Time-resolved 3D imaging opportunities with XMPI at ForMAX.

X-rays are commonly used in imaging experiments due to their penetration power, which enables non-destructive resolution of internal structures in samples that are opaque to visible light. Time-resolved X-ray tomography is the state-of-the-art method for obtaining volumetric 4D (3D + time) information by rotating the sample and acquiring projections from different angular viewpoints over time. This method enables studies to address a plethora of research questions across various scientific disciplines. However, it has several limitations, such as incompatibility with single-shot experiments, challenges in rotating complex sample environments that restrict the achievable rotation speed or range, and the introduction of centrifugal forces that can affect the sample's dynamics. These limitations can hinder and even preclude the study of certain dynamics. Here, we present an implementation of an alternative approach, X-ray multi-projection imaging (XMPI), which eliminates the need for sample rotation. Instead, the direct incident X-ray beam is split into beamlets using beam splitting X-ray optics. These beamlets intersect at the sample position from different angular viewpoints, allowing multiple projections to be acquired simultaneously. We commissioned this setup at the ForMAX beamline at MAX IV, the first operational diffraction-limited storage ring. We present projections acquired from two different sample systems - fibers under mechanical load and particle suspension in multiphase flow - with distinct spatial and temporal resolution requirements. We demonstrate the capabilities of the ForMAX XMPI setup using the detector's full analog-to-digital converter range for the relevant sample-driven spatiotemporal resolutions: (i) at least 12.5 kHz frame rates with 4 µm pixel sizes (fibers) and (ii) 40 Hz acquisitions with 1.3 µm pixel sizes (multiphase flows). The presented setup and results form the basis for a permanent XMPI endstation at ForMAX, offering flexibility to adapt to the spatiotemporal requirements of the studied dynamics.