This paper presents a method for optimizing parameters affecting the image quality in plane wave imaging. More specifically, the number of emissions and steering angles is optimized to attain the best images with the highest frame rate possible. The method is applied to a specific problem, where image quality for a λ -pitch transducer is compared with a λ /2-pitch transducer. Grating lobe artifacts for λ -pitch transducers degrade the contrast in plane wave images, and the impact on frame rate is studied. Field II simulations of plane wave images are made for all combinations of the parameters, and the optimal setup is selected based on Pareto optimality. The optimal setup for a simulated 4.1-MHz λ -pitch transducer uses 61 emissions and a maximum steering angle of 20° for depths from 0 to 60 mm. The achieved lateral full-width at half-maximum (FWHM) is 1.5λ and the contrast is -29 dB for a scatterer at 9 mm ( 24λ ). Using a λ /2-pitch transducer and only 21 emissions within the same angle range, the image quality is improved in terms of contrast, which is -37 dB. For imaging in regions deeper than 25 mm ( 66λ ), only 21 emissions are optimal for both the transducers, resulting in a -36 dB contrast at 34 mm ( 90λ ). Measurements are performed using the experimental SARUS scanner connected to a λ -pitch and λ /2-pitch transducer. A wire phantom and a tissue mimicking phantom containing anechoic cysts are scanned and show the performance using the optimized sequences for the transducers. FWHM is 1.6λ and contrast is -25 dB for a wire at 9 mm using the λ -pitch transducer. For the λ /2-pitch transducer, contrast is -29 dB. In vivo scans of the carotid artery of a healthy volunteer show improved contrast and present fewer artifacts, when using the λ /2-pitch transducer compared with the λ -pitch. It is demonstrated with a frame rate, which is three times higher for the λ /2-pitch transducer.

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Abstract-Synthetic Aperture Sequential Beamforming (SASB) is a technique with low complexity and the ability to yield a more uniform lateral resolution with range. However, the presence of speckle artifacts in ultrasound images degrades the contrast. In conventional imaging speckle is reduced by using spatial compounding at the cost of a reduced frame rate. The objective is to apply spatial compounding to SASB and evaluate if the images have a reduced speckle appearance and thereby an improved image quality in terms of contrast compared to ordinary SASB. Using the simulation software Field II, RF data are acquired for a phantom with cysts at different sizes and scattering levels. 192 scanlines are recorded for five steering angles (0, ±2, ±4 degrees) using a 192 element linear array transducer. SASB is performed for each angle using a rectangular grid in the second stage beamformation. After envelope detection the five second stage images are added to form the compounded image. Using a ProFocus scanner and the 8804 linear array transducer (BK Medical, Herlev, Denmark) measurements of a phantom containing water filled cysts are obtained to validate the simulation results. The setup is the same as in the simulations and SASB second stage beamformation data are processed offline for each of the five angles. Contrast-to-noise ratio (CNR) and speckle-to-noise ratio (SNR) are extracted for the compounded image and the reference image (ordinary SASB). CNR was calculated for the simulated cysts at depths of 40, 50, 60, 70 and 80 mm. On average the CNR was improved by 33.2% compared to the values obtained from the reference image. For regions of increasing depth SNR was on average increased by 9.3%. Results from the simulation were confirmed by calculations on the measured data. CNR of cysts at depths from 18 to 78 mm with a separation of 10 mm was on average improved by 45.9%. On average an improvement of 16.6% in SNR was obtained. The calculations along with visual inspection revealed larger improvements in deeper regions, and the compounded image for the measured phantom showed a 3 mm diameter cyst not detectable in the reference image. Compounding applied to SASB improves CNR and SNR results in images with a reduced speckle appearance. This was shown for simulations and confirmed on measured data.

To obtain accurate blood flow velocity estimates it is important to remove the clutter signal originating from tissue. Conventionally, the clutter signal has been separated from the blood signal based on the difference of their spectral frequencies. However, this approach is not enough for obtaining vector flow measurements, since the spectra overlaps at high beam-to-flow angles. In this work a distinct approach is proposed, where the energy of the velocity spectrum is used to differentiate among the two signals. The energy based method is applied by limiting the amplitude of the velocity spectrum function to a predetermined threshold. The effect of the clutter filtering is evaluated on a plane wave (PW) scan sequence in combination with transverse oscillation (TO) and directional beamforming (DB) for velocity estimation. The performance of the filter is assessed by comparison of the velocity estimates of the proposed filter against a conventional moving average clutter filter. The effect of tissue motion is investigated using a Field II simulation of a straight vessel with moving wall, while the direct effect of the filter on the velocity estimates is evaluated on a CFD model of a carotid bifurcation with a fixed vessel wall. The results show that the proposed filter outperformed the moving average during moving vessel wall conditions, where standard deviations from the velocity magnitudes and angles were kept consistently below 6% and 6 • compared to 63% and 48 • on the moving average filter. The results on the CFD showed that on non-moving conditions the velocity estimates had minor statistical differences with errors on the magnitude of-7.95±10.1% and angles of 0.15±6.65 • for the proposed filter compared to-5.83±9.08% and-0.12±4.48 • .