title: Apparent exchange rate imaging in anisotropic systems.
abstract: OBJECTIVE
  Double-wave diffusion experiments offer the possibility of probing correlation between molecular diffusion at multiple time points. It has recently been shown that this technique is capable of measuring the exchange of water across cellular membranes. The aim of this study was to investigate the effect of macroscopic tissue anisotropy on the measurement of the apparent exchange rate (AXR) in multicompartment systems.
  
  
  METHODS
  AXR data were collected from yeast and perfusion-fixated brain tissue at high angular resolution on a preclinical imaging system. The AXR was expanded for anisotropic systems by calculating scalar AXR values along the principal directions of the diffusion tensor.
  
  
  RESULTS
  In yeast, both the AXR and diffusivity were rotational invariant, whereas in fixated brain tissue, the measured AXR was sensitive to the orientation of anisotropic structures. AXR, especially in white matter, was robustly estimated along the first and second principal directions of the diffusion tensor, but increasing noise was seen in the AXR estimates along the third principal direction of the diffusion tensor.
  
  
  CONCLUSIONS
  Our results indicate that tissue anisotropy must be considered for AXR estimates in complex biological systems.
authors.affiliation: Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital Hvidovre, Hvidovre, Denmark.
authors.forename: Casper K
authors.initials: CK
authors.lastname: Sønderby
authors.forename: Henrik M
authors.initials: HM
authors.lastname: Lundell
authors.forename: Lise V
authors.initials: LV
authors.lastname: Søgaard
authors.forename: Tim B
authors.initials: TB
authors.lastname: Dyrby
mesh_terms: D000818:Animals; D016880:Anisotropy; D001921:Brain / Q000033:anatomy & histology*; D002522:Chlorocebus aethiops; D056324:Diffusion Tensor Imaging / Q000379:methods*; D015003:Yeasts / Q000166:cytology*
keywords: AXR; DTI; cell; exchange; membrane