aInstitute of Biological Chemistry, Biophysics and Bioengineering, Heriot-Watt University, EH14 4AS Edinburgh, United KingdombSchool of Clinical Sciences, Centre of Cardiovascular Science, University of Edinburgh, EH16 4TJ Edinburgh, United KingdomcCenter for Fast Ultrasound Imaging, Department of Electrical Engineering, Technical University of Denmark, DK-2800 Lyngby, Denmark
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HIGHLIGHTSExperimental performance of the sub-band Minimum Variance beamformer is studied.Lateral FWHM was measured to 16.7 μm (λ/12), from isolated wire-targets.This result demonstrates 24-fold improvement compared to conventional beamforming.No resolution benefits were found for imaging continuous targets.No significant differences were found between temporal and sub-band MV beamforming.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 7 MHz 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 52 mm, is 16.7 Symbolm (0.08Symbol) 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 −41 dB using the MVS approach, while the values from the DAS and MVT beamforming are above −24 dB and −33 dB, 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 30 mm are at best similar for MVS and DAS, with values ranging between −29 dB and −30 dB, 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.