Toward real-time shear-wave imaging: Ultradense magnetic sources enable rapid diffuse field correlations

Perfectly diffuse wavefields are the underlying assumption for noise correlation tomography in seismology, non-destructive testing and elastography. However, perfectly diffuse fields are rarely encountered in real world applications. We show that homogeneously distributed magnetic micro-particles allow to instantaneously generate a diffuse wavefield, which is imaged using a clinical probe connected to a fully-programmable ultrasound scanner. The particles are placed inside a bi-layered hydrogel and act as elastic wave sources on excitation by a magnetic pulse. Using ultrafast ultrasound imaging coupled to phase tracking, the diffuse elastic wave-field is imaged. This allows measuring the local wave velocity everywhere on the image through noise-correlation algorithms inspired by seismology. Thanks to this instantaneous diffuse wavefield, a very short acquisition time is sufficient to retrieve the wave speed contrast of the bilayered phantom. The correlation time window can be shrunk down to 3 time samples, which we show in a numerical simulation mimicking the experimental conditions. Our experimental and numerical results are consistent with theoretical predictions made by information theory, and pave the way for real-time elasticity imaging. This is of particular interest for medical treatment monitoring through real-time tissue elasticity assessment, and is applicable in related fields such as seismology or non-destructive testing.