title: Gravity-dependent signal gradients on MR images of the lung in supine and prone positions: a comparison with isogravitational signal variability.
abstract: OBJECTIVE
  To investigate the tendency of proton MR signal intensity (SI) gradients to be steeper in the supine than in the prone body position, and to quantify the relation between gravity-related and isogravitational changes of SI on proton MR images of the lung.
  
  
  METHODS
  In eight healthy volunteers, MR images were obtained in the supine and prone positions using a multiple inversion recovery turbo spin-echo (TSE) sequence. The variation in SI along the gravity-dependent direction and within isogravitational planes was measured on a pixel-by-pixel basis. Ratios of slopes were calculated for comparisons among volunteers. Comparisons of ratios were made using Fisher's exact test. Isogravitational variability was compared with the mean SI, the signal-to-noise ratio (SNR), and the image noise.
  
  
  RESULTS
  The average ratios of slopes showed that the overall SI gradient was steeper in the supine than the prone position, with a substantial difference in the supine/prone ratios between inspiration (1.21) and expiration (1.72). In both the supine and prone positions, gravity-dependent gradients were steeper in expiration than in inspiration (P = 0.001). The SI variability along the gravitational direction was larger than the isogravitational variability. The isogravitational variability in turn was larger than the image noise but smaller than the mean SI of the MR images.
  
  
  CONCLUSIONS
  Gravity-dependent gradients in proton MR SI are steeper in the supine than in the prone position. The magnitudes of these gradients were larger than the isogravitational signal variability, showing that MRI is sensitive to gravitationally induced effects.
authors.affiliation: Department of Radiology, Medical University of Vienna, Vienna, Austria. alexander.bankier@meduniwien.ac.at
authors.forename: Alexander A
authors.initials: AA
authors.lastname: Bankier
authors.forename: Pippa
authors.initials: P
authors.lastname: Storey
authors.forename: Vu M
authors.initials: VM
authors.lastname: Mai
authors.forename: Robert R
authors.initials: RR
authors.lastname: Edelman
authors.forename: Qun
authors.initials: Q
authors.lastname: Chen
mesh_terms: D000328:Adult; D000465:Algorithms; D016477:Artifacts; D017352:Echo-Planar Imaging / Q000379:methods*; D005260:Female; D006112:Gravitation; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008168:Lung / Q000033:anatomy & histology*; D008297:Male; D008875:Middle Aged; D016684:Prone Position; D012016:Reference Values; D012307:Risk Factors; D012494:Sampling Studies; D012680:Sensitivity and Specificity; D016683:Supine Position