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PurposeHeating of gradient coils and passive shim components is a common cause of instability in the B0 field, especially when gradient intensive sequences are used. The aim of the study was to set a benchmark for typical drift encountered during MR spectroscopy (MRS) to assess the need for real-time field-frequency locking on MRI scanners by comparing field drift data from a large number of sites.MethodA standardized protocol was developed for 80 participating sites using 99 3T MR scanners from 3 major vendors. Phantom water signals were acquired before and after an EPI sequence. The protocol consisted of: minimal preparatory imaging; a short pre-fMRI PRESS; a ten-minute fMRI acquisition; and a long post-fMRI PRESS acquisition. Both pre- and post-fMRI PRESS were non-water suppressed. Real-time frequency stabilization/adjustment was switched off when appropriate. Sixty scanners repeated the protocol for a second dataset. In addition, a three-hour post-fMRI MRS acquisition was performed at one site to observe change of gradient temperature and drift rate. Spectral analysis was performed using MATLAB. Frequency drift in pre-fMRI PRESS data were compared with the first 5:20 minutes and the full 30:00 minutes of data after fMRI. Median (interquartile range) drifts were measured and showed in violin plot. Paired t-tests were performed to compare frequency drift pre- and post-fMRI. A simulated in vivo spectrum was generated using FID-A to visualize the effect of the observed frequency drifts. The simulated spectrum was convolved with the frequency trace for the most extreme cases. Impacts of frequency drifts on NAA and GABA were also simulated as a function of linear drift. Data from the repeated protocol were compared with the corresponding first dataset using Pearson's and intraclass correlation coefficients (ICC).ResultsOf the data collected from 99 scanners, 4 were excluded due to various reasons. Thus, data from 95 scanners were ultimately analyzed. For the first 5:20 min (64 transients), median (interquartile range) drift was 0.44 (1.29) Hz before fMRI and 0.83 (1.29) Hz after. This increased to 3.15 (4.02) Hz for the full 30 min (360 transients) run. Average drift rates were 0.29 Hz/min before fMRI and 0.43 Hz/min after. Paired t-tests indicated that drift increased after fMRI, as expected (p < 0.05). Simulated spectra convolved with the frequency drift showed that the intensity of the NAA singlet was reduced by up to 26%, 44 % and 18% for GE, Philips and Siemens scanners after fMRI, respectively. ICCs indicated good agreement between datasets acquired on separate days. The single site long acquisition showed drift rate was reduced to 0.03 Hz/min approximately three hours after fMRI.DiscussionThis study analyzed frequency drift data from 95 3T MRI scanners. Median levels of drift were relatively low (5-min average under 1 Hz), but the most extreme cases suffered from higher levels of drift. The extent of drift varied across scanners which both linear and nonlinear drifts were observed.

Purpose: We investigated the correlation, reproducibility, and effect of white matter fiber orientation for three myelin‐sensitive MRI techniques: magnetization transfer ratio (MTR), inhomogeneous magnetization transfer ratio (ihMTR), and gradient and spin echo–derived myelin water fraction (MWF). Methods: We measured the three metrics in 17 white and three deep grey matter regions in 17 healthy adults at 3 T. Results: We found a strong correlation between ihMTR and MTR (r = 0.70, p < 0.001) and ihMTR and MWF (r = 0.79, p < 0.001), and a weaker correlation between MTR and MWF (r = 0.54, p < 0.001). The dynamic range in white matter was greatest for MWF (2.0%–27.5%), followed by MTR (14.4%–23.2%) and then ihMTR (1.2%–5.4%). The average scan–rescan coefficient of variation for white matter regions was 0.6% MTR, 0.3% ihMTR, and 0.7% MWF in metric units; however, when adjusted by the dynamic range, these became 6.3%, 6.1% and 2.8%, respectively. All three metrics varied with fiber direction: MWF and ihMTR were lower in white matter fibers perpendicular to B0 by 6% and 1%, respectively, compared with those parallel, whereas MTR was lower by 0.5% at about 40°, with the highest values at 90°. However, separating the apparent orientation dependence by white matter region revealed large dissimilarities in the trends, suggesting that real differences in myelination between regions are confounding the apparent orientation dependence measured using this method. Conclusion: The strong correlation between ihMTR and MWF suggests that these techniques are measuring the same myelination; however, the larger dynamic range of MWF may provide more power to detect small differences in myelin. [ABSTRACT FROM AUTHOR]

Purpose: Myelin water imaging (MWI) provides a valuable biomarker for myelin, but clinical application has been restricted by long acquisition times. Accelerating the standard multi‐echo T2 acquisition with gradient echoes (GRASE) or by 2D multi‐slice data collection results in image blurring, contrast changes, and other issues. Compressed sensing (CS) can vastly accelerate conventional MRI. In this work, we assessed the use of CS for in vivo human MWI, using a 3D multi spin‐echo sequence. Methods: We implemented multi‐echo T2 relaxation imaging with compressed sensing (METRICS) and METRICS with partial Fourier acceleration (METRICS‐PF). Scan‐rescan data were acquired from 12 healthy controls for assessment of repeatability. MWI data were acquired for METRICS in 9 m:58 s and for METRICS‐PF in 7 m:25 s, both with 1.5 × 2 × 3 mm3 voxels, 56 echoes, 7 ms ΔTE, and 240 × 240 × 170 mm3 FOV. METRICS was compared with a novel multi‐echo spin‐echo gold‐standard (MSE‐GS) MWI acquisition, acquired for a single additional subject in 2 h:2 m:40 s. Results: METRICS/METRICS‐PF myelin water fraction had mean: repeatability coefficient 1.5/1.1, coefficient of variation 6.2/4.5%, and intra‐class correlation coefficient 0.79/0.84. Repeatability metrics comparing METRICS with METRICS‐PF were similar, and both sequences agreed with reference values from literature. METRICS images and quantitative maps showed excellent qualitative agreement with those of MSE‐GS. Conclusion: METRICS and METRICS‐PF provided highly repeatable MWI data without the inherent disadvantages of GRASE or 2D multi‐slice acquisition. CS acceleration allows MWI data to be acquired rapidly with larger FOV, higher estimated SNR, more isotropic voxels and more echoes than with previous techniques. The approach introduced here generalizes to any multi‐component T2 mapping application. [ABSTRACT FROM AUTHOR]

Background and Purpose: Acquiring and interpreting quantitative myelin-specific MRI data at an individual level is challenging because of technical difficulties and natural myelin variation in the population. To overcome these challenges, we used multiecho T2 myelin water imaging (MWI) to create T2 metric healthy population atlases that depict the mean and variation of myelin water fraction (MWF), and intra- and extracellular water mobility as described by geometric mean T2 (IEGMT2 ).Methods: Cervical cord MWI was performed at 3T on 20 healthy individuals (10M/10F, mean age: 36 years) and 3 relapsing remitting multiple sclerosis (RRMS) participants (1M/2F, age: 39/42/37 years). Anatomical data were collected for the purpose of image segmentation and registration. Atlases were created by coregistering and averaging T2 metrics from all controls. Voxel-wise z-score maps from 3 RRMS participants were produced to demonstrate the preliminary utility of the MWF and IEGMT2 atlases.Results: The average MWF atlas provides a representation of myelin in the spinal cord consistent with well-known spinal cord anatomical characteristics. The IEGMT2 atlas also depicted structural variations in the spinal cord. Z-score analysis illustrated distinct abnormalities in MWF and IEGMT2 in the 3 RRMS cases.Conclusions: Our findings highlight the potential for using a quantitative T2 relaxation metric atlas to visualize and detect pathology in spinal cord. Our MWF and IEGMT2 atlases (URL: https://sourceforge.net/projects/mwi-spinal-cord-atlases/) can serve as normative references in the cervical spinal cord for other studies. [ABSTRACT FROM AUTHOR]

Radiation necrosis mostly occurs in and near the radiation field. We used magnetic resonance imaging to study radiation‐induced necrosis of atypical onset, severity, and extent following stereotactic radiosurgery for a symptomatic arteriovenous malformation. Susceptibility‐sensitive imaging, T1‐relaxation, myelin water imaging, and magnetic resonance spectroscopy were acquired three times up to 52 months postradiosurgery. Increasing water content outside the radiation field, contralateral neuronal loss, and gliosis were detected over time. Our findings suggest that radiation‐induced vasculopathic changes spread more diffusely than previously described. An autoimmune response to brain antigens could underlie white matter changes outside the initial radiation field. [ABSTRACT FROM AUTHOR]

Abstract: Multiecho T2 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 T2 (GMT2) 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 T2 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 GMT2 measures in a target population; and (3) to detect differences in these measures between a younger cohort (20–30years of age) and an older cohort (50–75years 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 GMT2; and 0.74 and 0.73 for the global GMT2. 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 GMT2 times (DC p=0.008; LC p=0.042), however, the global GMT2 times did not show any differences. These changes in MWF and IE GMT2 times, but not in global GMT2 times, indicate that multiecho T2 relaxation measures are sensitive to changes in myelin integrity and cell morphology that may not be apparent on conventional T2 weighted images. [ABSTRACT FROM AUTHOR]

Regional variations in glutamate levels across the cingulate cortex, decreasing rostral-to-caudal, have been observed previously in healthy volunteers using proton magnetic resonance spectroscopy ( 1 H-MRS) at 7T. This study sought to explore cingulate cortex glutamate trends further by investigating whether a similar gradient could be detected at 3T, the effect of sex, as well as whether individual variations gave rise to more than one regional glutamate pattern. 1 H-MRS at 3T (Phillips Elition, semi-Localization by Adiabatic Selective Refocusing, TE/TR = 32/5000) was acquired in four cingulate regions: the anterior, mid-anterior, mid-posterior, and posterior cortices, in 50 healthy participants (26F) scanned at a fixed time of day and with controlled food intake. K-means clustering was used to characterize the presence of distinct regional patterns, which were then compared between sex and clusters. In addition, cortical thickness was compared between clusters, and in relation to glutamate. Aligned with 7T findings, we demonstrated that average glutamate levels decreased rostral-to-caudal in the healthy cingulate cortex. No effect of sex was found, suggesting similar resting glutamate levels in both sexes. Interestingly, the majority of participants were characterized by glutamate levels that did not significantly change across the cingulate (65%). Different regional patterns in cortical thickness between clusters offer further evidence into these distinct glutamate variations and suggest both a neuroanatomical and functional role may lead to these findings. This study provides a much needed foundation for further research to determine the implications of neurotransmission patterns in health and disease.

Background and Purpose: Acquiring and interpreting quantitative myelin-specific MRI data at an individual level is challenging because of technical difficulties and natural myelin variation in the population. To overcome these challenges, we used multiecho T 2 myelin water imaging (MWI) to create T 2 metric healthy population atlases that depict the mean and variation of myelin water fraction (MWF), and intra- and extracellular water mobility as described by geometric mean T 2 (IEGMT 2 )., Methods: Cervical cord MWI was performed at 3T on 20 healthy individuals (10M/10F, mean age: 36 years) and 3 relapsing remitting multiple sclerosis (RRMS) participants (1M/2F, age: 39/42/37 years). Anatomical data were collected for the purpose of image segmentation and registration. Atlases were created by coregistering and averaging T 2 metrics from all controls. Voxel-wise z-score maps from 3 RRMS participants were produced to demonstrate the preliminary utility of the MWF and IEGMT 2 atlases., Results: The average MWF atlas provides a representation of myelin in the spinal cord consistent with well-known spinal cord anatomical characteristics. The IEGMT 2 atlas also depicted structural variations in the spinal cord. Z-score analysis illustrated distinct abnormalities in MWF and IEGMT 2 in the 3 RRMS cases., Conclusions: Our findings highlight the potential for using a quantitative T 2 relaxation metric atlas to visualize and detect pathology in spinal cord. Our MWF and IEGMT 2 atlases (URL: https://sourceforge.net/projects/mwi-spinal-cord-atlases/) can serve as normative references in the cervical spinal cord for other studies., (© 2019 by the American Society of Neuroimaging.)

Purpose: To assess the extent of demyelination in cervical spondylotic myelopathy (CSM) using myelin water imaging (MWI) and electrophysiologic techniques., Methods: Somatosensory evoked potentials (SSEPs) and MWI were acquired in 14 patients with CSM and 18 age-matched healthy controls. MWI was performed on a 3.0T whole body magnetic resonance scanner. Myelin water fraction (MWF) was extracted for the dorsal columns and whole cord. SSEPs and MWF were also compared with conventional MRI outcomes, including T2 signal intensity, compression ratio, maximum spinal cord compression (MSCC), and maximum canal compromise (MCC)., Results: Group analysis showed marked differences in T2 signal intensity, compression ratio, MSCC, and MCC between healthy controls and patients with CSM. There were no group differences in MWF and SSEP latencies. However, patients with CSM with pathologic SSEPs exhibited reduction in MWF ( p < 0.05). MWF was also correlated with SSEP latencies., Conclusion: Our findings provide evidence of decreased myelin content in the spinal cord associated with impaired spinal cord conduction in patients with CSM. While conventional MRI are of great value to define the extent of cord compression, they show a limited correlation with functional deficits (i.e., delayed SSEPs). MWI provides independent and complementary readouts to spinal cord compression, with a high specificity to detect impaired conduction., (© 2017 American Academy of Neurology.)