Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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In this work, diffusion weighting and parallel imaging is combined with a vertical gradient and spin echo data readout. This sequence was implemented and evaluated on healthy volunteers using a 1.5 and a 3 T whole-body MR system. As the vertical gradient and spin echo trajectory enables a higher k-space velocity in the phase-encoding direction than single-shot echo planar imaging, the geometrical distortions are reduced. ⋯ However, this combination of a diffusion preparation and multiple refocusing pulses during the vertical gradient and spin echo readout, generally violates the Carr-Purcell-Meiboom-Gill condition, which leads to interferences between echo pathways. To suppress the stimulated echo pathway, refocusing pulses with a sharper slice profiles and an odd/even crusher variation scheme were implemented and evaluated. Being a single-shot acquisition technique, the reconstructed images are robust to rigid-body head motion and spatially varying brain motion, both of which are common sources of artifacts in diffusion MRI.
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This study demonstrates the dependence of non-local susceptibility effects on object orientation in gradient echo MRI and the reduction of non-local effects by deconvolution using quantitative susceptibility mapping. Imaging experiments were performed on a 3T MRI system using a spoiled 3D multi-echo GRE sequence on phantoms of known susceptibilities, and on human brains of healthy subjects and patients with intracerebral hemorrhages. Magnetic field measurements were determined from multiple echo phase data. ⋯ Phantom and human data demonstrated that the hypointense region in GRE magnitude image corresponding to a susceptibility source increased in volume with TE and varied with the source orientation. The induced magnetic field extended beyond the susceptibility source and varied with its orientation. In quantitative susceptibility mapping, these blooming artifacts, including their dependence on object orientation, were reduced, and the material susceptibilities were quantified.
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The aim of this study was to investigate T₂* in the Achilles tendon (AT), in vivo, using a three-dimensional ultrashort time echo (3D-UTE) sequence, to compare field strength differences (3 and 7 T) and to evaluate a regional variation of T₂* in healthy and pathologic tendon. Ten volunteers with no history of pain in the AT and five patients with chronic Achilles tendinopathy were recruited. 3D-UTE images were measured with the following echo times, at echo time = [0.07, 0.2, 0.33, 0.46, 0.59, 0.74, 1.0, 1.5, 2.0, 4.0, 6.0, and 9.0 ms]. T₂* values in the AT were calculated by fitting the signal decay to biexponential function. ⋯ The results of this study suggest that the regional variability of AT can be quantified by T₂* in in vivo conditions. Advanced quantitative imaging of the human AT using a 3D-UTE sequence may provide additional information to standard clinical imaging. Finally, as the preliminary patient data suggest, T(2s)* may be a promising marker for the diagnosis of pathological changes in the AT.
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Recently, spatially two-dimensional selective radiofrequency excitations based on the PROPELLER trajectory have been presented and were applied to minimize partial volume effects in single-voxel MR spectroscopy. Thereby, residual side excitations appeared due to limitations of the Voronoi diagram that was used to consider the nonconstant sampling density, and trajectory distortions caused by eddy currents varying between the differently rotated blades. ⋯ Thus, signal contributions from outside the desired region-of-interest are completely avoided. The feasibility of this approach to acquire single-voxel MR spectra of anatomically defined regions-of-interest is demonstrated in the human brain in vivo on a 3T whole-body MR system.
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Uncertainty in arterial input function (AIF) estimation is one of the major errors in the quantification of dynamic contrast-enhanced MRI. A blind source separation algorithm was proposed to determine the AIF by selecting the voxel time course with maximum purity, which represents a minimal contamination from partial volume effects. Simulations were performed to assess the partial volume effect on the purity of AIF, the estimation accuracy of the AIF, and the influence of purity on the derived kinetic parameters. ⋯ The derived kinetic parameters in the tumor were more susceptible to the changes in purity when compared with those in the muscle. The animal experiment demonstrated good reproducibility in blind source separation-AIF derived parameters. In conclusion, the blind source separation method is feasible and reproducible to identify the voxel with the tracer concentration time course closest to the true AIF.