Quantitative MRI volumetry, diffusivity, cerebrovascular flow, and cranial hydrodynamics during head-down tilt and hypercapnia: the SPACECOT study

J Appl Physiol (1985). 2017 May 1;122(5):1155-1166. doi: 10.1152/japplphysiol.00887.2016. Epub 2017 Feb 16.


To improve the pathophysiological understanding of visual changes observed in astronauts, we aimed to use quantitative MRI to measure anatomic and physiological responses during a ground-based spaceflight analog (head-down tilt, HDT) combined with increased ambient carbon dioxide (CO2). Six healthy, male subjects participated in the double-blinded, randomized crossover design study with two conditions: 26.5 h of -12° HDT with ambient air and with 0.5% CO2, both followed by 2.5-h exposure to 3% CO2 Volume and mean diffusivity quantification of the lateral ventricle and phase-contrast flow sequences of the internal carotid arteries and cerebral aqueduct were acquired at 3 T. Compared with supine baseline, HDT (ambient air) resulted in an increase in lateral ventricular volume (P = 0.03). Cerebral blood flow, however, decreased with HDT in the presence of either ambient air or 0.5% CO2 (P = 0.002 and P = 0.01, respectively); this was partially reversed by acute 3% CO2 exposure. Following HDT (ambient air), exposure to 3% CO2 increased aqueductal cerebral spinal fluid velocity amplitude (P = 0.01) and lateral ventricle cerebrospinal fluid (CSF) mean diffusivity (P = 0.001). We concluded that HDT causes alterations in cranial anatomy and physiology that are associated with decreased craniospinal compliance. Brief exposure to 3% CO2 augments CSF pulsatility within the cerebral aqueduct and lateral ventricles.NEW & NOTEWORTHY Head-down tilt causes increased lateral ventricular volume and decreased cerebrovascular flow after 26.5 h. Additional short exposure to 3% ambient carbon dioxide levels causes increased cerebrovascular flow associated with increased cerebrospinal fluid pulsatility at the cerebral aqueduct. Head-down tilt with chronically elevated 0.5% ambient carbon dioxide and acutely elevated 3% ambient carbon dioxide causes increased mean diffusivity of cerebral spinal fluid within the lateral ventricles.

Keywords: dMRI; head-down tilt; hypercapnia; intracranial hypertension; microgravity; space analog.

Publication types

  • Randomized Controlled Trial

MeSH terms

  • Adult
  • Carbon Dioxide
  • Carotid Arteries / physiology
  • Cerebrospinal Fluid / physiology
  • Cerebrovascular Circulation / physiology*
  • Cross-Over Studies
  • Double-Blind Method
  • Head-Down Tilt / physiology*
  • Humans
  • Hydrodynamics
  • Hypercapnia / physiopathology*
  • Magnetic Resonance Imaging / methods
  • Male
  • Space Flight / methods


  • Carbon Dioxide