title: Theory of Q-ball imaging redux: Implications for fiber tracking.
abstract: Q-ball imaging is widely used to determine fiber directions for fiber tracking. In q-ball imaging the directional dependence of water diffusion in tissue is described by Tuch's orientation distribution function (ODF); a different function, the q-ball orientation distribution function, is measured using high angular resolution magnetic resonance diffusion imaging (HARDI). Tuch's ODF is assumed to be well approximated by the q-ball ODF. In this study it is shown that: 1) the q-ball ODF is not a good approximation to Tuch's ODF; 2) the properties of the q-ball ODF depend strongly on q, the area of the diffusion sensitization gradients; and 3) the q-ball ODF for a composite system is the weighted average of the q-ball ODFs for each subsystem, but the weighting factor is the product of the percent composition and a renormalization factor. In addition, a derivation is presented of the q-ball ODF for a system described by a Gaussian distribution and expressions are derived for both the dependence of the angular resolution on q and for the relation between the angular resolution and the signal loss. These findings might be useful in the design and interpretation of fiber-tracking experiments.
authors: National Institute of Mental Health, Clinical Brain Disorders Branch, Bethesda, MD 20892, USA. asb@nih.gov
authors: Alan
authors: A
authors: Barnett
mesh_terms: D000465:Algorithms; D001185:Artificial Intelligence; D001921:Brain / Q000033:anatomy & histology*; D003198:Computer Simulation; D056324:Diffusion Tensor Imaging / Q000379:methods*; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D021621:Imaging, Three-Dimensional / Q000379:methods*; D008959:Models, Neurological; D009413:Nerve Fibers, Myelinated / Q000648:ultrastructure*; D010363:Pattern Recognition, Automated / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity