NMR in biomedicine
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White matter (WM) perfusion has great potential as a physiological biomarker in many neurological diseases. Although it has been demonstrated previously that arterial spin labeling magnetic resonance imaging (ASL-MRI) enables the detection of the perfusion-weighted signal in most voxels in WM, studies of cerebral blood flow (CBF) in WM by ASL-MRI are relatively scarce because of its particular challenges, such as significantly lower perfusion and longer arterial transit times relative to gray matter (GM). Recently, ASL with a spectroscopic readout has been proposed to enhance the sensitivity for the measurement of WM perfusion. ⋯ Perfusion measurements by te-pCASL PRESS and conventional EPI showed no significant difference for quantitative WM CBF values (Student's t-test, p = 0.19) or temporal SNR (p = 0.33 and p = 0.81 for GM and WM, respectively), whereas GM CBF values (p = 0.016) were higher using PRESS than EPI readout. WM CBF values were found to be 18.2 ± 7.6 mL/100 g/min (PRESS) and 12.5 ± 5.5 mL/100 g/min (EPI), whereas GM CBF values were found to be 77.1 ± 11.2 mL/100 g/min (PRESS) and 53.6 ± 9.6 mL/100 g/min (EPI). This study demonstrates the feasibility of te-pCASL PRESS for the quantification of WM perfusion changes in a highly time-efficient manner, but it does not result in improved temporal SNR, as does traditional te-pCASL EPI, which remains the preferred option because of its flexibility in use.
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The reproducibility of gamma-aminobutyric acid (GABA) quantification results, obtained with MRSI, was determined on a 3 T MR scanner in healthy adults. In this study, a spiral-encoded, GABA-edited, MEGA-LASER MRSI sequence with real-time motion-scanner-instability corrections was applied for robust 3D mapping of neurotransmitters in the brain. In particular, the GABA+ (i.e. ⋯ For both inter-subject and intra-subject repeated measurement sessions a low coefficient of variation (CV) and a high intraclass correlation coefficient (ICC) were found for GABA+ and Glx ratios across all evaluated voxels (intra-/inter-subject: GABA+ ratios, CV ~ 8%-ICC > 0.75; Glx ratios, CV ~ 6%-ICC > 0.70). The same was found in selected brain regions for Glx ratios versus GABA+ ratios (CV varied from about 5% versus about 8% in occipital and parietal regions, to about 8% versus about 10% in the frontal area, thalamus, and basal ganglia). These results provide evidence that 3D mapping of GABA+ and Glx using the described methodology provides high reproducibility for application in clinical and neuroscientific studies.
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Acidity within the extracellular milieu is a hallmark of cancer. There is a current need for fast, high spatial resolution pH imaging techniques for clinical evaluation of cancers, including gliomas. Chemical exchange saturation transfer (CEST) MRI targeting fast-exchanging amine protons can be used to obtain high-resolution pH-weighted images, but conventional CEST acquisition strategies are slow. ⋯ Results showed that the Z-spectrum was highly dependent on saturation pulse shape, repetition time, saturation amplitude, magnetic field strength, and T2 within bulk water; however, the Z-spectrum was only minimally influenced by saturation pulse duration and the specific relaxation rates of amine protons. Results suggest that a Gaussian saturation pulse train consisting of 3 × 100 ms pulses using the minimum allowable repetition time is optimal for achieving over 90% available contrast across all tissues. Results also demonstrate that high saturation pulse amplitude and scanner field strength (>3 T) are necessary for adequate endogenous pH-weighted amine CEST contrast. pH-weighted amine CEST contrast increased with increasing tumor grade, with glioblastoma showing significantly higher contrast compared with WHO II or III gliomas.