50 research outputs found
On multi-path longitudinal spin relaxation in brain tissue
The purpose of this paper is to confirm previous reports that identified
magnetization transfer (MT) as an inherent driver of longitudinal relaxation in
brain tissue by asserting a substantial difference between the relaxation
times of the free and the semi-solid spin pools. Further, we aim to identify an
avenue towards the quantification of these relaxation processes on a
voxel-by-voxel basis in a clinical imaging setting, i.e. with a nominal
resolution of 1mm isotropic and full brain coverage in 12min. To this end, we
optimized a hybrid-state pulse sequence for mapping the parameters of an
unconstrained MT model. We scanned 4 people with relapsing-remitting multiple
sclerosis (MS) and 4 healthy controls with this pulse sequence and estimated
s and s for the free and semi-solid
spin pool of healthy WM, respectively, confirming previous reports and
questioning the commonly used assumptions or s.
Further, we estimated a fractional size of the semi-solid spin pool of , which is larger than previously assumed. An analysis of
in normal appearing white matter revealed statistically significant differences
between individuals with MS and controls. In conclusion, we confirm that
longitudinal spin relaxation in brain tissue is dominated by MT and that the
hybrid state facilitates a voxel-wise fit of the unconstrained MT model, which
enables the analysis of subtle neurodegeneration
A hierarquia monetĂĄria e suas implicaçÔes para as taxas de cĂąmbio e de juros e a polĂtica econĂŽmica dos paĂses perifĂ©ricos
Permanent and Selective Capital Account Management Regimes as an Alternative to Self-Insurance Strategies in Emerging-Market Economies
4D flow imaging with 2D-selective excitation
PURPOSE: 4D flow MRI permits to quantify non-invasively time-dependent velocity vector fields, but it demands long acquisition times. 2D-selective excitation allows to accelerate the acquisition by reducing the FOV in both phase encoding directions. In this study, we investigate 2D-selective excitation with reduced FOV imaging for fast 4D flow imaging while obtaining correct velocity quantification. METHODS: Two different 2D-selective excitation pulses were designed using spiral k-space trajectories. Further, their isophase time point was analyzed using simulations that considered both stationary and moving spins. On this basis, the 2D-selective RF pulses were implemented into a 4D flow sequence. A flow phantom study and seven 4D flow in vivo measurements were performed to assess the accuracy of velocity quantification by comparing the proposed technique to non-selective and conventional 1D slab-selective excitation. RESULTS: The isophase time point for spiral 2D-selective RF pulses was found to be located at the end of excitation for both stationary and moving spins. Based on that, 2D-selective excitation with reduced FOV allowed us to successfully quantify velocities both in a flow phantom and in vivo. In a flow phantom, the velocity difference Îv = (0.8 ± 5.3)cm/s between the smaller reduced FOV and the reference scan was similar to the inter-scan variability of Îv = (â1.0 ± 2.3)cm/s . In vivo, the differences in flow (P = 0.995) and flow volume (P = 0.469) between the larger reduced FOV and the reference scan were non-significant. By reducing the FOV by two-thirds, acquisition time was halved. CONCLUSION: A reduced field-of-excitation allows to limit the FOV and therefore shorten 4D flow acquisition times while preserving successful velocity quantification