Development of a Controlled, Pneumatic Rotation Device to Measure Brain Deformation using Tagged MRI

Introduction: : Traumatic brain injury (TBI) is a major global health concern and a leading cause of death and disability across all ages. In the United States, the CDC reported above 223,000 TBI-related hospitalizations and over 69,000 TBI-related deaths in 2021. Such a high incidence necessitates a deeper and more comprehensive understanding of the conditions that lead to TBI, both to improve prevention strategies and to enhance the quality and bio-fidelity of computational brain injury models (Zhang et al., 2004). Tagged magnetic resonance imaging (tMRI) is presently the leading method of visualizing and quantifying brain tissue deformation in living humans. In tMRI, spatially periodic harmonic “tag lines” are applied and follow the motion of underlying tissue during imaging. Such lines allow the brain’s deformation to be tracked and observed. tMRI has been applied to measure brain deformation in vivo using a custom-built MRI-safe head rotation device (Knutsen et al. 2020, Chan et al., 2018). In current experiments, a rotation device was used to impart non-injurious head accelerations; at a peak of 200 rad/ s², 10-15% of the acceleration experienced in “heading” a soccer ball (Naunheim et al., 2003). Head motions must be repeated multiple times in order to build up 3D, full-field images of the brain. This study aims to develop a compatible tMRI axial head rotation device that induces controlled, higher accelerations in cadaveric and surrogate-head phantom models, with the goal of acquiring high temporal resolution 3D brain deformation of the brain under rotational motion.

Materials and

Methods: : The MRI-compatible system consisted of two distinct assemblies: an in-bore, non-ferrous mechanical unit, and a remote electronics module. The in-bore unit was comprised of a head-rotation cradle and axle, 3D-printed from carbon-fiber nylon, (Markforged, Watertown, MA) and anchored within a self-lubricating Delrin bearing block. To transmit position data and manage MRI sequence timing, an MRI-safe optical position encoder (Micronor, Camarillo, CA) was linked to the rotating axle. A pin mechanism held the cradle level; once released by the operator, a 3D-printed polylactic acid (PLA) counterweight, fixed at a 42° incline, rotated downward, driving the cradle through a 35° rotation until contact with a sorbothane pad, upon release. An MRI-safe pressure chamber made of polyvinyl chloride (PVC) and carbon-fiber-reinforced polyphthalamide (PPA-CF) (Bambu Lab, Shenzhen, China) was angled 45° below the counterweight and triggered after 2° of travel to enable controlled and increased angular velocities. The input pressure was calibrated based on an analytical solution of the system and desired angular kinematics (a range of 3-12 rad/s and 200-1200 rad/s2).
Encoder signals (within the MRI suite) were transmitted via a fiber-optic cable to an optical control box (Micronor, Camarillo, CA) and a fast-sampling Teensy 4.1 microcontroller (PJRC, Portland, OR) activating a solenoid (Tailonz Pneumatic, Bellevue, WA) that drove the pneumatic piston actuation. Both pneumatic and fiber optic transmissions were routed through long lead lines to simulate isolation from the MRI room.
Trials were conducted with three device arrangements: single‑counterweight (SW) non-piston, double‑counterweight (DW) non-piston, and SW armed piston, pressurized to 40 psi.

Results, Conclusions, and Discussions:: All configurations yielded greater peak velocity, angular acceleration, and lower variability than those reported in previous tMRI experiments. The DW configuration provided the best repeatability, evident in Figure 1 and Table 1,  with acceleration and velocity standard deviations of 0.03 rad/s and 14 rad/ s², respectively. The piston-assisted configuration (Figure 2) produced the highest accelerations, generating 1425 rad/ s² at impact -327% above those found in (Chan et al. 2018) and 215% higher than the DW mode. Additional metrics for each configuration are provided in Table 1.

The prototype axial rotation device provides substantially higher loading conditions compared to published tMRI head rotation rigs. These gains were achieved through MRI-safe methods: all in-scanner components were non-ferromagnetic, and controller electronics were designed for use within the operating room. The DW's higher per-trial stability offers a functional advantage for extended 3-D acquisitions (for higher temporal resolutions), while piston assistance enables accelerations approaching the lower bound of physiological results. Data trends indicate that combining piston actuation with DW could raise peak accelerations past 2000 rad/s², producing brain deformation patterns reflecting those observed in TBI events. Despite promising results, this study has several limitations. Tests were performed on a rigid head form rather than cadaveric tissue or patients, which may lower accelerations due to neck resistance and inter-patient head variability. In-scanner MRI testing is also required to quantify image quality and repeatability. Additionally, while the current acceleration values exceed prior benchmarks, they still fall short of replicating typical impacts associated with TBI. Nonetheless, the potential for further enhancements across all key parameters holds considerable promise for achieving more physiologically relevant impact conditions.

This MR‑compatible head‑rotation device delivers repeatable, axial rotations up to 1425 rad/s² — over 300% higher than previous tMRI studies — while maintaining sub‑degree kinematic precision. The design paves the way for high‑acceleration, multi‑rotation tMRI experiments that more faithfully reproduce real‑world traumatic loading and thereby help refine next‑generation computational brain‑injury models. Ongoing work to validate performance with cadaveric and patient use, as well as merge piston and DW actuation, will further establish the utility of this approach in tMRI brain deformation studies.

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Research reported in this study was supported by the National Institute of Neurological Disorders and Stroke of the National Institutes of Health under Award Number R01NS136056. The authors have no conflicts of interest to disclose. The opinions and assertions expressed herein are those of the author(s) and do not reflect the official policy or position of the Henry M. Jackson Foundation for the Advancement of Military Medicine, Inc., the Uniformed Services University of the Health Sciences, or the Department of Defense.