38 research outputs found
Effects of nongaussian diffusion on "isotropic diffusion measurements'': an ex-vivo microimaging and simulation study
Designing novel diffusion-weighted pulse sequences to probe tissue
microstructure beyond the conventional Stejskal-Tanner family is currently of
broad interest. One such technique, multidimensional diffusion MRI, has been
recently proposed to afford model-free decomposition of diffusion signal
kurtosis into terms originating from either ensemble variance of isotropic
diffusivity or microscopic diffusion anisotropy. This ability rests on the
assumption that diffusion can be described as a sum of multiple Gaussian
compartments, but this is often not strictly fulfilled. The effects of
nongaussian diffusion on single shot isotropic diffusion sequences were first
considered in detail by de Swiet and Mitra in 1996. They showed theoretically
that anisotropic compartments lead to anisotropic time dependence of the
diffusion tensors, which causes the measured isotropic diffusivity to depend on
gradient frame orientation. Here we show how such deviations from the multiple
Gaussian compartments assumption conflates orientation dispersion with ensemble
variance in isotropic diffusivity. Second, we consider additional contributions
to the apparent variance in isotropic diffusivity arising due to
intracompartmental kurtosis. These will likewise depend on gradient frame
orientation. We illustrate the potential importance of these confounds with
analytical expressions, numerical simulations in simple model geometries, and
microimaging experiments in fixed spinal cord using isotropic diffusion
encoding waveforms with 7.5 ms duration and 3000 mT/m maximum amplitude.Comment: 26 pages, 9 figures. Appearing in J. Magn. Reso
Incorporating white matter microstructure in the estimation of magnetic susceptibility in ex-vivo mouse brain
Accurate estimation of microscopic magnetic field variations induced in
biological tissue can be valuable for mapping tissue composition in health and
disease. Here, we present an extension to Quantitative susceptibility mapping
(QSM) to account for local white matter (WM) microstructure by using our
previously presented model for solid cylinders with arbitrary orientations to
describe axons in terms of concentric cylinders. We show how multi-gradient
echo (MGE) and diffusion MRI (dMRI) images can be combined to estimate an
apparent scalar susceptibility. Experiments in mouse brains acquired at
ultrahigh field shows the mesoscopic contribution due to WM microstructure to
be substantial, with a magnitude up to 70% of the total frequency shift in
highly anisotropic WM. This in turn changed estimated susceptibility values up
to 56% in WM compared to standard QSM. Our work underscores how microstructural
field effects impact susceptibility estimates, and should not be neglected when
imaging anisotropic tissue such as brain WM.Comment: 33 pages, 7 figure
To mask or not to mask? Improving QSM quality by accounting for spatial frequency distributions and susceptibility sources
Estimating magnetic susceptibility using MRI depends on inverting a forward
relationship between the susceptibility and measured Larmor frequency. However,
an often-overlooked constraint in susceptibility fitting is that the Larmor
frequency is only measured inside the sample, and after background field
removal, susceptibility sources should only reside inside the same sample. Here
we test the impact of accounting for such effects in susceptibility fitting and
demonstrate that such effects should not be ignored.Comment: 22 pages, 5 figure
The Larmor frequency shift of a white matter magnetic microstructure model with multiple sources
Magnetic susceptibility imaging may provide valuable information about
chemical composition and microstructural organization of tissue. However, its
estimation from the MRI signal phase is particularly difficult as it is
sensitive to magnetic tissue properties ranging from the molecular to
macroscopic scale. The MRI Larmor frequency shift measured in white matter (WM)
tissue depends on the myelinated axons and other magnetizable sources such as
iron-filled ferritin. We have previously derived the Larmor frequency shift
arising from a dense media of cylinders with scalar susceptibility and
arbitrary orientation dispersion. Here we extend our model to include
microscopic WM susceptibility anisotropy as well as spherical inclusions with
scalar susceptibility to represent subcellular structures, biologically stored
iron etc. We validate our analytical results with computer simulations and
investigate the feasibility of estimating susceptibility using simple iterative
linear least squares without regularization or preconditioning. This is done in
a digital brain phantom synthesized from diffusion MRI (dMRI) measurements of
an ex vivo mouse brain at ultra-high field.Comment: 70 pages, 14 figure
ADC Histograms from Routine DWI for Longitudinal Studies in Cerebral Small Vessel Disease: A Field Study in CADASIL.
Diffusion tensor imaging (DTI) histogram metrics are correlated with clinical parameters in cerebral small vessel diseases (cSVD). Whether ADC histogram parameters derived from simple diffusion weighted imaging (DWI) can provide relevant markers for long term studies of cSVD remains unknown. CADASIL patients were evaluated by DWI and DTI in a large cohort study overa6-year period. ADC histogram parameters were compared to those derived from mean diffusivity (MD) histograms in 280 patients using intra-class correlation and Bland-Altman plots. Impact of image corrections applied to ADC maps was assessed and a mixed effect model was used for analyzing the effects of scanner upgrades. The results showed that ADC histogram parameters are strongly correlated to MD histogram parameters and that image corrections have only limited influence on these results. Unexpectedly, scanner upgrades were found to have major effects on diffusion measures with DWI or DTI that can be even larger than those related to patients' characteristics. These data support that ADC histograms from daily used DWI can provide relevant parameters for assessing cSVD, but the variability related to scanner upgrades as regularly performed in clinical centers should be determined precisely for longitudinal and multicentric studies using diffusion MRI in cSVD