title: The influence of spatial patterns of capillary networks on transverse relaxation.
abstract: Tissue-inherent relaxation parameters offer valuable information about the arrangement of capillaries: in an external field, capillaries act as magnetic perturbers to generate local inhomogeneous fields due to the susceptibility difference of deoxygenated blood and the surrounding tissue. These field inhomogeneities influence the free induction decay in a characteristic way, and, conversely, the above tissue parameters can be recovered by multi-parametric fits of adequate theoretical models to experimentally sampled free induction decays. In this work we study the influence of different spatial patterns of capillary positions on the free induction decay. Starting from the standard single capillary approximation (Krogh cylinder) for a symmetric array of capillaries, the free induction decay is analyzed for increasingly random capillary positions, using a previously described Gibbs point field model. The effects of diffusion are implemented with a flexible and fast random walk simulation. We find that the asymmetric form of the obtained frequency distribution is more robust against variations of capillary radii than against shifts of capillary positions, and further that, for an inclusion of diffusion effects, the single capillary approximation models the uniform alignment of capillaries in the hexagonal lattice to great accuracy. An increase in randomization of capillary positions then leads to a significant change in relaxation times. This effect, however, is found less pronounced than that of changes in the off-resonance field strengths which are controlled by the oxygen extraction fraction, thus indicating that observed changes in BOLD imaging are more likely to be attributed to changes in oxygenation than to capillary alignment.
authors.affiliation: Heidelberg University Hospital, Department of Neuroradiology, Heidelberg, Germany; German Cancer Research Center, Radiology E010, Heidelberg, Germany. Electronic address: felix.kurz@med.uni-heidelberg.de.
authors.forename: F T
authors.initials: FT
authors.lastname: Kurz
authors.affiliation: Heidelberg University Hospital, Department of Neuroradiology, Heidelberg, Germany; German Cancer Research Center, Radiology E010, Heidelberg, Germany.
authors.forename: C H
authors.initials: CH
authors.lastname: Ziener
authors.affiliation: Humboldt University Berlin, Department of Physics, Berlin, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Rückl
authors.affiliation: Heidelberg University Hospital, Department of Neuroradiology, Heidelberg, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Hahn
authors.affiliation: Heidelberg University Hospital, Department of Neuroradiology, Heidelberg, Germany; German Cancer Research Center, Radiology E010, Heidelberg, Germany.
authors.forename: V J F
authors.initials: VJF
authors.lastname: Sturm
authors.affiliation: Heidelberg University Hospital, Department of Neuroradiology, Heidelberg, Germany; German Cancer Research Center, Radiology E010, Heidelberg, Germany.
authors.forename: K
authors.initials: K
authors.lastname: Zhang
authors.affiliation: Heidelberg University Hospital, Department of Neuroradiology, Heidelberg, Germany; German Cancer Research Center, Radiology E010, Heidelberg, Germany.
authors.forename: L R
authors.initials: LR
authors.lastname: Buschle
authors.affiliation: Heidelberg University Hospital, Department of Neuroradiology, Heidelberg, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Bendszus
authors.affiliation: Heidelberg University Hospital, Department of Neuroradiology, Heidelberg, Germany.
authors.forename: S
authors.initials: S
authors.lastname: Heiland
authors.affiliation: German Cancer Research Center, Radiology E010, Heidelberg, Germany.
authors.forename: H P
authors.initials: HP
authors.lastname: Schlemmer
authors.affiliation: University Hospital Würzburg, Department of Internal Medicine I, Würzburg, Germany.
authors.forename: W R
authors.initials: WR
authors.lastname: Bauer
authors.affiliation: University Hospital Würzburg, Division of Neuroradiology, Würzburg, Germany; University of Würzburg, Department of Physics V, Würzburg, Germany.
authors.forename: T
authors.initials: T
authors.lastname: Kampf
mesh_terms: D002196:Capillaries / Q000502:physiology*; D004058:Diffusion; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008280:Magnetics; D008962:Models, Theoretical; D010100:Oxygen / Q000097:blood*
chemical_list: D010100:Oxygen
keywords: BOLD; Capillary arrangement/distribution; Magnetic susceptibility; Spin dephasing; Transverse relaxation