In dynamic focusing in receive, the F-number (F) dictates the number of elements (aperture) used for beam-forming:  
\[F=\frac{d}{l_{app}} \tag{1}\]  
where \(d\) and \(l_{app}\) are the depth of the reconstruction point and aperture width, respectively. Furthermore, apodization can be applied to weigh the signals (e.g., by Hamm or Hann function) to reconstruct  
Figure 1: Illustrations of: (**a**) a focused acquisition series in which the aperture (active elements in light gray) is shifted to acquire data line-by-line by transmitting a focused ultrasound beam and receiving the reflected signal for each aperture position; (**b**) a plane-wave acquisition, where an unfocused ultrasound is transmitted and reflected signals are received by the full transducer aperture. each point and so expected angle sensitivity and back-scatter signal intensities can be incorporated. Apodization is often applied to increase contrast in B-mode images. Another approach is beam-forming in the Fourier domain by Lu's-fk or Stolt's-fk which are computationally less expensive and can run around 25 times faster compared to DaS [13]. In both methods, 2-D element data in the spatial-temporal domain are transformed to the fk-space. Next, a migration method is applied to transform the data into the complete k-space and finally the migrated data are transformed back into the spatial-temporal domain (Figure 3). A migration method of plane-wave ultrasound was first carried out by [PERSON] et al. from the analysis of limited diffraction beams [11; 12]. [PERSON] et al. introduced the mapping rule throughout the transmission-scattering-receiving process. More recently, [PERSON] et al. proposed another method by adapting Stolt's-fk seismic wave migration to the plane wave ultrasound scenario [13]. In this modification, the plane wave's backscattering is fitted with waves spontaneously radiating from sources. Comparing Stolt's-fk and Lu's-fk, the two methods share a similar spectrum content at small angles, while their spectra differ more significantly in the mapping of large angles. Similar to apodization in DaS, angular weighting can be applied in Fourier domain beam-forming to increase contrast in the final B-mode image [14]. In this method, developed in our group, the Fourier spectrum after the migration (k-space) is multiplied with a template which is designed such that reflected waves originating from directions closely aligned with the plane-wave steering angle pass unaltered, whereas waves from wave directions deviating from the plane-wave propagation direction are attenuated according to a Hann function (Figure 3). This template can be applied for both migration methods (Lu's-fk and Stolt's-fk). Since the beam-forming in plane-wave imaging is executed using software, the design of the beam-forming reconstruction grid (USGrid) is more flexible compared to conventional focused line-by-line scanners in which beam-forming is executed by hardware. Consequently, more lines per pitch can be reconstructed, which might result in more accurate displacement estimates. For focused imaging, it has already been shown that more accurate lateral displacement estimates can be obtained when increasing the RF line density by interpolation [15; 16]. Plane-wave imaging and displacement and strain imaging have been combined in multiple studies. Besides a paper from our group [17] which investigated the effect of line density on displacement  
Figure 2: Schematic overview of delay-and-sum (DaS) beam-forming: for each reconstruction point, the time-of-flight (ToF) can be calculated by the transmit (\(\tau_{\text{T}}\)) and receive time towards each element (\(\tau_{\text{R}}\)). These ToF values are used to delay and sum the element data; if required the data can be weighted (w) by an apodization function. \(\theta_{\text{Inc}}\) is the incidence angle of the signal. estimation for DaS, to our knowledge, the effects of different beam-forming strategies and line densities on displacement and strain estimation have not been explored.