Magnetic resonance spectroscopy enables insight into the chemical composition of spinal cord tissue. However, spinal cord magnetic resonance spectroscopy has rarely been applied in clinical work due to technical challenges, including strong susceptibility changes in the region and the small cord diameter, which distort the lineshape and limit the attainable signal to noise ratio. Hence, extensive signal averaging is required, which increases the likelihood of static magnetic field changes caused by subject motion (respiration, swallowing), cord motion, and scanner-induced frequency drift. To avoid incoherent signal averaging, it would be ideal to perform frequency alignment of individual free induction decays before averaging. Unfortunately, this is not possible due to the low signal to noise ratio of the metabolite peaks. In this article, frequency alignment of individual free induction decays is demonstrated to improve spectral quality by using the high signal to noise ratio water peak from non-water-suppressed proton magnetic resonance spectroscopy via the metabolite cycling technique. Electrocardiography (ECG)-triggered point resolved spectroscopy (PRESS) localization was used for data acquisition with metabolite cycling or water suppression for comparison. A significant improvement in the signal to noise ratio and decrease of the Cramér Rao lower bounds of all metabolites is attained by using metabolite cycling together with frequency alignment, as compared to water-suppressed spectra, in 13 healthy volunteers.

The use of water suppression for in vivo proton MR spectroscopy diminishes the signal intensities from resonances that undergo magnetization exchange with water, particularly those downfield of water. To investigate these exchangeable resonances, an inversion transfer experiment was performed using the metabolite cycling technique for non-watersuppressed MR spectroscopy from a large brain voxel in 11 healthy volunteers at 3.0 T. The exchange rates of the most prominent peaks downfield of water were found to range from 0.5 to 8.9 s 21, while the T 1 relaxation times in absence of exchange were found to range from 175 to 525 ms. The use of water suppression for in vivo proton MR spectroscopy (MRS) arose as a solution for difficulties with the limited dynamic range of analog-to-digital converters and baseline distortions caused by the several orders of magnitude difference in concentration between water and metabolite protons. However, the use of water suppression can also diminish signal from metabolites of interest including metabolites that exchange magnetization with water, such as those containing amide moieties, resulting in spectra with decreased information content. Acquiring spectra without water suppression enables automatic phase, frequency and eddy current correction of individual traces, as well as offers an internal reference signal for absolute quantification, without the requirement of additional measurements. Unfortunately, these advantages are hindered by the presence of gradient coil vibration artifacts, also known as ''sidebands,'' caused by temporal fluctuations in the local amplitude of static (polarizing) field (B 0 ) following gradient switching (1).Several experimental and post-acquisition processing methods have been proposed to eliminate sidebands from MR spectra acquired with clinical systems. Experimental methods proposed to date can be roughly categorized as (a) selective dephasing of water, (b) gradient cycling, (c) metabolite cycling, (d) B 0 compensation, or (e) sideband subtraction. Selective dephasing gradients (such as those found in the WATERGATE (WATER suppression by GRAdient Tailored Excitation) sequence) can be incorporated into the evolution period of a localization sequence to selectively dephase water before detection, thus minimizing time for magnetization exchange; however, selective pulses with sharp transition bands are relatively long and, hence, do not allow for short echo times (TEs) at clinical field strengths (2,3). (b) Gradient cycling involves alternately inverting the direction of the gradient pulses used in localization (4-7), since the phase of the sidebands depends on the sign of the gradients. Though the sideband artifacts are significantly reduced by this method, the extent of the reduction depends on the hardware details of the scanner and the sequence, and the resulting spectra contain a large water peak whose tail overlaps the metabolite regions necessitating further processing. (c) The metabolite cycling technique uses a frequency selective inversion p...

BACKGROUND AND PURPOSE: Myelin water imaging (MWI) is a magnetic resonance imaging technique that quantifies myelin in-vivo. Although MWI has been extensively applied to study myelin-related diseases in groups, clinical use in individual patients is challenging mainly due to population heterogeneity. The purpose of this study was twofold: (1) create a normative brain myelin water atlas depicting the population mean and regional variability of myelin content; and (2) apply the myelin atlas to assess the degree of demyelination in individuals with multiple sclerosis (MS). METHODS: 3T MWI was performed on 50 healthy adults (25 M/25 F, mean age 25 years [range 17-42 years]). The myelin water atlas was created by averaging coregistered myelin water fraction (MWF) maps from all healthy individuals. To illustrate the preliminary utility of the atlas, white matter (WM) regional MWF variations were evaluated and voxel-wise z-score maps (z < −1.96) from the MWI of three MS participants were produced to assess individually the degree of demyelination. RESULTS: The myelin water atlas demonstrated significant MWF variation across control WM. No significant MWF differences were found between male and female healthy participants. MS z-score maps revealed diffuse regions of demyelination in the two participants with Expanded Disability Status Scale (EDSS) = 2.0 but not in the participant with EDSS = 0. CONCLUSIONS:The myelin water atlas can be used as a reference (URL: https://sourceforge.net/projects/myelin-wateratlas/) to demonstrate areas of demyelination in individual MS participants. Future studies will expand the atlas age range, account for education, and other variables that may affect myelination.