NMR in biomedicine
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Signal loss observed in the brain by MRI following the administration of ultrasmall superparamagnetic particles of iron oxide (USPIO) has been correlated with immune cell activity in inflammatory areas during multiple sclerosis. Uptake of USPIO by circulating monocytes and their migration towards inflammatory areas have been considered as the most important mechanism for USPIO uptake by the brain parenchyma. However, the involvement of a damaged blood-brain barrier is also debated as a possible mechanism for cerebral USPIO uptake. ⋯ The current study demonstrates that USPIO enter the central nervous system directly after administration, pointing to the involvement of a damaged blood-brain barrier in the appearance of USPIO-associated MR abnormalities. Furthermore, a possible role of the cervical lymph nodes as a drainage pathway of USPIO is suggested. These data shed new light on the use of USPIO in neuroinflammatory diseases, identifying USPIO as a marker for both cellular infiltration and blood-brain barrier damage.
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The combination of flow-sensitive alternating inversion recovery (FAIR) and single-shot k-space-banded gradient- and spin-echo (kbGRASE) is proposed here to measure perfusion in the mouse brain with high sensitivity and stability. Signal-to-noise ratio (SNR) analysis showed that kbGRASE-FAIR boosts image and temporal SNRs by 2.01 ± 0.08 and 2.50 ± 0.07 times, respectively, when compared with standard single-shot echo planar imaging (EPI)-FAIR implemented in our experimental systems, although the practically achievable spatial resolution was slightly reduced. ⋯ The functional MRI time courses with kbGRASE-FAIR showed a more stable response to 5% CO(2) than did those with EPI-FAIR. The results establish kbGRASE-FAIR as a practical and robust protocol for quantitative CBF measurements in mice at 9.4 T.
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In vivo high-field MRI in the abdomen of small animals is technically challenging because of the small voxel sizes, short T(2) and physiological motion. In standard Cartesian sampling, respiratory and gastrointestinal motion can lead to ghosting artefacts. Although respiratory triggering and navigator echoes can either avoid or compensate for motion, they can lead to variable TRs, require invasive intubation and ventilation, or extend TEs. ⋯ The results were similar, with the exception that respiratory triggering was unable to exclude major motion artefacts as a result of the sensitisation to motion by the diffusion gradients. The PROPELLER data were of consistently higher quality. Considerations specific to the use of PROPELLER at high field are discussed, including the selection of practical blade widths and the effects on contrast, resolution and artefacts.
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In conventional diffusion tensor imaging (DTI), water diffusion distribution is described as a 2nd-order three-dimensional (3D) diffusivity tensor. It assumes that diffusion occurs in a free and unrestricted environment with a Gaussian distribution of diffusion displacement, and consequently that diffusion weighted (DW) signal decays with diffusion factor (b-value) monoexponentially. In biological tissue, complex cellular microstructures make water diffusion a highly hindered or restricted process. ⋯ Several recent rodent DKI studies from our group are summarized, and DKI and DTI compared for their efficacy in detecting neural tissue alterations. They demonstrate that DKI offers a more comprehensive approach than DTI in describing the complex water diffusion process in vivo. By estimating both diffusivity and kurtosis, it may provide improved sensitivity and specificity in MR diffusion characterization of neural tissues.
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The interaction ('cross terms') between diffusion-weighting gradients and susceptibility-induced background gradient fields around vessels has an impact on apparent diffusion coefficient (ADC) measurements and diffusion-weighted functional magnetic resonance imaging (DFMRI) experiments. Monte-Carlo (MC) simulations numerically integrating the Bloch equations for a large number of random walks in a vascular model were used to investigate to what extent such interactions would influence the extravascular signal change as well as the ADC change observed in DFMRI experiments. ⋯ In such a network, the cross terms result in the observation of a functional increase in ADC accompanied by a descending percent signal change with increasing diffusion weighting. It is shown that the twice-refocused spin-echo sequence permits sufficient yet not total suppression of such effects compared to the standard Stejskal-Tanner spin-echo diffusion weighting under experimentally relevant conditions.