Multidimensional correlation of nuclear relaxation and diffusion tensor size, shape, and orientation
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1
Lund University, Physical Chemistry, Sweden
The tissue volume corresponding to an MRI voxel in general contains hundreds of thousands of cells and possibly a few different tissue types, thus leading to the presence of a range of microscopic environments for the water within each voxel. By resolving, quantifying, and characterizing the various types of water, it could prove possible to determine the composition of the voxel in terms of tissue and cell types. With diffusion and relaxation correlation NMR,1 the signals from chemically identical molecules can be resolved if they reside in different environment wherein they have distinct values of the nuclear relaxation rates R1 and R2 and/or self-diffusion coefficients D. Complications arise for anisotropic materials where the observed value of D depends on the magnitude, anisotropy, and orientation of the microscopic diffusion tensor as well as on the details of the applied magnetic field gradients for diffusion encoding. Here we show that acquisition protocols incorporating isotropic diffusion encoding2 permit separation of components with distinct values of the isotropic diffusivity Diso, as well as estimation of the anisotropy and orientation distribution for each of the resolved components. Even in the absence of substantial differences in Diso, distinct values of R1 and/or R2 can be utilized to resolve components and determine their individual diffusion anisotropy and orientation. The new method relies on acquiring the signal as a function of six experimental variables: echo time TE, repetition time TR, and the magnitude b, anisotropy bΔ, and orientation (Θ,Φ) of the axisymmetric diffusion-encoding tensor b.3 Model-free data inversion then yields the weights of signal components with individual values of R1 and R2, as well as the size Diso, anisotropy DΔ, and orientation (Θ,Φ) of the assumedly axisymmetric microscopic diffusion tensors. Proof-of-principle experiments are carried out on microimaging equipment and composite samples with lyotropic liquid crystals, yeast cell suspensions, and polymer solutions.
References
[1] D. Bernin, D. Topgaard, NMR diffusion and relaxation correlation methods: New insights in heterogeneous materials, Curr. Opin. Colloid Interface Sci., 18 (2013) 166-172.
[2] S. Eriksson, S. Lasič, D. Topgaard, Isotropic diffusion weighting by magic-angle spinning of the q-vector in PGSE NMR, J. Magn. Reson., 226 (2013) 13-18.
[3] S. Eriksson, S. Lasič, M. Nilsson, C.-F. Westin, D. Topgaard, NMR diffusion encoding with axial symmetry and variable anisotropy: Distinguishing between prolate and oblate microscopic diffusion tensors with unknown orientation distribution, J. Chem. Phys., 142 (2015) 104201.
Keywords:
Diffusion,
MRI,
dMRI,
multidimensional,
diffusion encoding
Conference:
New dimensions in diffusion encoding, Fjälkinge, Sweden, 11 Jan - 14 Jan, 2016.
Presentation Type:
Oral presentation
Topic:
New Dimensions in Diffusion Encoding
Citation:
Topgaard
D
(2016). Multidimensional correlation of nuclear relaxation and diffusion tensor size, shape, and orientation.
Front. Phys.
Conference Abstract:
New dimensions in diffusion encoding.
doi: 10.3389/conf.FPHY.2016.01.00008
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Received:
07 Jul 2016;
Published Online:
07 Jul 2016.
*
Correspondence:
Prof. Daniel Topgaard, Lund University, Physical Chemistry, Lund, 221 00, Sweden, daniel.topgaard@fkem1.lu.se