  The field of contrast-enhanced ultrasound (CEUS) combines nonlinear, resonant
microbubbles (MBs) with dedicated pulse sequences to reveal the vascular
function of organs. Clinical ultrasound contrast agents consist of polydisperse
MB suspensions with diameters ranging from 0.5 to 10 um and resonance
frequencies ranging from 1 to 15 MHz. As a result, just a small fraction of MBs
resonates at a given ultrasound frequency. MB suspensions with narrow size
distributions can be tuned for a specific imaging frequency, boost CEUS
sensitivity and enable deeper vascular imaging. However, their enhanced
nonlinear behavior makes imaging susceptible to nonlinear wave propagation
artifacts. Here, we numerically investigate the impact of the acoustic
wavefront shape on the imaging of nonlinear, monodisperse MBs. Specifically,
our approach relies on an extension of the Iterative Nonlinear Contrast Source
(INCS) method that accounts for all nonlinear effects in CEUS. We demonstrate
that supersonic x-shaped wavefronts referred to as x-waves can be used to
generate ultrasound images of monodisperse MBs without nonlinear wave
propagation artifacts. On the contrary, imaging based on focused, planar and
diverging wavefronts leads to significant nonlinear artifacts. Taken together,
our results show that x-waves can harness the full potential of monodisperse
MBs by enabling their sensitive and specific detection in a tissue context.
