In vivo investigation of the multi‐exponential<i>T</i><sub>2</sub>decay in human white matter at 7 T: Implications for myelin water imaging at UHF

Wiggermann, Vanessa; MacKay, Alexander L.; Rauscher, Alexander; Helms, Gunther

doi:10.1002/nbm.4429

Cited by 6 publications

(6 citation statements)

References 59 publications

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“…Direct saturation is linked to short T 2 of the free pool as observed in myelin water and iron‐rich structures which decreases at higher field strength. However, the geometric mean T 2 s of different structures are proportionally shorter at 7T relative to those observed at 3T 56 . Hence, we do not observe tissue‐type specific effects.…”

Section: Discussionmentioning

confidence: 56%

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Mapping magnetization transfer saturation (MT_sat) in human brain at 7T: Protocol optimization under specific absorption rate constraints

Olsson

Andersen

Wirestam

et al. 2021

Magnetic Resonance in Med

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Purpose To optimize a whole‐brain magnetization transfer saturation (MTsat) protocol at 7T, focusing on maximizing obtainable MTsat under the constraints of specific absorption rate (SAR) and transmit field inhomogeneity, while avoiding bias and keeping scan time short. Theory and Methods MTsat is a semi‐quantitative metric, obtained by spoiled gradient‐echo MRI in the imaging steady‐state. Optimization was based on an established 7T dual flip angle protocol, and focused on MT pulse, readout flip angle, repetition time (TR), offset frequency (Δ), and correction of residual effects from transmit field inhomogeneities by separate flip angle mapping. Results A 100% SAR level was reached at a 180° MT pulse flip angle, using a compact sinc main lobe (4 ms duration) and minimum TR = 26.5 ms. The use of Δ = +2.0 kHz caused no discernible direct saturation, while Δ = −2.0 kHz resulted in 45% higher MTsat in white matter (WM) compared to Δ = +2.0 kHz. A 4° readout flip angle eliminated bias while yielding a good signal‐to‐noise ratio. Increased TR yielded only a little increase in MTsat, and TR = 26.5 ms (scan time 04:58 min) was thus selected. Post hoc transmit field correction clearly improved homogeneity, especially in WM. Conclusions The range of MTsat is limited at 7T, and this can partly be overcome by the exploitation of the asymmetry of the macromolecular lineshape through the sign of Δ. To reduce scan time, a compact MT pulse with a sufficiently narrow frequency response should be used. TR and readout flip angle should be kept short/small. Transmit field correction through separate flip angle mapping is required.

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Section: Discussionmentioning

confidence: 56%

“…However, the geometric mean T 2 s of different structures are proportionally shorter at 7T relative to those observed at 3T. 56 Hence, we do not observe tissue-type specific effects.…”

Section: Offset Frequencymentioning

confidence: 54%

Mapping magnetization transfer saturation (MT_sat) in human brain at 7T: Protocol optimization under specific absorption rate constraints

Olsson

Andersen

Wirestam

et al. 2021

Magnetic Resonance in Med

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“…The 2D MESE sequences tend to have lower SNR levels than 3D (∼300 in our experiments), which are under the sufficient values suggested for ideal MWI analysis (>700), 28 therefore, a compromise on ideal SNR is needed for 2D MESE work, which then points to the value of an independent B 1 + map. For 3D MESE methods, a higher SNR threshold is possible, but recent work with 3D GRASE has lower SNR (150‐300 range) 21,34,35 . The high SNR requirement (700) for multi‐component T 2 analysis of 3D MESE data is considered too strict in recent literature 21,27 …”

Section: Discussionmentioning

confidence: 99%

“…For 3D MESE methods, a higher SNR threshold is possible, but recent work with 3D GRASE has lower SNR (150-300 range). 21,34,35 The high SNR requirement (700) for multi-component T 2 analysis of 3D MESE data is considered too strict in recent literature. 21,27 Prospective studies could include a rapid B 1 + map, requiring only a small fraction of the time necessary for an MWI acquisition.…”

Section: Discussionmentioning

confidence: 99%

Myelin water fraction mapping from multiple echo spin echoes and an independent B₁⁺ map

Mehdizadeh

Wilman

2022

Magnetic Resonance in Med

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Purpose: Myelin water fraction (MWF) is often obtained from a multiple echo spin echo (MESE) sequence using multi-component T 2 fitting with non-negative least squares. This process fits many unknowns including B 1 + to produce a T 2 spectrum for each voxel. Presented is an alternative using a rapid B 1 + mapping sequence to supply B 1 + for the MWF fitting procedure.Methods: Effects of B 1 + errors on MWF calculations were modeled for 2D and 3D MESE using Bloch and extended phase graph simulations, respectively. Variations in SNR and relative refocusing widths were tested. Human brain experiments at 3 T used 2D MESE and an independent B 1 + map. MWF maps were produced with the standard approach and with the use of the independent B 1 + map. Differences in B 1 + and mean MWF in specific brain regions were compared.Results: For 2D MESE, MWF with the standard method was strongly affected by B 1 + misestimations arising from limited SNR and response asymmetry around 180 • , which decreased with increasing relative refocusing width. Using an independent B 1 + map increased mean MWF and decreased coefficient of variation. Notable differences in vivo in 2D MESE were in areas of high B 1 + such as thalamus and splenium where mean MWF increased by 88% and 31%, respectively (P < 0.001). Simulations also demonstrated the advantages of this approach for 3D MESE when SNR is <500. Conclusion:For 2D MESE, because of increased complexity of decay curves and limited SNR, supplying B 1 + improves MWF results in peripheral and central brain regions where flip angles differ substantially from 180 • .

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“…For a 48-echo scan at 3 T, repetition times are around 1 s. The much higher specific absorption rate at 7 T requires longer repetition times. 29 Furthermore, the T 2 relaxation times are shortened with increasing field strength. Initial work in human MWI at 7 T used the GraSE approach, 29 but it is anticipated that compressed sensing approaches will be more successful at ultra-high fields.…”

Section: Magnetic Resonance Myelin Imaging Methods and Their Biophysi...mentioning

confidence: 99%

Quantitative myelin imaging with MRI and PET: an overview of techniques and their validation status

Weijden

Biondetti

Gutmann

et al. 2022

Brain

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Myelin is the protective sheath wrapped around axons, consisting of a phospholipid bilayer with water between the wraps. The measurement of damage to the myelin sheaths, the evaluation of the efficacy of therapies aiming to promote remyelination as well as monitoring the degree of brain maturation in children require non-invasive quantitative myelin imaging methods. To date, various myelin imaging techniques have been developed. Five different MRI approaches can be distinguished based on their biophysical principles: (1) imaging of the water between the lipid bilayers directly (e.g. myelin water imaging), (2) imaging the non-aqueous protons of the phospholipid bilayer directly with ultrashort echo time techniques, (3) indirect imaging of the macromolecular content (e.g. magnetization transfer; inhomogeneous magnetization transfer), (4) mapping of the effects of the myelin sheath’s magnetic susceptibility on the MRI signal (e.g., quantitative susceptibility mapping), and (5) mapping of the effects of the myelin sheath on water diffusion. Myelin imaging with PET uses radioactive molecules with high affinity to specific myelin components, in particular myelin basic protein. This review aims to give an overview of the various myelin imaging techniques, their biophysical principles, image acquisition, data analysis, and their validation status.

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In vivo investigation of the multi‐exponentialT₂decay in human white matter at 7 T: Implications for myelin water imaging at UHF

Cited by 6 publications

References 59 publications

Mapping magnetization transfer saturation (MT_sat) in human brain at 7T: Protocol optimization under specific absorption rate constraints

Mapping magnetization transfer saturation (MT_sat) in human brain at 7T: Protocol optimization under specific absorption rate constraints

Myelin water fraction mapping from multiple echo spin echoes and an independent B₁⁺ map

Quantitative myelin imaging with MRI and PET: an overview of techniques and their validation status

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In vivo investigation of the multi‐exponentialT2decay in human white matter at 7 T: Implications for myelin water imaging at UHF

Cited by 6 publications

References 59 publications

Mapping magnetization transfer saturation (MTsat) in human brain at 7T: Protocol optimization under specific absorption rate constraints

Mapping magnetization transfer saturation (MTsat) in human brain at 7T: Protocol optimization under specific absorption rate constraints

Myelin water fraction mapping from multiple echo spin echoes and an independent B1+ map

Quantitative myelin imaging with MRI and PET: an overview of techniques and their validation status

Contact Info

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In vivo investigation of the multi‐exponentialT₂decay in human white matter at 7 T: Implications for myelin water imaging at UHF

Mapping magnetization transfer saturation (MT_sat) in human brain at 7T: Protocol optimization under specific absorption rate constraints

Mapping magnetization transfer saturation (MT_sat) in human brain at 7T: Protocol optimization under specific absorption rate constraints

Myelin water fraction mapping from multiple echo spin echoes and an independent B₁⁺ map