## 1 Introduction  
Ultrafast ultrasound imaging uses plane-wave transmissions instead of the conventional line-by-line focused beam transmissions, increasing the frame rate by at least a factor of 100 (typically >1000 frames per second) [1; 2]. This ultrafast imaging technology was an essential breakthrough for the field of Shear Wave Elastography (SWE), as it allowed real-time imaging of shear waves in soft tissues with a high temporal resolution [3; 4; 5]. Because of this, the technique was almost instantaneously applied and therefore less sensitive to respiratory and/or cardiac motion. This allowed local quantitative estimates of wave speed and therefore of tissue stiffness [6]. Initially, shear waveswere generated with a transient vibration originating from an external mechanical vibrator [3; 4]. However, as these vibrators were challenging to integrate in daily clinical practice, the excitation source was changed into a remote palpation induced by a radiation force of focused ultrasonic beam(s), unifying the shear wave excitation source and ultrafast imaging modality together in the ultrasound transducer [5; 7; 8; 9]. At the beginning, the ultrafast frame rates came at the cost of reduced image contrast and resolution compared to conventional transmissions as the transmit focusing step is skipped in the ultrafast imaging modality. However, this limitation was overcome by introducing coherent plane wave compounding [4; 10], which consists of sending out multiple tilted and non-tilted plane waves into the medium and coherently summing the backscattered echoes to compute the full image. In this manner, the image quality is improved compared to single plane wave imaging while still maintaining sufficiently high frame rates [10]. The concept of compounding has been applied to different ultrasound modalities [11; 12; 13; 14], and has become a key feature of ultrafast ultrasound imaging. Ultrafast imaging in SWE to assess tissue stiffness has been clinically applied in several areas such as breast cancer diagnosis [15] and liver fibrosis staging [16]. The ability of ultrafast imaging--with or without plane wave compounding--in displaying and characterizing the true biomechanical shear wave propagation has not been well studied yet, to the best of our knowledge. We are particularly interested in the performance of ultrafast imaging in tissues with thin and layered geometries and other intricate anisotropic material properties, as complex shear wave propagation phenomena such as wave guiding, mode conversions and dispersion are expected to arise [17; 18]. These wave features will complicate shear wave visualization, characterization and interpretation, eventually affecting SWE-based stiffness estimation. This may be especially true when plane wave compounding is applied, as the compounded image fuses temporal characteristics of the propagating shear wave at different time points. Indeed, a recent study in ex vivo thoracic aorta [19] has experimentally shown that certain SWE settings, such as pushing length and number of compounding angles, influenced the technique's accuracy to estimate phase velocity-based tissue stiffness. Therefore, the objective of this work was to establish a flexible framework that allows us to investigate the performance of ultrafast imaging in SWE in accurately displaying and characterizing the true biomechanical shear wave propagation. As actual SWE experiments do not provide access to a ground truth for imaged shear wave propagation, a multiphysics modeling approach combining computational solid mechanics (CSM) of the shear wave propagation [20; 21; 22] with ultrasound (US) modeling of ultrafast imaging was used for this purpose. The resulting wave mechanics from CSM provided the true mechanical shear wave propagation whereas the virtual images represented the imaged shear wave propagation. The multiphysics model was employed in combination with SWE experiments, for validation purposes.