Characterization of the spatial coherence function under plane-wave transmission: Simulation and experimental...

The spatial coherence function is widely used in ultrafast ultrasound imaging to characterize tissue microstructure and anisotropy. In coherent plane wave compounding, however, systematic characterization and experimental validation of how angular compounding and imaging depth jointly govern the effective transmit aperture and the resulting measured coherence remain limited. The spatial coherence function in plane wave imaging is revisited through theoretical analysis, simulations, and experiments. In a one-dimensional linear-array configuration, an expression for the spatial coherence function of synthetically focused signals obtained via coherent compounding is derived. This expression links the measured coherence to the scatterers' distribution and to an effective transmit aperture whose lateral extent is governed by the physical probe aperture and by a synthetic aperture that scales with imaging depth and the maximum steering angle. The influence of angular range, number of plane waves, and depth is then examined. The coherence-support width increases with both angular range and depth, whereas the angular step size mainly affects sampling-related modulation within the coherence curve. These findings provide intuitive insights and practical guidance for designing ultrafast imaging protocols to assess tissue anisotropy, and the framework can serve as a tool for optimizing coherence-based methods in high-frame-rate applications such as cardiac fiber orientation estimation.