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. 2020 Oct;67(10):2057-2068.
doi: 10.1109/TUFFC.2020.2998979. Epub 2020 Jun 1.

Parallel Receive Beamforming Improves the Performance of Focused Transmit-Based Single-Track Location Shear Wave Elastography

Rifat Ahmed, Marvin M Doyley

Parallel Receive Beamforming Improves the Performance of Focused Transmit-Based Single-Track Location Shear Wave Elastography

Rifat Ahmed et al. IEEE Trans Ultrason Ferroelectr Freq Control. 2020 Oct.

Abstract

Single-track location shear wave elastography (STL-SWEI) is robust against speckle-induced noise in shear wave speed (SWS) estimates; however, it is not immune to other incoherent sources of noise (such as electronic noise) that increases the variance in SWS estimates. Although estimation averaging enabled by parallel receive beamforming adequately suppresses these noise sources, these beamforming techniques often rely on broad transmit beams (plane or diverging). While broad beam approaches, such as plane-wave imaging, are becoming ubiquitous in research ultrasound systems, clinical systems usually employ focused transmit beams due to compatibility with hardware beamforming and deeper penetration. Consequently, improving the noise robustness of focused transmit-based STL-SWEI may enable easier translation to clinical scenarios. In this article, we experimentally evaluated the performance of parallel beamforming for STL-SWEI using fixed or multiple transmit focus. By imaging tissue-mimicking phantoms, we found that parallel beamforming improved the focal zone elastographic signal-to-noise ratio (SNRe) by 40.9%. For a receive line spacing equivalent to transducer pitch, averaging estimates from three parallel lines produced peak SNRe at the focal zone (25 mm), while, at the shallower regions (< 20 mm), a larger number of parallel lines (>7) were needed. Increasing the beamforming line density by a factor of 8 increased the focal zone SNRe only by 13.2%. When SWS quantification was desirable at a fixed depth (such as within the push focal depth), using a deeper tracking focal zone enabled higher parallel line count and improved the peak SNRe by 33%. The multifocusing strategy produced a lower SNRe than the single-focus configurations. For a fixed tracking focal zone, a depth-dependent averaging based on the simulated transmit intensity adequately accounted for the transmit beamwidth. The results in this work demonstrated that STL-SWEI can be implemented using focused transmit beams with robust noise-suppression capability.

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Figures

Fig. 1:
Fig. 1:
(a) Pulse sequence of STL-SWEI using parallel receive beamforming. SWS was reconstructed by tracking the shear waves generated from two push beams at a common location. Parallel receive lines were used to track the shear waves at multiple locations simultaneously and the redundant SWS estimates obtained from these parallel locations were averaged. The pair of push-detect ensembles were swept laterally to form a SWS image. The parallel receive lines were reconstructed from either (b) fixed focus, (c) multi-focus or (d) angular plane wave transmissions. Parallel receive lines from four temporally consecutive pulses were coherently summed. In (b), these four pulses were identical. In (c) and (d), they had different focal depths or angular wavefront tilts, respectively.
Fig. 2:
Fig. 2:
Quality of SWS estimates as a function of receive beam position. SWS maps obtained from a homogeneous elastic phantom using STL-SWEI with receive beams at different relative positions for (a) single focus and (b) multi-focus transmits. The receive beam position is defined relative to the axis of the transmit beams. The SWS maps are spatially co-registered and are shown prior to averaging. The depth-dependent SNRe of the SWS maps at different receive beam positions for (c) single focus and (d) multi-focus imaging.
Fig. 3:
Fig. 3:
Depth-dependent averaging based on transmit simulation. (a) and (b) Normalized transmit beam intensity and contour plots, respectively, simulated using FIELD II for an aperture with F#=2.0 in a medium with αA = 0.5 dB/cm/MHz. (c) Number of parallel lines that maximizes SNRe (optimum parallel lines) as a function of depth, which were either predicted from different intensity contours or evaluated experimentally on a elastically homogeneous phantom with αA = 0.5 dB/cm/MHz. (d)-(f) show the depth-dependent SNRe in three homogeneous phantoms achieved using DDA based on a −12 dB intensity contour. The maximum achievable SNRe at each depth is also displayed and was computed by evaluating different levels of track line averaging (within a range of 1 to 15). Each line represents a mean over five experiments and the shaded areas represent their standard deviations.
Fig. 4:
Fig. 4:
Comparison of single focus and multi-focus sequences. For both sequences, depth-dependent SNRe is displayed for different levels of track line averaging (N = 1, 3, 5 and 11) and DDA. Data was acquired from a homogeneous phantom with αA = 0.5 dB/cm/MHz. Each line represents a mean over five experiments and the shaded areas represent their standard deviations.
Fig. 5:
Fig. 5:
The impact of beamforming line density on the SNRe of SWS maps. Data was acquired using single-focus sequence (25 mm focus) and DDA was not applied during processing. The SNRe is reported as a function of total span of the track lines used in the SWS averaging. SNRe was evaluated within a 5 mm axial kernel at depths of 15, 20, and 25 mm (focal zone) as shown in the columns left to right, respectively. Results are reported for different beamforming line spacings (ΔL=pitch, pitch/2, pitch/4, and pitch/8). Each line represents a mean over five experiments and the shaded areas represent their standard deviations.
Fig. 6:
Fig. 6:
Effect of averaging level on the SNRe for different transmit F#. Data was acquired using single-focus sequence (25 mm focus) and DDA was not applied during processing. Results are reported for three different depths. SNRe was computed using a 5 mm axial kernel. The shaded areas represent standard deviation over five experiments.
Fig. 7:
Fig. 7:
The impact of using a focal zone deeper than the ROI. SNRe was evaluated at a fixed depth of 15 mm (within a 5 mm axial kernel) as a function of track line averaging. DDA was not applied during processing and the push beam was focused at 15 mm. Results are shown for single-focus acquisitions with different tracking focal depths (15, 20, 25, and 30 mm) and pSTL-SWEI. The shaded areas represent standard deviation over five experiments.
Fig. 8:
Fig. 8:
Elastograms obtained from three homogeneous phantoms using fixed focus STL-SWEI and pSTL-SWEI. First, second and third row show elastograms corresponding to a elastic phantom with αA = 0.5 dB/cm/MHz, a elastic phantom with αA = 0.7 dB/cm/MHz, and a viscoelastic phantom, respectively. Columns left to right, represent elastograms obtained using fixed focused STL-SWEI with no track line averaging, DDA, and pSTL-SWEI with 15 track line averaging, respectively.
Fig. 9:
Fig. 9:
Comparison of single focus STL-SWEI and pSTL-SWEI in terms of depth-dependent SNRe computed from elastograms in fig. 8. (a)-(c) show analysis performed using an elastic phantom with αA = 0.5 dB/cm/MHz, an elastic phantom with αA = 0.7 dB/cm/MHz, and a viscoelastic phantom, respectively. Shaded areas represent standard deviation over five experiments.
Fig. 10:
Fig. 10:
Elastograms of an inclusion phantom obtained using STL-SWEI and pSTL-SWEI. Elastograms in the rows 1 to 3 were obtained using STL-SWEI with single tracking focus at 15 mm, 25 mm, and pSTL-SWEI, respectively. Elastograms in the columns 1 to 4 were obtained using 1, 3, 5, and 15 track line averaging, respectively. Elastograms in the final column were computed using DDA for the single-focus sequences. In all cases, the push focus was set at 15 mm.
Fig. 11:
Fig. 11:
CNRe computed from the elastograms in fig. 10. (a) ROI used for the CNRe calculation. The circular region with 4 mm diameter was used as the inclusion region and the two half-circular regions were used as the background region. (b) CNRe of STL-SWEI elastograms obtained using STL-SWEI with single focus (SF) at 15 mm, 25 mm, and pSTL-SWEI. For each method, results are shown for 1, 3, 5, 7, 11, 15, and depth-dependent (only for single focus) track line averaging. The errorbars represent standard deviation over five experiments.
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