Beamforming

Throw a stone into a pond. You'll see circular waves travelling outwards from the point of impact. Throw two stones, and you'll see an interference pattern forming. At locations where maxima from the first stone coincide with the minima from the second stone, you'll get extiction, and where maxima meet maxima, you'll get amplification. Beamforming is about controlling this interference pattern, and, in most cases, it is about forming a beam-like interference pattern where the amplification occurs predominantly in one distinct direction.

In medical ultrasound, the stones are replaced by several small piezo-electric elements at the tip of an transducer, and the pond is replaced by the human body. If you apply an alternating voltage signal to an piezo-electric element, it will start vibrating and emit sound. If you select the spacing between your elements and the delay in the elements' signals just right, you can create an interference pattern that's to your benefit, in particular one in which the majority of the signal energy all goes out in one angular direction. When using the transducer to receive sound, the principles are the same. Received sound vibrations at the elements will be converted to an electric signal. Adjust the amplitude and delays of the received signal on each element, and you'll be able to receive from a chosen angular direction. If you transmit and receive along narrow beams in several adjacent directions and combine the received echo data, you can create an ultrasound image.

Radiation patterns. From top left: 1: Single element, 2: Two elements, 3: 20 elements, 4: 20 elements focused, 5: 20 elements focused and with hanning apodization (lowers the sidelobes at the expense of broader main lobe), 6: Beam steering, 7: Dynamic focusing (Can only be achieved on receive. The receive focus is following the pulse as it propagates), 8: Dynamic focusing and expanding aperture (Can only be achieved on receive. In addition to the focus following the pulse, the aperture expands as the pulse propagates to achieve a uniform lateral resolution throughout the depth of the image)

Beamforming visualization
In a recent student project a wave field simulator based on the Huygens principle was developed by Jon Petter Åsen. The simulator uses the graphics card to simulate and visualize arbitrary wave fields in real-time. It is described in more detail and available for download on its homepage. See below for a demonstration:

Research activities
The research activity the last years have mainly been within parallel beamforming. Using parallel beamforming, several receive beams are formed for each transmit event to increase the frame rate. Without any compensation, this technique is known to produce artefacts in the images. The following articles addresses the problems of parallel beamforming:

Bjåstad, Tore; Aase, Svein Arne; Torp, Hans. Velocity Sensitivity Mapping in Tissue Doppler Imaging. Ultrasonics Symposium 2005; 18.09.2005 - 21.09.2005

Hergum, Torbjørn; Bjåstad, Tore; Kristoffersen, Kjell; Torp, Hans. Parallel Beamforming using Synthetic Transmit Beams. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 2007;54(2):271-280

Bjåstad, Tore; Aase, Svein Arne; Torp, Hans. The Impact of Aberration on High Frame Rate Cardiac B-Mode Imaging. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 2007;54(1):32-41

The first article describes the impact of the warping-effect of high frame rate parallel beam forming in tissue Doppler imaging. The second article describes a method that removes the artefacts of parallel beamforming. The latter article describes the impact of aberration on parallel beamforming, and also shows that traditional compensation methods does not suffice in the presence of aberration.

Illustrasjonsbilde/FOTO

Hans Torp MSc, PhD

Professor

Phone: +47 728 28098
E-mail: hans.torp@ntnu.no
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