High frame rate ultrasound imaging using parallel beamforming

The human heart contracts and relaxes approximately once each second. This is a complex process where different parts of the cardiac tissue contract and relax at different times and at different rates. The accurate evaluation of this deformation with ultrasound requires the use of a high frame rate. The frame rate of a conventional ultrasound image is limited by the round trip propagation time of the sound pulse along each of the scan lines covering the imaged object. A common technique to increase the frame rate is multiple line acquisition, MLA. Using this technique, several scan lines are acquired in parallel for each transmitted pulse. This technique is therefore also called parallel beamforming. Although it increases the frame rate in proportion to the number of parallel beams, this technique also introduces block-like artifacts in the B-mode image. These artifacts severely degrade the image quality, and are especially visible in image sequences (movies). An aim of this thesis is to investigate methods to increase the frame rate using parallel beamforming without introducing such image artifacts. Investigations of the mechanisms of MLA image artifacts have shown that the misalignment of the transmit and receive beams causes distortions to the pulse-echo responses. These distortions result in a shift variant imaging system and image artifacts. This thesis is comprised of four papers that document several metrics that have been developed to evaluate the pulse-echo distortions, image artifacts and shift invariance property. Different methods for artifact reduction have been compared and evaluated. The two methods that have been most thoroughly investigated are steering compensation and the synthetic transmit beam method, STB. In the first method, the receive 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 the STB method, synthetic transmit beams are created in each receive direction through interpolation. This method performed well both with and without aberrations. Additionally, it has been shown that from the same STB acquisition pattern it is also possible to estimate velocities with an accuracy comparable to that of conventional TDI. This enables higher TDI frame rates or a larger field of view compared to conventional TDI, which requires separate acquisitions for B-mode and tissue Doppler