Introduction

There is increased evidence that exposure to evacuation relevant hypobaric conditions worsens neurological outcomes and potentially exacerbates secondary injury¹. We demonstrated that rats exposed to underbody blasts and then to hypobaria under 100% O2 exhibit increased axonal damage and impaired motor function compared to those subjected to blast and hypobaria under normoxic conditions². In another study we demonstrated significantly worsened cognitive deficits, hippocampal neuronal loss, and microglial/astrocyte activation when rats were exposed to hypobaria following fluid percussion injury³. These findings compelled us to closely examine the effects of hypobaria in the brain in vivo during a flight by simulating the experiences of a war-fighter being evacuated soon after injury which led us to the construction of a MRI compatible hypobaric chamber.

Methods – Hypobaric Chamber Construction

The hypobaric chamber was constructed by maximizing the utility of the standard rat bed on a 7.0 Tesla scanner from Bruker BioSpin as it contains the nose-cone and other apparatus essential for conducting the imaging experiments. A 1/4 inch thick, 82-mm diameter polycarbonate tubing was chosen as the body for the hypobaric chamber with two end caps that are removable (Fig 1A). On one end provisions were made to allow for a 4-channel phased array receiver coil and for providing anesthesia gas. On the other end cap provisions were made to allow for the insertion of a temperature probe, respiration probe, warm water circulation, tubing for vacuum generation, and anesthesia gas circulation (see Fig 1B & C ). Once all the specific items are in place, including the animal, the chamber is hermetically sealed and the pressure in the chamber controlled by adjusting the air flow from a constant vacuum generator, e.g., 12.7 psi (4,000ft altitude, 10.9 psi (8,000 ft altitude) and 4.4 psi (30000-ft altitude). A 86-mm circular-polarized volume is then slipped over the plastic chamber and positioned at the center of the rat head as shown in Fig 1B.

Results

The hypobaric chamber was bench tested to assess whether it could maintained a desired hypobaric condition from 12.7 psi, down to 4.4psi. Over several trials (minimum of 4) at each of the above three pressure levels, the chamber was stable and maintained under specific hypobaric condition. Then the chamber was tested inside of the Bruker Biospec 7.0 Tesla 30-cm horizontal bore scanner equipped with a BGA12S gradient system, which was interfaced to a Paravision 6.0 console. Animals were placed prone on an animal bed through 1-3 % isoflurane with 100 % oxygen administration at 1 L/min rate. The head was fixed with a byte bar and a pair of ear pins. The 4-channel RF receiver coil was centered and fixed over the head. The chamber tube was then slid over the animal bed and the two ends of the chamber were sealed with rubber plugs and plastic caps. An MR-compatible system was used to monitor respiration rate and body temperature was maintained at 35-37.5 ^oC, using a circulating warm water heater. Figure 2a demonstrates high-quality T₂-weighted images from the above setup from a female adult Sprague Dawley rat (300 grams) at hypobaric levels of 0.14 x100 kpa (4000 feet). Images were obtained using the RARE sequence at a TR/TE=3000/22 ms, RARE factor=4, slice thickness (st)=1mm, field of view (FOV)=3.5 x 3.5 cm², in-plane resolution=137 x 137 μm², average=2. The signal-to-noise of the images from the hypobaric chamber was comparable to those obtained without the use of the chamber. The proton spectra (Fig 2b) obtained from the cortex using a short-TE PRESS (3.5 x 1.5 x 4 mm³), TR/TE=2500/10 ms, and 400 averages clearly demonstrates that the apparatus does not impose additional field inhomogeneity and that excellent quality spectra can be obtained within the chamber. Figure 3, shows fractional anisotropy maps obtained from diffusion tensor images obtained at a TR/TE=2500/24.5 ms, 30 directions, b-value=1000 and 2000 s/mm², FOV=3.5 x 3.5 cm², st=1 mm, Matrix size=128 x 128, and 5 bo images to improve the signal to noise of the DTI maps. Clearly the hypobaric chamber does not introduce any additional image distortion.

Conclusion

The ability to monitor changes in the brain by simulating an aeromedical environment immediately following brain injury allows one to carefully assess the ramifications of such evacuation on the long term sequelae. In addition other interventions such as normoxic and hyperoxic conditions could be assessed and injury types such as with and without hemorrhagic shock could be assessed while still remaining in the hypobaric condition. We have demonstrated here that such experiments are possible through the use of this hypobaric chamber to study brain changes in vivo.

Acknowledgements

We thank the Core for Translational Research in Imaging @ Maryland (C-TRIM) provided in vivo neuroimaging service for the study. We also thank USAMDA medical prototype laboratory to build the hypobaric chamber for the study. This material is based on research sponsored by 711 HPW/XPT under Cooperative Agreement number FA8650-11-2-6142 and FA8650-17-2-6H13. The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright notation thereon. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of 711HPW/XPT or the U.S. Government.