Abstract
Recent progress in adaptive beamforming techniques for medical ultrasound has shown that current resolution limits can be surpassed. One method of obtaining improved lateral resolution is the Minimum Variance (MV) beamformer. The frequency domain implementation of this method effectively divides the broadband ultrasound signals into sub-bands (MVS) to conform with the narrow-band assumption of the original MV theory. This approach is investigated here using experimental Synthetic Aperture (SA) data from wire and cyst phantoms. A 7MHz linear array transducer is used with the SARUS experimental ultrasound scanner for the data acquisition. The lateral resolution and the contrast obtained, are evaluated and compared with those from the conventional Delay-and-Sum (DAS) beamformer and the MV temporal implementation (MVT). From the wire phantom the Full-Width-at-Half-Maximum (FWHM) measured at a depth of 52mm, is 16.7μm (0.08λ) for both MV methods, while the corresponding values for the DAS case are at least 24 times higher. The measured Peak-Side-lobe-Level (PSL) may reach −41dB using the MVS approach, while the values from the DAS and MVT beamforming are above −24dB and −33dB, respectively. From the cyst phantom, the power ratio (PR), the contrast-to-noise ratio (CNR), and the speckle signal-to-noise ratio (sSNR) measured at a depth of 30mm are at best similar for MVS and DAS, with values ranging between −29dB and −30dB, 1.94 and 2.05, and 2.16 and 2.27 respectively. In conclusion the MVS beamformer is not suitable for imaging continuous targets, and significant resolution gains were obtained only for isolated targets.
Highlights
Adaptive beamforming techniques have been used for decades in numerous applications of array processing [1,2,3,4] in fields such as sonar, radar, and seismology
Beamformed responses of individual wire targets at increasing depths are shown in Fig. 1 for Boxcar, Hanning, MVT, and Minimum Variance Sub-band (MVS) apodizations
MVS responses with two different L values and a single MVT case were selected for display
Summary
Adaptive beamforming techniques have been used for decades in numerous applications of array processing [1,2,3,4] in fields such as sonar, radar, and seismology. Adaptive beamformers aim to maximize the signal strength from a particular location and suppress signals from all other locations. This is accomplished by processing the received responses of an array to obtain constructive and destructive interference respectively. Such research includes the linearly constrained adaptive beamformer [9,10], the adaptive beamformers suggested by Viola and Walker [11], and the Minimum Variance (MV) beamformer [12,13,14,15] The latter was originally developed by Capon [16] for use with seismic arrays with the objective of localizing earthquakes with greater precision. The MV beamformer is intended to provide unit gain in a selected direction and minimize the signal power for all other directions that are normally contributions from side-lobes
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