## 1 Introduction  
Limited resolution and a restricted field of view (FOV) are two of the main challenges in ultrasound (US) imaging that, in principle, are fundamentally limited by the extent of the transmitting and receiving apertures. The image lateral resolution of ultrasonography is diffraction limited and mostly depends on the wavelength of the transmission wave, the imaging depth and the aperture size (measured by the F-number) [1]. However, since high frequency acoustic waves are easily absorbed and attenuated by tissues, the use of low frequencies is the only choice when the imaging region is located in deep areas in the body. Consequently, a large aperture size is desired to decrease the F-number and improve resolution and FOV. Nevertheless, it is not clear that a large aperture will overcome the expected resolution loss through depth in the presence of acoustically heterogeneous tissues [2]. Inhomogeneities and tissue layers with different acoustic properties cause phase errors restricting the improvements provided by large arrays [3]. Transabdominal US imaging is particularly difficult in obese patients because of the increased imaging depth with longer attenuation paths and the presence of clutter associated with boundaries between regions with dissimilar properties [4; 5]. Difficult abdominal imaging tasks require an array design that provides sufficient penetration for the chosen target depth, adequate resolution for the diagnostic task and tolerance to acoustic clutter effects that degrade the backscattered echo signals. The need for the development of large arrays for high resolution abdominal diagnostic imaging has been already suggested by previous studies, where a large array was synthesized through a synthetic aperture method by combining the received data from multiple positions of the same probe [6; 7]. However, in such a method, the final coherent aperture is limited by the long acquisition times, and the image quality is very sensitive to calibration errors and noise in the tracking system. Despite a large aperture being desirable, the practical aperture size is often limited by the complexity and cost of scanners and probes based on large channel count arrays and the low flexibility for some applications. Using collections of conventional existing arrays to extend the aperture may be a way to approach this problem. Indeed, the use of multiple conventionally sized arrays to create a large aperture instead of using a single big array may be more flexible for diverse applications, e.g., for intercostal imaging, where the acoustic windows are narrow. In practice, multiple probes can be manipulated using a single and potentially adjustable holder that allows the operator to hold multiple probes with only one hand while keeping them directed to the same region of interest [8]. In that first study, images were compounded incoherently, which, as is well known, allows the exploitation of a diversity of points of views, but does not directly achieve the gains in resolution and sensitivity that could be realised if the multiple probes were fully and coherently combined into a one large effective aperture. Hence, coherent multi-transducer ultrasound (CoMTUS) imaging, which combines all radio frequency (RF) data received by multiple synchronized transducers that take turns transmitting plane waves (PWs) into a common FOV, was introduced by our group to further improve ultrasound (US) imaging performance [9; 10]. CoMTUS yields images with an extended FOV and higher resolution, while preserving, at the same time, a reasonably high frame rate. It provides a first step toward large aperture imaging using independently placed transducers to form discontinuous extended apertures. In contrast to other proposed techniques [6; 7], CoMTUS achieves the subwavelength localization accuracy required to combine information from multiple transducer poses, without the use of any external tracking device by optimizing the beamforming parameters [10].