Time-domain Brillouin scattering (TDBS) uses ultrashort laser pulses to (i) generate coherent acoustic pulses of picoseconds duration in a solid sample and (ii) follow their propagation in order to image material inhomogeneities with the axial resolution that can be deeply sub-optical, to nm-scale, and the lateral one down to the optical diffraction limit (half the optical wavelength of the probe laser). TDBS permits highly resolved 3D-imaging of grains in polycrystalline transparent samples with unlimited lateral sizes and thicknesses of at least 10 m also when samples are orientationally textured and/or located in devices permitting access along one direction and from one side only. This optical technique presents, accordingly, clear advantages compared to any x-ray based computed tomography (neither back-projection algorithm nor multiple viewpoints of the sample are needed) and classical spectroscopic methods. Here, we applied TDBS to the 3D-imaging of a sample of polycrystalline water ice containing two high-pressure phases. The imaging, accomplished via a simultaneous detection of quasi-longitudinal and quasi-shear waves, provided shape, coordinates, phase content, and crystallographic orientation of resolved crystallites in a common coordinate system. Monitoring of acoustic pulses simultaneously propagating in two neighboring grains provided a new tool for the localization of grain boundaries.