Beamforming for novel matrix arrays

Abstract

Currently a lot of effort is put into developing matrix arrays which allow for volumetric imaging and new applications. There are however multiple problems. Compared to arrays currently in use which have in the order of 128 elements, matrix arrays can easily contain 1000 to 10000 elements. If all elements would be connected independently, the cable would become very thick. Therefore, beamforming methods are required that can operate with fewer transmit and receive channels. Furthermore, the room for electronics on the chip is limited. So, the required electronics for the beamforming methods should be kept simple.

In this thesis we will propose beamforming methods that are able to operate with fewer channels. This will be done separately for the transmit and receive part, but they do in no way exclude each other. To be able to focus pulsed waves in transmit we propose a method based on Fresnel zone plates which are used in optics to focus continuous wave light. Our method only requires a single continuous-wave excitation signal to be present, which is connected and disconnected on demand to each element. We have evaluated our method with measurements and simulations. As compared to the conventional focusing method, the spatial resolution is not affected by our method, but the Contrast-to-Noise ratio is 5\% lower for shallow depths and up to 20\% lower deeper into the medium. Overall though, the differences were relatively small and so it is clear that our new focusing method works very well. If needed, better results can be obtained by trading in frame rate. In this case the results are almost indistinguishable from the conventional focusing method.

To solve the problem with image formation, we have developed a frequency domain two stage beamforming method for use with matrix arrays, which does not require all element data to be present. This has been done for two matrix types. For the first method we have confirmed with simulations that it performs similar to the respective results obtained with two 2D frequency domain two stage beamforming method that have already been experimentally verified. For the second method we have evaluated the performance with simulations and measurements. Our method was able to obtain a 25\% better spatial resolution as compared to Dynamic Receive Focusing(DRF), without additional artefacts. As an alternative to the last method, we have also developed a frequency domain beamforming method that does require all element data, but only requires a single insonification by a spherical wave. This method did perform worse than the method discussed before in both simulations and measurements, but it does outperform DRF applied to spherical wave data.