  X-ray time-resolved tomography is one of the most popular X-ray techniques to
probe dynamics in three dimensions (3D). Recent developments in time-resolved
tomography opened the possibility of recording kilohertz-rate 3D movies.
However, tomography requires rotating the sample with respect to the X-ray
beam, which prevents characterization of faster structural dynamics. Here, we
present megahertz (MHz) X-ray multi-projection imaging (MHz-XMPI), a technique
capable of recording volumetric information at MHz rates and micrometer
resolution without scanning the sample. We achieved this by harnessing the
unique megahertz pulse structure and intensity of the European X-ray
Free-electron Laser with a combination of novel detection and reconstruction
approaches that do not require sample rotations. Our approach enables
generating multiple X-ray probes that simultaneously record several angular
projections for each pulse in the megahertz pulse burst. We provide a
proof-of-concept demonstration of the MHz-XMPI technique's capability to probe
4D (3D+time) information on stochastic phenomena and non-reproducible processes
three orders of magnitude faster than state-of-the-art time-resolved X-ray
tomography, by generating 3D movies of binary droplet collisions. We anticipate
that MHz-XMPI will enable in-situ and operando studies that were impossible
before, either due to the lack of temporal resolution or because the systems
were opaque (such as for MHz imaging based on optical microscopy).
