NMR in biomedicine
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With the advent of ultra-high field MRI scanners in clinical research, susceptibility based MRI has recently gained increasing interest because of its potential to assess subtle tissue changes underlying neurological pathologies/disorders. Conventional, but rather slow, three-dimensional (3D) spoiled gradient-echo (GRE) sequences are typically employed to assess the susceptibility of tissue. 3D echo-planar imaging (EPI) represents a fast alternative but generally comes with echo-time restrictions, geometrical distortions and signal dropouts that can become severe at ultra-high fields. ⋯ The susceptibility maps obtained were comparable with regard to QSM values and geometric distortions to those calculated from a conventional 4 min 3D GRE scan using the same QSM processing pipeline. Copyright © 2016 John Wiley & Sons, Ltd.
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Quantitative susceptibility mapping (QSM) estimates the underlying tissue magnetic susceptibility from the gradient echo (GRE) phase signal through background phase removal and dipole inversion steps. Each of these steps typically requires the solution of an ill-posed inverse problem and thus necessitates additional regularization. Recently developed single-step QSM algorithms directly relate the unprocessed GRE phase to the unknown susceptibility distribution, thereby requiring the solution of a single inverse problem. ⋯ These acquisitions employ the rapid three-dimensional echo planar imaging (3D EPI) and Wave-CAIPI (controlled aliasing in parallel imaging) trajectories for signal-to-noise ratio-efficient whole-brain imaging. Herein, we also demonstrate the multi-echo capability of the Wave-CAIPI sequence for the first time, and introduce an automated, phase-sensitive coil sensitivity estimation scheme based on a 4-s calibration acquisition. Copyright © 2016 John Wiley & Sons, Ltd.
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Quantitative susceptibility mapping (QSM) measures tissue magnetic susceptibility and typically relies on time-consuming three-dimensional (3D) gradient-echo (GRE) MRI. Recent studies have shown that two-dimensional (2D) multi-slice gradient-echo echo-planar imaging (GRE-EPI), which is commonly used in functional MRI (fMRI) and other dynamic imaging techniques, can also be used to produce data suitable for QSM with much shorter scan times. However, the production of high-quality QSM maps is difficult because data obtained by 2D multi-slice scans often have phase inconsistencies across adjacent slices and strong susceptibility field gradients near air-tissue interfaces. ⋯ The experimental results show that this new 2D EPI-based QSM technique can produce quantitative susceptibility measures that are comparable with those of 3D GRE-based QSM across different brain regions (e.g. subcortical iron-rich gray matter, cortical gray and white matter). This new 2D EPI QSM reconstruction method is implemented within STI Suite, which is a comprehensive shareware for susceptibility imaging and quantification. Copyright © 2016 John Wiley & Sons, Ltd.
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The two-fold benefit of 1 H magnetic resonance spectroscopy (MRS) at high B0 fields - enhanced sensitivity and increased spectral dispersion - has been used previously to study dynamic changes in metabolite concentrations in the human brain in response to visual stimulation. In these studies, a strong visual on/off stimulus was combined with MRS data acquisition in a voxel location in the occipital cortex determined by an initial functional magnetic resonance imaging experiment. However, 1) to exclude the possibility of systemic effects (heartbeat, blood flow, etc.), which tend to be different for on/off conditions, a modified stimulation condition not affecting the target voxel needs to be employed, and 2) to assess important neurotransmitters of low concentration, in particular γ-aminobutyric acid (GABA), it may be advantageous to analyze steady-state, rather than dynamic, conditions. ⋯ The observed reduction in GABA can be interpreted as reduced neuronal inhibition during voxel activation, whereas the increase in lactate hints at an intensification of anaerobic glycolysis. Differences from previous studies, notably the absence of any changes in glutamate, are attributed to the modified experimental conditions. This study demonstrates that the use of advanced 1 H MRS methodology at 7 T allows the detection of subtle changes in metabolite concentrations involved in neuronal activation and inhibition.
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In ultrahigh-field MRI, such as 7 T, the signal-to-noise ratio (SNR) increases while transmit (Tx) field (B1+ ) can be degraded due to inhomogeneity and elevated specific absorption rate (SAR). By applying new array coil concepts to both Tx and receive (Rx) coils, the B1+ homogeneity and SNR can be improved. In this study, we developed and tested in vivo a new RF coil system for 7 T breast MRI. ⋯ For the in vivo comparisons between 3 T and 7 T, an approximately fourfold increase of SNR was measured with 7 T imaging. The B1+ field distributions in the breast model, phantom and in vivo were in reasonable agreement. High-quality 7 T in vivo breast MRI was successfully acquired at 0.6 mm isotropic resolution using the newly developed RF coil system.