Synthetic Aperture Sequential Beamforming and other Beamforming Techniques in Ultrasound Imaging

This thesis consists of various subjects and applications within beamforming in general, and subjects within synthetic aperture focusing. An insight into the software architecture and beamformer design principles of a software beamformation toolbox is given. Some of the many considerations in relation to beamformer development and implementation are shared.

In the delay-and-sum beamformer the sample index is not necessarily discrete. Some form of interpolation is needed and the influence on image quality has been investigated. The interpolation schemes investigated include linear, polynomial, and up-sampling and FIR filtering. Directional beamforming for angle estimation of the velocity vector has been investigated using simulations and measurements.

Using the measurements more than 96% valid estimates were produced for the flow angles q = {60±,75±,90±} and with a bias below 2± and a standard deviation below 5±. The two synthetic aperture imaging techniques described in this thesis are both candidates for a realistic implementation in a commercial scanner.

In one technique synthetic aperture focusing (SAF) is applied to 2-dimensional imaging with a single rotating mechanically focused concave element. Emission and reception are done while the transducer element continuously rotates and the received RF signals are stored. The geometrical focal point can be considered as a point source emitting a spherical wave in a limited angular region.

For each image point in a high resolution image line (HRL) it must be determined which emissions that have a wave field that encompasses the image point. These emissions contribute to the HRL, and samples from each of them are selected according to the focusing delays, and added together. Due to the rotation, the synthesized aperture only experiences a moderate expansion.

This is not sufficient to reduce the extent of the wide point spread function of a single emission. The advantage of SAF is the increase in SNR. For the setup with focal depth at 20 mm the SAF SNR gain is 11 dB. The other synthetic aperture focusing technique is similar but has been revised toward linear array imaging.

The technique is realized using two beamformers, and denoted Synthetic Aperture Sequential Beamforming (SASB). The VS is now created from an electronic focused subaperture in the first beamformer. Receive focusing is a simple fixed focusing with receive focal point in the transmit focal point, and the first beamformer could easily be analog and thereby save many ADC’s.

The focused RF-lines from the first beamformer are stored and transferred to the 2nd beamformer. Here it is exploited that a single image point is represented in multiple output lines from the first beamformer. There is an substantial improvement in lateral resolution using SASB compared to dynamic receive focusing (DRF).

The improvement in FWHM is at least a factor of 2 and the improvement at -40 dB is at least a factor of 3. At depths until 20 mm the FWHM is superior with DRF.With SASB the resolution is almost constant throughout the range. For DRF the FWHM increases almost linearly with range and the resolution at -40 dB is fluctuating with range.

SASB has been applied to data acquired with a commercial scanner and a tissue phantom with wire targets. The images confirm the results from the simulations. At the center of the image the resolution of SASB is superior to DRF and is practically range independent. The resolution in the near field is slightly better for DRF.

A decrease in performance at the transducer edges occur for both DRF and SASB. They are more profound for SASB and especially at greater depths it is obvious that the lateral resolution is laterally dependent.