Time-resolved 3D imaging opportunities with XMPI at ForMAX

TL;DR

This work addresses the limitations of time-resolved tomography that rely on rotating samples, which hinder studies of rotation-sensitive dynamics and complex in situ environments. It presents X-ray Multi-Projection Imaging (XMPI) as a rotation-free approach at the ForMAX beamline that uses beam-splitting crystals to produce multiple simultaneous viewpoints at the sample position. The authors demonstrate two regimes, achieving frame rates of at least 12.5 kHz with about 8 µm spatial resolution and 40 Hz with about 2.6 µm resolution, respectively, and show reconstruction using sparse projections with 4D-ONIX and X-Hexplane. They discuss current limitations and future improvements, highlighting the potential for broad 4D imaging applications across rotation-sensitive samples and single-shot dynamics.

Abstract

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 faces 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. We present projections acquired from two different sample systems - fibers under mechanical load and particle suspension in multi-phase flow - with distinct spatial and temporal resolution requirements. We demonstrate the capabilities of the ForMAX XMPI setup using the detector's full dynamical range for the relevant sample-driven spatiotemporal resolutions: i) at least 12.5 kHz framerates with 4 micrometer pixel sizes (fibers) and ii) 40 Hz acquisitions with 1.3 micrometer pixel sizes (multi-phase flows), setting the basis for a permanent XMPI endstation at ForMAX.

Figures (5)

• Figure 1: The XMPI setup at ForMAX. (a) The XMPI setup consists of beam splitters mounted on nanopositioners (i) for the generation of beamlets through spectral and amplitude splitting, that intersect at the sample environment position (ii), and an indirect detector system for each projection (iii). This illustration showcases an example study where the dynamics of interest concern the flow of particles through a capillary. The setup was commissioned upstream of the main sample station of ForMAX by modifying the box-marked segment in the right-hand side beamline sketch. (b) A photograph of the setup in the ForMAX beamline with annotated projections. (c) Examples of samples that can be examined using XMPI: Fiber (left) and fluid samples (right).
• Figure 2: Reconstructed tomography slice of a bamboo rod recorded with each beamlet individually. The dataset was acquired with an acquisition speed of 6 kHz and a rotation speed of 72 deg/s. For the reconstruction, 349 projections over 180$^\circ$ were used follwing the Crowther criterion.
• Figure 3: XMPI projections of wood fiber failure recorded at 12.5 kHz with an energy of 16.5 keV. The failure region is highlighted by a box. Three representative time stamps were selected (0 ms, 80 ms and 160 ms). The projections were flat field corrected using conventional flat field correction. Projection #1 with $-17.0^\circ$, #2 with $13.7^\circ$ and #3 with $30.7^\circ$ with respect to the primary beam axis were generated with a recombiner (Si-111 + Ge-400), Si-111 and Ge-400, respectively.
• Figure 4: XMPI projections of SHGS suspended in glycerol recorded at 40 Hz with an energy of 16.5 keV. Three representative time stamps were selected (0 ms, 400 ms and 800 ms). The projections were flat field corrected using conventional flat field correction. Projection #1 with $-17.0^\circ$ and #3 with $30.7^\circ$ with respect to the primary beam axis were generated with a recombiner (Si-111 + Ge-400) and Ge-400, respectively. The same particle is highlighted in red in each frame to illustrate its movement with time.
• Figure 5: Flat field images of two different experiments acquired at 12.5 kHz (a) and 40 Hz (b) with corresponding histograms. The x-axis is log-scaled with base 2, and the tick labels indicate the exponent. (a) The high temporal resolution configuration covers an ADC range of 12 bit (Projection #1), 12 bit (Projection #2) and 11 bit (Projection #3). The full ADC range of the Photron Nova S16 is 12 bit. (b) The high spatiotemporal resolution configuration covers an ADC range of 16 bits for both projections, i.e., the full ADC range of the Andor Zyla 5.5.