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Direction-averaged diffusion-weighted MRI signal using different axisymmetric B-tensor encoding schemes

Afzali, Maryam, Aja-Fernandez, Santiago and Jones, Derek K. 2020. Direction-averaged diffusion-weighted MRI signal using different axisymmetric B-tensor encoding schemes. Magnetic Resonance in Medicine 84 (3) , pp. 1579-1591. 10.1002/mrm.28191

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Purpose: It has been shown, theoretically and in vivo, that using the Stejskal-Tannerpulsed-gradient, or linear tensor encoding (LTE), and in exhibiting a ’stick-like’ diffusion geometry,the direction-averaged diffusion-weighted MRI signal at high b-values (7000 < b <10000 s=mm2) follows a power-law, decaying as 1=pb. It has also been shown, theoretically,that for planar tensor encoding (PTE), the direction-averaged signal decays as 1=b. We aimedto confirm this theoretical prediction in vivo. We then considered the direction-averaged signalfor arbitrary b-tensor shapes and different tissue substrates to look for other conditions underwhich a power-law exists.Methods: We considered the signal decay for high b-values for encoding geometries rangingfrom 2-dimensional PTE, through isotropic or spherical tensor encoding (STE) to LTE. Whena power-law behaviour was suggested, this was tested using in silico simulations and in vivousing ultra-strong gradients (300 mT/m).Results: Our in vivo results confirmed the predicted 1/b power law for PTE. Moreover, ouranalysis showed that using an axisymmetric b-tensor a power-law only exists under very specificconditions: (a) the tissue must have ’stick-like’ geometry; and (b) the waveform must bepurely LTE or purely PTE.Conclusion: A complete analysis of the power-law dependencies of the diffusion-weightedsignal at high b-values has been performed. Only two specific forms of encoding result in apower-law dependency, pure linear and pure planar tensor encoding and when the microstructuralgeometry is ’stick-like’. The different exponents of these encodings could be used toprovide independent validation of the presence of stick-like geometries in vivo.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Psychology
Cardiff University Brain Research Imaging Centre (CUBRIC)
Publisher: Wiley
ISSN: 0740-3194
Funders: Wellcome Trust, EPSRC
Date of First Compliant Deposit: 13 January 2020
Date of Acceptance: 9 January 2020
Last Modified: 03 Aug 2020 13:16

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