RESUMEN
PURPOSE: A new sequence combining chemical-exchange saturation-transfer (CEST) with traditional MRS is used to simultaneously determine metabolite content and effects of magnetization exchange. METHODS: A CEST saturation block consisting of a train of RF-pulses is placed before a metabolite-cycled semi-LASER single-voxel spectroscopy sequence. The saturation parameters are adjustable to allow optimization of the saturation for a specific target. Data were collected in brain from 20 subjects in experiments with different B1 -settings (0.4-2.0 µT) on a 3T MR scanner. CEST Z-spectra were calculated from water intensities and fitted with a multi-pool Lorentzian model. Interrelated metabolite spectra were fitted in fitting tool for arrays of interrelated datasets (FiTAID). RESULTS: Evaluation of traditional Z-spectra from water revealed exchange effects from amides, amines, and hydroxyls as well as an upfield nuclear Overhauser effect. The magnetization transfer effect was evaluated on metabolites and macromolecules for the whole spectral range and for the different B1 levels. A correction scheme for direct saturation on metabolites is proposed. Both magnetization-transfer and direct saturation proved to differ for individual metabolites. CONCLUSION: Using non-water-suppressed spectroscopy offers time-saving simultaneous recording of the traditional CEST Z-spectrum from water and the metabolite spectrum under frequency-selective saturation. In addition, exchange and magnetization-transfer effects on metabolites and macromolecules can be detected, which might offer additional possibilities for quantification or give further insight into the composition of the traditional CEST Z-spectrum. Apparent magnetization-transfer effects on macromolecular signals in the 1 H-MR spectrum have been found. Detailed knowledge of magnetization-transfer effects is also relevant for judging the influence of water-suppression on the quantification of metabolite signals.
Asunto(s)
Neoplasias Encefálicas , Imagen por Resonancia Magnética , Algoritmos , Encéfalo/diagnóstico por imagen , Humanos , Espectroscopía de Resonancia MagnéticaRESUMEN
PURPOSE: To perform exchange-rate measurements on the in vivo human brain downfield spectrum (5-10 ppm) at 9.4 T and to compare the variation in concentrations of the downfield resonances and of known upfield metabolites to determine potential peak labels. METHODS: Non-water-suppressed metabolite cycling was used in combination with an inversion transfer technique in two brain locations in healthy volunteers to measure the exchange rates and T1 values of exchanging peaks. Spectra were fitted with a heuristic model of a series of 13 or 14 Voigt lines, and a Bloch-McConnell model was used to fit the exchange rate curves. Concentrations from non-water-inverted spectra upfield and downfield were compared. RESULTS: Mean T1 values ranged from 0.40 to 0.77 s, and exchange rates from 0.74 to 13.8 s-1 . There were no significant correlations between downfield and upfield concentrations, except for N-acetylaspartate, with a correlation coefficient of 0.63 and P < 0.01. CONCLUSIONS: Using ultrahigh field allowed improved separation of peaks in the 8.2 to 8.5 ppm amide proton region, and the exchange rates of multiple downfield resonances including the 5.8-ppm peak, previously tentatively assigned to urea, were measured in vivo in human brain. Downfield peaks consisted of overlapping components, and largely missing correlations between upfield and downfield resonances-although not conclusive-indicate limited contributions from metabolites present upfield to the downfield spectrum. Magn Reson Med 79:2863-2873, 2018. © 2017 International Society for Magnetic Resonance in Medicine.
Asunto(s)
Encéfalo/diagnóstico por imagen , Procesamiento de Imagen Asistido por Computador/métodos , Espectroscopía de Resonancia Magnética , Agua/química , Adulto , Algoritmos , Ácido Aspártico/análogos & derivados , Ácido Aspártico/química , Femenino , Humanos , Magnetismo , Masculino , Adulto JovenRESUMEN
PURPOSE: To characterize the downfield spectrum at 5-10 ppm in the human brain at a high magnetic field of 7 T. Knowledge of relaxation parameters is of interest for spectroscopy as well as chemical exchange-dependent saturation transfer experiments. METHODS: Water-suppressed spectra were recorded as echo time and inversion time series in healthy volunteers to investigate T2 and T1 values of downfield peaks in gray matter at 7T. The spectra were fitted in a two-dimensional fashion to a heuristic model of a series of Voigt lines, and the relaxation times were obtained for 12 peaks of interest. RESULTS: The mean T2 values averaged over the volunteers ranged from 24 to 158 ms, whereas the mean T1 values ranged from 0.22 to 2.40 s. Spectra of specific inversion and echo times revealed superposition of the amide peaks of N-acetylaspartate with short T2 and an inhomogeneously broadened component with longer T2 . CONCLUSIONS: T2 values were shorter than expected for most peaks, whereas T1 values had a very wide range; shorter relaxation times for some peaks suggests the presence of macromolecules. Most of the larger peaks seemed to be composed of overlapping components, because the Gaussian widths in the Voigt line shape descriptions were larger than expected based on field inhomogeneities. Magn Reson Med 78:11-19, 2016. © 2017 International Society for Magnetic Resonance in Medicine.
Asunto(s)
Algoritmos , Ácido Aspártico/análogos & derivados , Biopolímeros/análisis , Química Encefálica , Sustancia Gris/química , Modelos Químicos , Procesamiento de Señales Asistido por Computador , Ácido Aspártico/análisis , Simulación por Computador , Humanos , Modelos Neurológicos , Reproducibilidad de los Resultados , Sensibilidad y EspecificidadRESUMEN
Multiecho T(2) relaxation measurements offer specific information about myelin content through the myelin water fraction (MWF), as well as about the water environments through the intra- and extra-cellular (IE), and global, geometric mean T(2) (GMT(2)) times. While these measurements have yielded new insights into brain development and pathologies, they have yet to be thoroughly investigated in the spinal cord. The goals of this study were: (1) to apply a new 3D multiecho T(2) relaxation measurement in the cervical spine with sufficient axial resolution to distinguish grey and white matter; (2) to perform a pilot reliability assessment of the resulting MWF and GMT(2) measures in a target population; and (3) to detect differences in these measures between a younger cohort (20-30 years of age) and an older cohort (50-75 years of age) of healthy adults. The results demonstrated that the MWF in younger healthy adults follows the known pattern of lower myelin content in grey matter (mean (95% confidence interval)) (0.049 (0.030-0.067)) as compared to white matter (0.296 (0.275-0.317), p<0.001). The reliability coefficients were 0.65 and 0.82 for the MWF in the dorsal (DC) and lateral column (LC) white matter, respectively; 0.79 and 0.52 for the IE GMT(2); and 0.74 and 0.73 for the global GMT(2). Significantly lower MWF were found in the older adults than in the younger adults (DC p=0.014; LC p=0.012), as well as lower IE GMT(2) times (DC p=0.008; LC p=0.042), however, the global GMT(2) times did not show any differences. These changes in MWF and IE GMT(2) times, but not in global GMT(2) times, indicate that multiecho T(2) relaxation measures are sensitive to changes in myelin integrity and cell morphology that may not be apparent on conventional T(2) weighted images.