γ‐Aminobutyric acid (GABA) and glutamate (Glu), major neurotransmitters in the brain, are recycled through glutamine (Gln). All three metabolites can be measured by magnetic resonance spectroscopy in vivo, although GABA measurement at 3 T requires an extra editing acquisition, such as Mescher–Garwood point‐resolved spectroscopy (MEGA‐PRESS). In a GABA‐edited MEGA‐PRESS spectrum, Glu and Gln co‐edit with GABA, providing the possibility to measure all three in one acquisition. In this study, we investigated the reliability of the composite Glu + Gln (Glx) peak estimation and the possibility of Glu and Gln separation in GABA‐edited MEGA‐PRESS spectra. The data acquired in vivo were used to develop a quality assessment framework which identified MEGA‐PRESS spectra in which Glu and Gln could be estimated reliably. Phantoms containing Glu, Gln, GABA and N‐acetylaspartate (NAA) at different concentrations were scanned using GABA‐edited MEGA‐PRESS at 3 T. Fifty‐six sets of spectra in five brain regions were acquired from 36 healthy volunteers. Based on the Glu/Gln ratio, data were classified as either within or outside the physiological range. A peak‐by‐peak quality assessment was performed on all data to investigate whether quality metrics can discriminate between these two classes of spectra. The quality metrics were as follows: the GABA signal‐to‐noise ratio, the NAA linewidth and the Glx Cramer–Rao lower bound (CRLB). The Glu and Gln concentrations were estimated with precision across all phantoms with a linear relationship between the measured and true concentrations: R 1 = 0.95 for Glu and R 1 = 0.91 for Gln. A quality assessment framework was set based on the criteria necessary for a good GABA‐edited MEGA‐PRESS spectrum. Simultaneous criteria of NAA linewidth <8 Hz and Glx CRLB <16% were defined as optimum features for reliable Glu and Gln quantification. Glu and Gln can be reliably quantified from GABA‐edited MEGA‐PRESS acquisitions. However, this reliability should be controlled using the quality assessment methods suggested in this work.
Purpose Glutathione (GSH) is an important intracellular antioxidant in the brain. A number of studies report its measurement by localized 1H spectroscopy using PRESS and STEAM. This study evaluates the reliability and accuracy of GSH measurements from PRESS at 3T and compares the results to those obtained with MEGA-PRESS. Methods Phantoms containing brain metabolites, identical except for variable GSH concentration between 0mM and 24mM, were scanned using PRESS (TE=35ms) and MEGA-PRESS(optimized TE=130ms) at 3T. Spectra of the anterior cingulate cortex and occipital cortex in 7 healthy volunteers were also acquired. Results Phantom GSH concentrations from 0 to 3mM were unreliably quantified using PRESS although at 4mM and above there was a linear relationship between measured and true concentrations (R2=0.99). Using MEGA-PRESS, there was no signal detected at 0mM GSH, plus a linear relationship (R2=0.99) over the full range from 0 – 24mM. In brain, concentrations calculated from MEGA-PRESS and PRESS were significantly different in occipital cortex (P<0.001). Moreover only MEGA-PRESS reported significant differences in [GSH] between the two brain regions (P=0.003). Conclusion Due to uncertainties in GSH quantification raised by the study, authors conclude that physiological concentrations (<4mM) of GSH cannot be reliably quantified from PRESS (TE=35ms) spectra at 3T.
Purpose: Magnetic resonance spectroscopic imaging (MRSI) provides complementary information to conventional magnetic resonance imaging. Acquiring high resolution MRSI is time consuming and requires complex reconstruction techniques.Methods: In this paper, a patch-based super-resolution method is presented to increase the spatial resolution of metabolite maps computed from MRSI. The proposed method uses high resolution anatomical MR images (T1-weighted and Fluid-attenuated inversion recovery) to regularize the super-resolution process. The accuracy of the method is validated against conventional interpolation techniques using a phantom, as well as simulated and in vivo acquired human brain images of multiple sclerosis subjects.Results: The method preserves tissue contrast and structural information, and matches well with the trend of acquired high resolution MRSI.Conclusions: These results suggest that the method has potential for clinically relevant neuroimaging applications.
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