Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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Non-2DFT k-space readout strategies are useful in fast imaging but prone to blurring when reconstructed off resonance. Field inhomogeneities or susceptibility variations, coupled with a long readout time, are the major sources of this artifact. Correction methods based on a priori off-resonance information such as an acquired field map have been proposed in the literature. ⋯ This paper introduces an improved algorithm for field map estimation which is both faster and more robust than the existing method. It uses a multi-stage estimation of the field map, starting from a coarse estimate both in frequency and space and proceeds towards higher resolution. The new algorithm is applied to phantom and in vivo images acquired with radial and spiral sequences to give sharper images.
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A method for volume selective proton spectroscopy is presented based on a multiecho sequence with short refocusing interval tcp. It is demonstrated, that by appropriate choice of tcp on the order of 4-6 ms, signals from overlapping multiplets like the glutamine and glutamate (Glu/Gln) resonances in spectra of the human brain are considerably increased compared with a conventional PRESS volume selection scheme. Thus proton spectra from J-coupled multiplet signals can be acquired with TE on the order of 20-30 ms avoiding the baseline problems arising at shorter echo times due to broad resonances. This allows to selectively acquire spectra from substances with longer T2 without the confounding effects from J-coupling occurring in conventional volume selection techniques.
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A method for the acquisition of localized 2D shift-correlated spectra, based on the combination of the stimulated-echo volume-selection and gradient-enhanced COSY experiments, is described. The sequence can be modified to perform a number of localized experiments including HOHAHA and DQF-COSY. ⋯ The major cerebral metabolites are represented in the in vivo COSY brain spectrum, including N-acetylaspartate, glutamate/glutamine, total creatine, aspartate, and myo-inositol. Difficulties in the implementation of localized shift-correlation spectroscopy, including water suppression and T2 relaxation, are discussed.
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A number of pulse sequence techniques, including magnetization-prepared gradient echo (MP-GRE), segmented GRE, and hybrid RARE, employ a relatively large number of variable pulse sequence parameters and acquire the image data during a transient signal evolution. These sequences have recently been proposed and/or used for clinical applications in the brain, spine, liver, and coronary arteries. Thus, the need for a method of deriving optimal pulse sequence parameter values for this class of sequences now exists. ⋯ Using our algorithm, the values of 35 sequence parameters, including the magnetization-preparation RF pulse flip angle and delay time, 32 flip angles in the variable flip angle gradient-echo acquisition sequence, and the magnetization recovery time, were derived. Optimized 3D MP-RAGE achieved up to a 130% increase in white matter-gray matter signal difference compared with optimized 3D RF-spoiled FLASH with the same total acquisition time. The simulated annealing approach was effective at deriving optimal parameter values for a specific 3D MP-RAGE imaging objective, and may be useful for other imaging objectives and sequences in this general class.
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The 1H NMR spectrum of the macromolecule fraction of rat brain cytosol was investigated following centrifugation and dialysis to remove low molecular weight metabolites and peptides (< 3500 daltons). At least seven well resolved resonances were detected between 0.9 and 3.0 ppm in the 1H NMR spectrum of rat brain cytosol after dialysis, several of which cannot be observed in vivo due to overlap with N-acetylaspartate, glutamate, glutamine, creatine, and gamma-aminobutyric acid. Several cross-peaks detected in 2D COSY spectra of the cytosolic macromolecule fraction coincided with those measured in a previous study of rat brain tissue in vitro and in situ (K. ⋯ Treatment of the cytosolic macromolecule fraction with a nonspecific protease permitted partial assignments of resonances in the 1H NMR spectrum to specific amino acids. Fractionation of the dialyzed cytosol of rat brain by gel filtration yielded qualitatively similar 1H NMR spectra for elution volumes corresponding to molecular masses from 12.5 kDa to over 100 kDa. The results indicate that many of the background nonmetabolite resonances observed in the 1H NMR spectrum of normal brain tissue arise from cytosolic proteins.