###### Abstract  
Coherent multi-transducer ultrasound (CoMTUS) imaging creates an extended effective aperture through the coherent combination of multiple arrays, which results in images with enhanced resolution, extended field-of-view, and higher sensitivity. However, this also creates a large discontinuous effective aperture that presents additional challenges for current beamforming methods. The discontinuities may increase the level of grating and side lobes and degrade contrast. Also, direct transmissions between multiple arrays, happening at certain transducer relative positions, produce undesirable cross-talk artifacts. Hence, the position of the transducers and the scan sequence play key roles in the beamforming algorithm and imaging performance of CoMTUS. This work investigates the role of the distribution of the individual arrays and the scan sequence in the imaging performance of a coherent dual-array system. First, the imaging performance for different configurations was assessed numerically using the point-spread-function, and then optimized settings were tested on a tissue mimicking phantom. Finally, a subset of the proposed optimum imaging schemes was experimentally validated on two synchronized ULA OP-256 systems equipped with identical linear arrays. Results show that CoMTUS imaging performance can be enhanced by optimizing the relative position of the arrays and the scan sequence together, and that the use of apodization can reduce cross-talk artifacts without degrading spatial resolution. Adding weighted compounding further decreases artifacts and helps to compensate for the differences in the brightness across the image. Setting the maximum steering angle according to the spatial configuration of the arrays reduces the sidelobe energy up to 10 dB plus an extra 4 dB reduction is possible when increasing the number of PWs compounded. beamforming; imaging; large-aperture; multi-transducers; plane waves; ultrasound +
Footnote †: journal: Applied  
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An ideal imaging system with a large and flexible aperture would have the potential to overcome the fundamental US limitations and lead to significant imaging improvements. However, fabrication of flexible probes that can conform to the body has not been successfully implemented because coherent image formation would require continuous estimation of the transducer shape. Previous attempts at flexible arrays are mostly restricted to nondestructive testing and evaluation of specimens that are still and with simple geometries [7], and the transducer shape calibration relies on external positional sensors [8]. Alternative methods based on image contrast or entropy optimization have also been investigated in medical US but they have been only tested for small linear arrays and homogeneous media [9; 10]. One way to extend the aperture of the imaging system while keeping some geometrical flexibility is coherent multi-transducer ultrasound (CoMTUS) imaging [11]. CoMTUS enables the use of multiple synchronized arrays, which take turns to transmit plane waves (PWs) into a common FOV, and together acting as one large effective aperture. Coherent combination of all signals received by the extended aperture improves resolution and sensitivity, and extends the FOV, while flexible placement of individual arrays preserves compatibility with different body shapes and parallel operation can preserve time resolution. In contrast to previous works on multiple probes that rely on a fixed geometry [12; 13; 14; 15], CoMTUS utilizes multiple standard US arrays that can be positioned flexibly and combined into an extended dynamically self-calibrating large aperture. This calibration is done by optimizing the beamforming parameters: the average sound speed in the medium, and the location of the transducers [11].