiallyvisible in image sequences (movies). An aim of this thesis is to investigate methodsto increase the frame rate using parallel beamforming without introducing such imageartifacts.Investigations of the mechanisms of MLA image artifacts have shown that themisalignment of the transmit and receive beams causes distortions to the pulse-echoresponses. These distortions result in a shift variant imaging system and imageartifacts. This thesis is comprised of four papers that document several metrics thathave been developed to evaluate the pulse-echo distortions, image artifacts and shiftinvariance property. Different methods for artifact reduction have been compared andevaluated. The two methods that have been most thoroughly investigated are steeringcompensation and the synthetic transmit beam method, STB. In the first method, thereceive beams are additionally steered to partially avoid the pulse-echo distortion.Applying this method reduced image artifacts under ideal conditions. However,the performance was heavily reduced in realistic scenarios with aberrations. In theSTB method, synthetic transmit beams are created in each receive direction throughinterpolation. This method performed well both with and without aberrations.Additionally, it has been shown that from the same STB acquisition pattern it is alsopossible to estimate velocities with an accuracy comparable to that of conventionalTDI. This enables higher TDI frame rates or a larger field of view compared toconventional TDI, which requires separate acquisitions for B-mode and tissue Doppler.