Effect Of Simulated Microgravity On Human Brain Gray Matter Density And Cerebral Perfusion

Background: There are many different environmental factors during space flight.Microgravity is the most important of all. Microgravity has different influences on eachsystem of human body, including the central nervous system. Cerebral perfusion(cerebral blood flow per100g brain per minute) and brain morphology are two importantaspects of the central nervous system. For many years they had been the key issues onwhich many specialists and scholars had research and made some improvement.However, previous experimental studies also had corresponding limitation. Mostreported researches about cerebral perfusion in simulated microgravity environmentwere associated with animal experiments and these results can’t truly reflect humanbrain cerebral perfusion in simulated microgravity environment. Blood flowmeasurement using transcranial doppler(TCD) in simulated microgravity environmentcan only provide data for cerebral blood flow from large intracranial and extracranialvessels and can’t measure and reflect perfusion of brain parenchyma. In term of brainmorphology, most of previous researches used to draw outline manually on interestedregions(ROI). That is a time consuming method with poor repeatability. Focus on ROImight ignore the general change of brain morphology.With the rapid development of medical equipments and continuous improvementof MR imaging technology, ASL(arterial spin labeling, ASL) and VBM(voxel-basedmorphometry, VBM) are widely used in the clinical works and scientific researches.For the present experiment, we used ASL technology to acquire perfusion data of thewhole brain parenchyma with the advantages of no invasion, no injection of contrastagent, no radiation, and good spatial and temporal resolution. So ASL can reflect perfusion of brain parenchyma in the general and become the best means forresearching real-time perfusion of human brain. Millimeter high resolution magneticresonance imaging technology provided reliable data for morphological researches ofliving human brain. VBM used automatic computer post-processing technology tomake automatical, comprehensive, objective analysis based at the level of image voxel.ASL and VBM provide us feasibility and credibility for this present study in the fieldof simulated microgravity environment.Objective:(1)To make clear if simulated microgravity environment can induce changeof normal human brain gray matter density.(2)To understand the change of cerebralperfusion in simulated microgravity environment.(3)To explore the relationshipbetween human brain gray matter density and cerebral perfusion.Methods: Fourteen healthy male volunteers were recruited by astronaut researchtraining center and underwent-6°head-down bed rest(HDBR) for1month simulatedmicrogravity. High-resolution brain anatomical imaging data and cerebral perfusionimages were collected by using a3.0T superconducting MR imaging system before andafter HDBR. With VBM module of analysis software package SPM8, the pre-andpost-HDBR image data of whole brain gray matter density and cerebral perfusion werecalculated and compared.Results:(1)After HDBR, the gray matter density was significantly decreased in thebilateral thalamus, bilateral gyrus lingualis and right supramarginal gyrus(p＜0.005).Among them, left thalamus was the most remarkable area. In contrast, the gray matterdensity was increased in right hippocampus(p＜0.005).(2)After HDBR, the cerebralperfusion was significantly decreased in the right insula, right middle temporal gyrusand left posterior cingulate cortex(p＜0.005). Among them, left posterior cingulate cortex was the most remarkable area. While the cerebral perfusion was increased in leftsuperior temporal gyrus(p＜0.005).(3)The changed regions of brain gray matter densitydidn’t agree with changed regions of cerebral perfusion.Conclusion: The experimental group data suggest:(1)Simulated microgravityenvironment may induce change of gray matter density for adult volunteers brain. Theregions of gray matter density decrease are closely related to the functional domains ofsensation, motor, vision, and memory, which is thought to be anatomical factor for thechange of sensory, cognitive, visual and coordinated function in previous simulatedmicrogravity researches. The right hippocampus appears gray matter density increase,which may be related to persistent neurogenesis.(2)Simulated microgravityenvironment may induce change of cerebral perfusion for adult volunteers brain. Theregions of cerebral perfusion increase and decrease can be found in the trial, but thewhole cerebral perfusion appears increase. These results are consistent with the mostprevious similar researches of cerebral perfusion increase in simulated microgravityenvironment.(3)The changed regions of brain gray matter density don’t agree withchanged regions of cerebral perfusion, which points out change of gray matter density isnot significantly correlated with cerebral perfusion. Gray matter density change may bedue the decrease of neurons impulse when activities are restricted in HDBR.