PZFlex COMBINED TRANSDUCER AND NONLINEAR TISSUE PROPAGATION SIMULATIONS
Ultrasonic imaging system evaluation is often based on models of the
transducer as a distribution of baffled piston sources, and of the tissue as a
homogeneous, linear acoustic medium, e.g., Jensen’s Field code. In
reality, these are fairly gross idealizations, since the transducer exhibits
more complicated response modes and real tissue is inhomogeneous and
nonlinear. Greater model fidelity would be useful, especially in the
context of transducer design qualification, second harmonic imaging, and
acoustic power indices. To this end we combine 2D finite element
models of transducer dynamics with highly accurate 2D finite difference
propagation models in the large-scale inhomogeneous tissue crosssections.
Transducer models employ the time-domain code, PZFlex, and
tissue models utilize a new pseudospectral solver to be included in
PZFlex. The pseudospectral algorithm solves the inhomogeneous
acoustic wave equation using FFTs for high order approximation of the
spatial differential operator and a fourth-order, explicit time integrator.
Second-order (B/A) nonlinearity and frequency-accurate, causal
absorption are included. We describe the algorithmic and modeling
issues, and present a suite of simulations in lossy, nonlinear abdominal
cross sections and tissue showing coupling of the 1D medical array to the
tissue model and scattering from deeper inhomogeneities and back to the
transducer. In contrast to paraxial schemes, like the KZK method, details
of the field transmitted from the transducer and all backscatter within the
model are included. However, models are currently limited to 2D (plane
or axisymmetric) on readily available hardware.
Key Words: Imaging, transducers, tissue, simulations, aberrations, finite
elements, pseudospectral, scattering, second harmonic, acoustic power