The Fullwave simulation tool, which solves a modified Westervelt equation, can model 3D ultrasound propagation in the human body. It includes the effects of nonlinearity, frequency dependent attenuation, and scattering. This high order finite difference simulation has a high dynamic range and ability to include sub-resolution scattering physics enabling it to tackle the computationally challenging problem of generating ultrasound images directly from the first principles of propagation and reflection. Three dimensional acoustical maps of the human body are derived from the Visible Human project, which provides the high degree of anatomical fidelity required to model sources of image degradation such as reverberation clutter and aberration. A three dimensional intercostal imaging scenario shows how the ribs degrade the image quality via reverberation and yet can improve the beam shape via apodization. It is shown that the in situ Mechanical Index (MI) in the human body differs significantly from derated estimates in a homogeneous medium. Due to tissue heterogeneities, the peak MI is close to the transducer and far from the focal region. Finally, we present a numerical model of subresolution displacements by using an impedance flow model applied to a scatterer composed of two elements. This shows that the numerical model can support displacements that are over three orders of magnitude smaller than the grid spacing. Applications to acoustic radiation force, elastography, and shear shock wave tracking are discussed.