## 1 Introduction  
High frame rate imaging has recently gained increased attention in the field of echocardiography given its potential to reveal new areas of myocardial mechanics and blood flow analysis [1]. Among high frame rate imaging approaches, plane wave or diverging wave imaging are popular research topics due to their capacity to produce very high frame rates by scanning a given field-of-view (i.e., a 90-degree sector) with only a few transmissions [2; 3; 4]. However, the signal-to-noise ratio (SNR), spatial and contrast resolution of the resulting images are degraded due to the lack of focusing. To compensate this limitation, spatial coherent compounding is generally required in which the same region is interrogated several times from different directions and the final image is an average of all acquisitions [5]. As a drawback, the effective gain in frame rate drops by a factor equal to the number of the compounded images. Moreover, motion artifacts can often occur during compound. Additionally, plane waves or diverging waves spread energy over a large area leading to small acoustic pressure amplitudes. Hence, it is technically challenging to make harmonic imaging. As an alternative, multi-line transmit beamforming (MLT) has also been proposed [6; 7]. In this approach, multiple focused beams are simultaneously transmitted into different directions leading to a gain in frame rate equal to the number of MLT beams. Typically, to simultaneously obtain multiple focused beams, the pulses that would be applied to individual elements to sequentially generate focused beams at different directions during several transmit events can be literally superimposed and be applied to those elements during a single transmit event. As an example, Figure 1 shows the pulses that would be used to generate four ultrasound beams either sequentially (Figure 1a-d) or simultaneously (i.e., 4-MLT) (Figure 1e). Figure 1f presents the transmit beam pattern corresponding to a case of 4 MLT (Figure 1e). In contrast to plane wave/diverging wave imaging, MLT utilizes focused beams. This implies that the resulting SNR and spatial resolution [8; 9] can be preserved as well as the possibility of a second harmonic imaging [10]. Despite these advantages, MLT beams may potentially introduce crosstalk that would appear as ghost-like artifacts on the images (Figure 2a). Intrinsically, crosstalk artifacts are the results of the interference between MLT beams in different directions. It has been demonstrated that such crosstalk artifacts can be largely suppressed by using a Tukey (\(\alpha\) = 0.5)-Tukey (\(\alpha\) = 0.5) (TT) transmit and receive apodization scheme; so that the resulting MLT images look competitive to the conventional single line transmit beamforming (SLT) (Figure 2b) [8; 9; 11]. Nonetheless, despite these promising results, residual crosstalk artifacts can sometimes be detected [9]. Recently, alternative approaches, such as minimum variance (MV) receive beamforming [12], low complexity adaptive (LCA) receive apodization [13], and filtered delay multiply and sum (F-DMAS) [14] have been proposed to reduce receive crosstalk. Indeed, MV adaptive beamforming could not obtain the same crosstalk reduction as simply using a Tukey apodization when received, though a better spatial resolution could be obtained [12]. Similar, the LCA adaptive apodization method using a modified predefined apodization bank could have slightly better crosstalk reduction while improving the contrast and spatial resolution, but artifacts remained visible when hyperechoic structures were presented (for instance the pericardium) [13]. The F-DMAS method could provide a better receive crosstalk suppression but the contrast-to-noise ratio would be degraded [14]. Nonetheless, more attempts for better reduction of crosstalk remain desired.