title: How does angular resolution affect diffusion imaging measures?
abstract: A key question in diffusion imaging is how many diffusion-weighted images suffice to provide adequate signal-to-noise ratio (SNR) for studies of fiber integrity. Motion, physiological effects, and scan duration all affect the achievable SNR in real brain images, making theoretical studies and simulations only partially useful. We therefore scanned 50 healthy adults with 105-gradient high-angular resolution diffusion imaging (HARDI) at 4T. From gradient image subsets of varying size (6<or=N<or=94) that optimized a spherical angular distribution energy, we created SNR plots (versus gradient numbers) for seven common diffusion anisotropy indices: fractional and relative anisotropy (FA, RA), mean diffusivity (MD), volume ratio (VR), geodesic anisotropy (GA), its hyperbolic tangent (tGA), and generalized fractional anisotropy (GFA). SNR, defined in a region of interest in the corpus callosum, was near-maximal with 58, 66, and 62 gradients for MD, FA, and RA, respectively, and with about 55 gradients for GA and tGA. For VR and GFA, SNR increased rapidly with more gradients. SNR was optimized when the ratio of diffusion-sensitized to non-sensitized images was 9.13 for GA and tGA, 10.57 for FA, 9.17 for RA, and 26 for MD and VR. In orientation density functions modeling the HARDI signal as a continuous mixture of tensors, the diffusion profile reconstruction accuracy rose rapidly with additional gradients. These plots may help in making trade-off decisions when designing diffusion imaging protocols.
authors.affiliation: Laboratory of Neuro Imaging, Department of Neurology, UCLA School of Medicine, 635 Charles E. Young Drive South, Suite 225E, Los Angeles, CA 90095-7332, USA.
authors.forename: Liang
authors.initials: L
authors.lastname: Zhan
authors.forename: Alex D
authors.initials: AD
authors.lastname: Leow
authors.forename: Neda
authors.initials: N
authors.lastname: Jahanshad
authors.forename: Ming-Chang
authors.initials: MC
authors.lastname: Chiang
authors.forename: Marina
authors.initials: M
authors.lastname: Barysheva
authors.forename: Agatha D
authors.initials: AD
authors.lastname: Lee
authors.forename: Arthur W
authors.initials: AW
authors.lastname: Toga
authors.forename: Katie L
authors.initials: KL
authors.lastname: McMahon
authors.forename: Greig I
authors.initials: GI
authors.lastname: de Zubicaray
authors.forename: Margaret J
authors.initials: MJ
authors.lastname: Wright
authors.forename: Paul M
authors.initials: PM
authors.lastname: Thompson
mesh_terms: D000465:Algorithms; D016880:Anisotropy; D003198:Computer Simulation; D003337:Corpus Callosum / Q000502:physiology*; D004058:Diffusion; D038524:Diffusion Magnetic Resonance Imaging / Q000379:methods*; D005260:Female; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods; D008297:Male; D008959:Models, Neurological; D055815:Young Adult