Search

Purpose: To evaluate the benefits of fast spin echo (FSE) imaging over rapid gradient-echo (RAGE) for magnetization-prepared inhomogeneous magnetization transfer (ihMT) imaging., Methods: A 3D FSE sequence was modified to include an ihMT preparation (ihMT-FSE) with an optional CSF suppression based on an inversion-recovery (ihMT-FLAIR). After numeric simulations assessing SNR benefits of FSE and the potential impact of an additional inversion-recovery, ihMT-RAGE, ihMT-FSE, and ihMT-FLAIR sequences were compared in a group of six healthy volunteers, evaluating image quality, thermal, and physiological noise as well as quantification using an ihMT saturation (ihMTsat) approach. A preliminary exploration in the cervical spinal cord was also conducted in a group of three healthy volunteers., Results: Several fold improvements in thermal SNR were observed with ihMT-FSE in agreement with numerical simulations. However, we observed significantly higher physiological noise in ihMT-FSE compared to ihMT-RAGE that was mitigated in ihMT-FLAIR, which provided the best total SNR (+74% and +49% compared to ihMT-RAGE in the white and gray matter, P ≤ 0.004). IhMTsat quantification was successful in all cases with strong correlation between all sequences (r 2  > 0.75). Early experiments showed potential for spinal cord imaging., Conclusions: FSE generally offers higher SNR compared to gradient-echo based acquisitions for magnetization-prepared contrasts as illustrated here in the case of ihMT. However, physiological noise has a significant effect, but an inversion-recovery-based CSF suppression was shown to be efficient in mitigating effects of CSF motion., (© 2022 International Society for Magnetic Resonance in Medicine.) more...

Inhomogeneous Magnetization Transfer (ihMT) is a development from the MT MRI technique. IhMT can be considered as a dipolar order relaxation time (T 1D ) weighted imaging modality whose signal has shown an enhanced selectivity for myelin-rich structures. However, a formal validation of the ihMT sensitivity relative to a gold standard myelin density measurement has not yet been reported. To address this need, we compared ihMT MRI with green fluorescence protein (GFP) microscopy, in a study performed on genetically-modified plp-GFP mice, considered as a reference technique for myelin-content assessment. Various ihMT protocols consisting of variable T 1D -filtering and radiofrequency power temporal distributions, were used for comparison with fluorescence microscopy. Strong and significant linear relationships (r 2 (0.87-0.96), p < 0.0001) were found between GFP and ihMT ratio signals across brain regions for all tested protocol variants. Conventional MT ratios showed weaker correlations (r 2 (0.24-0.78), p ≤ 0.02) and a much larger signal fraction unrelated to myelin, hence corresponding to a much lower specificity for myelin. T 1D -filtering reduced the ihMT signal fraction not attributed to myelin by almost twofold relative to zero filtering suggesting that at least half of the unrelated signal has a substantially shorter T 1D than myelin. Overall, these results strongly support the sensitivity of ihMT to myelin content., (Copyright © 2019 Elsevier Inc. All rights reserved.) more...

Purpose: This paper describes a technique that can be used in vivo to measure the dipolar relaxation time, T 1D , of macromolecular protons contributing to magnetization transfer (MT) in tissues and to produce quantitative T 1D maps., Theory and Methods: The technique builds upon the inhomogeneous MT (ihMT) technique that is particularly sensitive to tissue components with long T 1D . A standard ihMT experiment was altered to introduce a variable time for switching between positive and negative offset frequencies for RF saturation. A model for the dependence of ihMT was developed and used to fit data acquired in vivo., Results: Application of the method to images from brains of healthy volunteers produced values of T 1D  = (5.9 ± 1.2) ms in gray matter and T 1D  = (6.2 ± 0.4) ms in white matter regions and provided maps of the T 1D parameter., Conclusion: The model and experiments described provide access to a new relaxation characteristic of tissue with potentially unique diagnostic information. Magn Reson Med 78:1362-1372, 2017. © 2016 International Society for Magnetic Resonance in Medicine., (© 2016 International Society for Magnetic Resonance in Medicine.) more...