  Simulations of three dimensional ultrasound propagation in heterogeneous
media are computationally intensive due to the constraints arising from the
large size of the domain, which is on the order of hundreds of wavelengths, and
the small size of scatterers, which are much smaller than a wavelength.
Consequently, three dimensional ultrasound imaging simulations are currently
based on models that simplify the propagation physics. Here the full three
dimensional wave physics is simulated with finite differences to generate
ultrasound images of the human body based directly on the first principles of
propagation and backscattering. The Visible Human project, a 3D data set of the
human body that was generated with photographs of 0.33 mm cryosections, is
converted into 3D acoustical maps. A full-wave nonlinear acoustic simulation
tool is used to propagate ultrasound into the liver with a 2D intercostal
ultrasound array in a $93 \times 39 \times 22$ mm domain with $6\times10^8$
points. Imaging metrics, based on the beamplots, root-mean-square phase
aberration, spatial coherence lengths, and contrast-to-noise ratio are used to
characterize the image quality. It is shown that the harmonic image quality is
better than the fundamental image quality due, in part, to a narrower beam
profile. The root-mean-square estimate of aberration after propagation through
the simulated body wall is shown to be low (23.4 ns), consistently with
previous reports of aberration measured experimentally in a human body wall.
The spatial coherence measured at the transducer surface indicates that a
transducer array element size of $<0.81 \lambda$ would be required to fully
sample the acoustic field. These simulated three dimensional ultrasound images
based directly on propagation physics provide a platform to investigate the
sources of image degradation in three dimensions (included in Part II).
