  An extended aperture has the potential to greatly improve ultrasound imaging
performance. This work extends the effective aperture size by coherently
compounding the received radio frequency data from multiple transducers. A
framework is developed in which an ultrasound imaging system consisting of $N$
synchronized matrix arrays, each with partly shared field of view, take turns
to transmit plane waves. Only one individual transducer transmits at each time
while all $N$ transducers simultaneously receive. The subwavelength
localization accuracy required to combine information from multiple transducers
is achieved without the use of any external tracking device. The method
developed in this study is based on the study of the backscattered echoes
received by the same transducer and resulting from a targeted scatterer point
in the medium insonated by the multiple ultrasound probes of the system. The
current transducer locations along with the speed of sound in the medium are
deduced by optimizing the cross-correlation between these echoes. The method is
demonstrated experimentally in 2-D using ultrasound point and anechoic lesion
phantoms and a first demonstration of a free-hand experiment is also shown.
Results demonstrate that the coherent multi-transducer imaging has the
potential to improve ultrasound image quality, improving resolution and target
detectability. Lateral resolution, contrast and contrast-to-noise ratio
improved from 0.67 mm, -6.708 dB and 0.702, respectively, when using a single
probe, to 0.18 mm, -7.251 dB and 0.721 in the coherent multi-transducer imaging
case.
