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Reply:
======

* Donna R. Roberts
* Joe Tidwell
* J. Andrew Taylor

We thank Wostyn et al1 for their comments and for the alternative explanation, based on venous congestion, that they provide for the perivascular space (PVS) volume increase observed in the bed rest subjects in our study.2 Indeed, altered cerebral venous hemodynamics have been documented in astronauts,3,4 and we have shown an increase in PVS volumes in astronauts and cosmonauts after spaceflight.5 Although altered venous outflow would be expected to have occurred in the bed rest subjects participating in our study, it was not measured. Taken together, these results suggest that an altered cerebrovascular physiology occurs both in the ground-based spaceflight analog of head-down tilt bed rest and during long-term spaceflight. The relative contributions that altered arterial or venous flow may have made individually or collectively to increase the PVS volume are unclear, suggesting further studies are needed to disentangle the underlying mechanisms involved. In that regard, advanced MR imaging techniques including MRA, MRV, and SWI sequences (which enhance the visualization of small venous structures) should be included in future MR imaging protocols used in head-down tilt bed rest studies and for pre- and postflight astronaut brain MR imaging.6

## References

1.  1.Wostyn P, Mader TH, Gibson CR, et al. Enlargement of perivascular spaces as a downstream consequence of spaceflight analog-induced alterations in cerebral venous hemodynamics. AJNR Am J Neuroradiol 2024;45:E6 doi:10.3174/ajnr.A8062 
    
    [FREE Full Text](http://www.ajnr.org/lookup/ijlink/YTozOntzOjQ6InBhdGgiO3M6MTQ6Ii9sb29rdXAvaWpsaW5rIjtzOjU6InF1ZXJ5IjthOjQ6e3M6ODoibGlua1R5cGUiO3M6NDoiRlVMTCI7czoxMToiam91cm5hbENvZGUiO3M6NDoiYWpuciI7czo1OiJyZXNpZCI7czo3OiI0NS81L0U2IjtzOjQ6ImF0b20iO3M6MTg6Ii9ham5yLzQ1LzUvRTcuYXRvbSI7fXM6ODoiZnJhZ21lbnQiO3M6MDoiIjt9) 

2.  2.Tidwell JB, Taylor JA, Collins HR, et al. Longitudinal changes in cerebral perfusion, perivascular space volume, and ventricular volume in a healthy cohort undergoing a spaceflight analog. AJNR Am J Neuroradiol 2023;44:1026–31 doi:10.3174/ajnr.A7949 pmid:37562828
    
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3.  3.Marshall-Goebel K, Laurie SS, Alferova IV, et al. Assessment of jugular venous blood flow stasis and thrombosis during spaceflight. JAMA Netw Open 2019;2:e1915011 doi:10.1001/jamanetworkopen.2019.15011 pmid:31722025
    
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4.  4.Rosenberg M, Coker MA, Taylor JA, et al. Post-flight dural venous sinus congestion in astronauts with spaceflight-associated neuro-ocular syndrome. JAMA Netw Open 2021;4:e2131465 doi:10.1001/jamanetworkopen.2021.31465 pmid:34705011
    
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5.  5.Barisano G, Sepehrband F, Collins HR, et al. The effect of prolonged spaceflight on cerebrospinal fluid and perivascular spaces of astronauts and cosmonauts. Proc Natl Acad Sci U S A 2022;119:e2120439119 doi:10.1073/pnas.2120439119
    
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6.  6.Roberts DR, Stahn AC, Seidler RD, et al. Towards understanding the effects of spaceflight on the brain. Lancet Neurol 2020;19:808 doi:10.1016/S1474-4422(20)30304-5 pmid:32949538
    
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*   © 2024 by American Journal of Neuroradiology