Assessment Of Human Lower Limb Fat Volume Using MRI DIXON Technology In Simulated Microgravity Experiment

Background: When the astronaut in the weightlessness environment during space flight,the gravity loading on human sport organs disappeared. The astronaut will no longerneed to fight with the effect of gravity on his body for maintaining a position and/ormoving. Thus, effects of microgravity on the human body’s physiological systemsappeared, including body fluid loss, weight loss, orthostatic tolerance drop, muscleatrophy, progressive bone loss. As for the changing of human body fat volume inmicrogravity, only few previous publications are available with inconsistent results, fatincrease[1,2]and constant[3]both are reported.As the important energy storing place, the fat plays a very important role inmaintaining a normal human physiological function. So if we want to thoroughly studythe effects of simulated microgravity on human fat metabolism, we must understand theinfluence of simulated microgravity on human body fat volume and distribution. Dualenergy X-ray absorptiometry (DXA) is used in most of previous studies measuringhuman fat volume. Though the modality provides an accurate measurement for fatvolume in different parts of human body, which is radioactive and harmful to humanbody.The rapid development of Magnetic Resonance Imaging (MRI) technologyprovides us a reliable new tool for the accurate measurement of human body fat volume.With the advantage of no invasion, no radiation, easy to repeat, multiple choice of scansequences and parameters, as well as high spatial and temporal resolution, MRItechnology can be used to quantitatively measure the human body fat volume. MRI water/fat separation (Dixon) technology is one specially designed scan sequence, whichcan separately form a pure fat proton image and/or a pure water proton image withinone scan based on the different proton resonance frequency of water and fat. It alreadyhas been widely used in clinical work. Dixon technology improves the image quality byreducing the interaction of muscle and fat. Thus a more accurate measurement reflectingthe human body fat volume changing in lower limb could be performed with less error.Moreover, Dixon technique provides the possibility of a separate measurement ofmuscle volume and fat volume based on high quality image data, which helps us inunderstanding the effects of microgravity.Objective: To assess the dynamic lower limb fat volume changing under simulatedmicrogravity using advanced imaging technology, and providing the scientific basis forthe future research of fat metabolism change in microgravity environment, as well asmaking rehabilitation plan for astronaut.Methods: Fourteen healthy male volunteers were recruited by Astronaut ResearchTraining Center. The volunteers underwent-6°head-down bed rest (HDBR) for30daysunder simulated microgravity. Anatomical imaging data of lower limb were collected byusing GRE water/fat separation technology on a Siemens Verio3.0T MRI system withbody surface coil before and after HDBR. Imaging field was from the inferior border ofbilateral femoral head to superior border of ankle. The pre-and post-experimentalimage data of human total lower limb fat volume, thigh fat volume, lower leg fatvolume were automatically calculated by using a home made software which waswritten by Information Science and Technology Institute at Beijing Normal University.The significance of measured lower limbs fat volume changing before and after theexperiment was tested through paired t test using SPSS16.0software. Results: After-6°HDBR, the total lower limb fat volume increased significantly by15.67±11.19%(P<0.001), while the thigh fat volume increased by20.05±13.61%(P<0.001). Lower leg fat volume showed an increasing trend by7.91±10.30%, however,without statistical significance (P＞0.05).Conclusion: The present experimental data suggested:(1)-6°30days HDBR simulated microgravity environment may induce increase of thetotal lower limb fat volume and the thigh fat volume. Lower leg fat volume showed anincreasing trend, however, no statistical significance.(2) MRI water/fat separation technology can be used to assess the human lower limb fatvolume with the advantage of high image contrast, no radiation, easily repeat, whichprovides’ an ideal imaging modality for the dynamic fat studies.