Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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A method for estimating T1 using a single breath-hold, segmented, inversion recovery prepared, true fast imaging with steady-state precession (sIR-TrueFISP) acquisition at low flip angle (FA) was implemented in this study. T1 values measured by sIR-TrueFISP technique in a Gd-DTPA-doped water phantom and the human brain and abdomen of healthy volunteers were compared with the results of the standard IR fast spin echo (FSE) technique. A good correlation between the two methods was observed (R2=0.999 in the phantom, and R2=0.943 in the brain and abdominal tissues). The T1 values of the tissues agreed well with published results. sIR-TrueFISP enables fast measurements of T1 to be obtained within a single breath-hold with good accuracy, which is particularly important for chest and abdominal imaging.
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Previous studies have shown the relative importance of physiological noise and thermal noise in 2D MR images. Since physiological noise is proportional to the signal, it can be the dominant component at the center of k-space. In this study we demonstrate that the signal-to-noise ratio (SNR) efficiency and temporal resolution for 3D functional MRI (fMRI) are increased by the use of a partial-k-space acquisition method. ⋯ Thus, the partial-k-space 3D method sacrifices much less SNR than is expected from the thermal noise model, and the SNR efficiency is increased compared to a full-k-space acquisition since more time frames can be collected for the same scan time. Accordingly, the temporal resolution can be increased in 3D acquisitions because only partial coverage of k-space is necessary. Experimental results confirm that more activation with a higher average t-score is detected by this method.
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The radial trajectory has found applications in cardiac imaging because of its resilience to undersampling and motion artifacts. Recent work has shown that interleaved and weighted radial imaging can produce images with multiple contrasts from a single data set. This feature was investigated for inversion recovery imaging of scar using a radial technique. ⋯ The interleaved acquisition provided better image quality than the noninterleaved radial acquisition. Interleaving with weighting provided better quality when the inversion time (TI) was shorter than optimal; otherwise, interleaving without weighting was superior. These methods enable a radial trajectory to be employed in conjunction with preparation pulses for viability imaging.
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Comparative Study
Measurement of brain glutamate and glutamine by spectrally-selective refocusing at 3 Tesla.
A new single-voxel proton NMR spectrally-selective refocusing method for measuring glutamate (Glu) and glutamine (Gln) in the human brain in vivo at 3T is reported. Triple-resonance selective 180 degrees RF pulses with a bandwidth of 12 Hz were implemented within point-resolved spectroscopy (PRESS) for selective detection of Glu or Gln, and simultaneous acquisition of creatine singlets for use as a reference in phase correction. The carriers of the spectrally-selective 180 degrees pulses and the echo times (TEs) were optimized with both numerical and experimental analyses of the filtering performance, which enabled measurements of the target metabolites with negligible contamination from N-acetylaspartate and glutathione. The concentrations of Glu and Gln in the prefrontal cortex were estimated to be 9.7+/-0.5 and 3.0+/-0.7 mM (mean+/-SD, N=7), with reference to Cr at 8 mM.
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Both ultrashort echo-time STEAM and MEGA-PRESS-edited spectroscopy were used to validate noninvasive quantification of vitamin C (ascorbate) in the developing rat brain, where changes in ascorbate concentration have been reported. Despite strong overlap with resonances from glutamine, glutamate, glutathione, and macromolecules, reliable quantification of ascorbate (Cramer-Rao lower bounds<0.2 micromol/g) by LCModel analysis of STEAM (TE=2 ms) spectra was possible at 9.4 T. Ascorbate concentrations quantified from the STEAM spectra were in very good agreement with concentrations calculated from fully resolved ascorbate resonances in MEGA-PRESS-edited spectra measured from identical volumes of interest. Ascorbate concentrations measured using STEAM decreased with increasing postnatal rat age, in agreement with published brain ascorbate concentrations measured in vitro using high-performance liquid chromatography (HPLC).