Quantification of the relative contribution of phase aberration and reverberation in transcranial ultrasound imaging: an experimentally calibrated fullwave study in 2D and 3D.

The skull significantly aberrates ultrasound imaging pulses due to its acoustic properties and morphology. However, in addition to aberration of sound waves, the large speed of sound and density mismatch between soft tissue and bone is responsible for multiple reverberations between tissue interfaces and the transducer. Even though a significant amount of research has been dedicated to measuring, characterizing, and correcting the phase aberration caused by the skull, comparatively few results exist on multiple reverberation. The objective of this paper is to quantify reverberation clutter in the brain and to compare degradation from clutter and aberration.A full-wave equation simulating nonlinear propagation in a heterogeneous medium is solved numerically to explore the degrading effects of the human skull. Simulations were performed using isovelocity and clutter subtraction simulations to compare the relative contributions of reverberation and aberration on point spread function degradation.From the performed simulations, it is shown that (a) reverberation is significant in transcranial imaging due to the inclusion of both transmit and receive pulses during imaging, (b) the effect of aberration on image degradation is independent of target brightness whereas the effect of reverberation is dependent on target brightness, (c) reverberation is depth dependent, and (d) the microstructure has little impact on overall reverberation properties in thin skull regions.From this study, it shown that to further improve transcranial ultrasound imaging, especially with respect to lower amplitude and shallower targets, both aberration and reverberation should be addressed.