  Ultrafast imaging modes, such as coherent plane-wave compounding (CPWC),
capture a large field of view in a single pulse-echo measurement using parallel
receive focusing. The number of foci or, equivalently, the number of volume
elements (voxels) in the image determines the computational costs and the
memory consumption of the image formation. Herein, 120{\deg} rhombic grids are
proposed to specify the voxel positions and reduce the number of voxels in
comparison to orthogonal grids. The proposed grids derive from the bivariate
sampling theorem and the spectral properties of the images formed by the
delay-and-sum algorithm in CPWC. A phantom experiment validated the proposed
grids and showed reductions in the number of voxels by 81.4 % and 14.7 % in
comparison to the usual and optimal orthogonal grids, respectively. Mean
structural similarity indices above 96.6 % and relative root mean-squared
errors below 6.8 % confirmed the visual equivalence of all images after
interpolations to the usual orthogonal grid.
