# A Sibelobe Suppressing Beamformer for Coherent Plane Wave Compounding
## 1 Introduction  
The emergence of ultrafast ultrasound has been made possible by the development of new imaging modalities [1; 2; 3]. Plane wave imaging (PWI) is a promising one which has attracted great attention. In the PWI, the entire region of interest (ROI) can be scanned using the full aperture in a single shot. Although plane waves enable covering a large field quickly, their imaging results have higher sidelobes compared with the conventional focused imaging. An example of sidelobes for a point target is given in Figure 1. High sidelobes may lower imaging contrast [4; 5] and make it difficult to detect the low scattering targets, such as small anechoic cysts. In the tissue characterization, high sidelobes may decrease the imaging resolution [6].  
Recently the concept of coherent plane wave compounding (CPWC) [7] has been proposed to improve the imaging quality of the single-shot PWI. It has been proved that CPWC improves imaging lateral resolution and contrast-to-noise ratio (CNR) while still preserving a speed advantage over a focused linear-can acquisition [8]. However, the sidelobes from different angles remain a problem and affect the imaging contrast.  
In general, a sidelobe suppression method suitable for PWI demands rapidity. To date, few beamformers can meet this requirement [9]. Generally, the sidelobe level can be reduced by applying apodization techniques, i.e., the Hamming window. However, the predefined weighting coefficients result in a wider mainlobe and thus reduce the lateral resolution which is intolerable in PWI. Adaptive beamformers [10], such as the Capon beamformer or minimum variance (MV) beamformer [11], have been proposed to compute the optimal weighting coefficients by using the information from the received raw channel data continually. Though effective for sidelobe rejection, this method also has several drawbacks for PWI. First, the computation load of the MV beamformer is intensive and not suitable in real-time processing [12]. Second, the MV beamformer lacks robustness and artifacts can hardly be avoided [13]. Other sidelobe suppression methods, such as dynamic focused reception, deconvolution, coherence weighting prior to the beam formation, and so on [14; 15; 16], have also been suggested. Most of these methods have been proven capable of improving the contrast, but they all need additional intensive computation on the radio-frequency (RF) data and are not very suitable for PWI or CPWC.  
In [17], a singular value decomposition (SVD) filter is proposed for clutter rejection in Doppler and micro-bubble imaging. A set of frames sampled in a short time possesses the similar background signal which has high coherence. By using the SVD, researchers stripped out the high coherent component and left the flowing information. Inspired by this work, we introduce a new low-complexity sidelobe suppression beamformer applied to CPWC in this paper. It is the first time that the SVD filter is introduced in sidelobe suppression in PWI. In CPWC, the frames from different angles are analogous to a time series to some degree. As sidelobes from different angles are uncorrelated, a coherence-based SVD filter is adopted before the final coherent summation. For the sake of efficiency, the three-dimensional dataset is first transformed into a lower dimensional form, after which the computational increase is only related to the number of angles and acceptable for real-time imaging.  
We assessed this new approach by simulation with the Field II program and phantom experiments with a Verasonics system on a commercial phantom. Results show that our method can reduce the sidelobes effectively with low additional computation. Additionally, its imaging quality including the lateral resolution is not affected compared to the original CPWC.