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In vivo investigation of the multi‐exponential T2 decay in human white matter at 7 T: Implications for myelin water imaging at UHF

Wiggermann, Vanessa ; MacKay, Alexander L ; Rauscher, Alexander and Helms, Gunther LU (2021) In NMR in Biomedicine 34(2).
Abstract
Introduction
Multi‐component T2 mapping using a gradient‐ and spin‐echo (GraSE) acquisition has become standard for myelin water imaging at 3 T. Higher magnetic field strengths promise signal‐to‐noise ratio benefits but face specific absorption rate limits and shortened T2 times. This study investigates compartmental T2 times in vivo and addresses advantages and challenges of multi‐component T2 mapping at 7 T.

Methods
We acquired 3D multi‐echo GraSE data in seven healthy adults at 7 T, with three subjects also scanned at 3 T. Stimulated echoes arising from B1+ inhomogeneities were accounted for by the extended phase graph (EPG) algorithm. We used the computed T2 distributions to determine T2 times that identify different... (More)
Introduction
Multi‐component T2 mapping using a gradient‐ and spin‐echo (GraSE) acquisition has become standard for myelin water imaging at 3 T. Higher magnetic field strengths promise signal‐to‐noise ratio benefits but face specific absorption rate limits and shortened T2 times. This study investigates compartmental T2 times in vivo and addresses advantages and challenges of multi‐component T2 mapping at 7 T.

Methods
We acquired 3D multi‐echo GraSE data in seven healthy adults at 7 T, with three subjects also scanned at 3 T. Stimulated echoes arising from B1+ inhomogeneities were accounted for by the extended phase graph (EPG) algorithm. We used the computed T2 distributions to determine T2 times that identify different water pools and assessed signal‐to‐noise and fit‐to‐noise characteristics of the signal estimation. We compared short T2 fractions and T2 properties of the intermediate water pool at 3 T and 7 T.

Results
Flip angle mapping confirmed that EPG accurately determined the larger B1+ inhomogeneity at 7 T. Multi‐component T2 analysis demonstrated shortened T2 times at 7 T compared with 3 T. Fit‐to‐noise and signal‐to‐noise ratios were improved at 7 T but depended on B1+ homogeneity. Adjusting the shortest T2 to 8 ms and the T2 threshold that separates different water compartments to 20 ms yielded short T2 fractions at 7 T that conformed to 3 T data. Short T2 fractions in myelin‐rich white matter regions were lower at 7 T than at 3 T, and higher in iron‐rich structures.

Discussion
Adjusting the T2 compartment boundaries was required due to the shorter T2 relaxation times at 7 T. Shorter echo spacing would better sample the fast decaying signal but would increase peripheral nerve stimulation. Multi‐channel transmission will improve T2 measurements at 7 T.

Conclusion
We used a multi‐echo 3D GraSE sequence to characterize the multi‐exponential T2 decay at 7 T. We adapted T2 parameters for evaluation of the short T2 fraction. Obtained 7 T multi‐component T2 maps were in good agreement with 3 T data. (Less)
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author
; ; and
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
EPG, gradient‐echo spin‐echo, ultra‐high field MRI, T2 relaxation, myelin, multi‐exponential
in
NMR in Biomedicine
volume
34
issue
2
article number
e4429
publisher
John Wiley and Sons
external identifiers
  • pmid:33118238
  • scopus:85094114745
ISSN
0952-3480
DOI
10.1002/nbm.4429
project
T2 mapping and myelin water imaging at 7T using 3D GraSe
language
English
LU publication?
yes
id
e95f3d4f-85d8-4664-be5b-a79902f350ec
date added to LUP
2020-10-30 11:46:29
date last changed
2021-01-19 09:03:59
@article{e95f3d4f-85d8-4664-be5b-a79902f350ec,
  abstract     = {Introduction<br/>Multi‐component T2 mapping using a gradient‐ and spin‐echo (GraSE) acquisition has become standard for myelin water imaging at 3 T. Higher magnetic field strengths promise signal‐to‐noise ratio benefits but face specific absorption rate limits and shortened T2 times. This study investigates compartmental T2 times in vivo and addresses advantages and challenges of multi‐component T2 mapping at 7 T.<br/><br/>Methods<br/>We acquired 3D multi‐echo GraSE data in seven healthy adults at 7 T, with three subjects also scanned at 3 T. Stimulated echoes arising from B1+ inhomogeneities were accounted for by the extended phase graph (EPG) algorithm. We used the computed T2 distributions to determine T2 times that identify different water pools and assessed signal‐to‐noise and fit‐to‐noise characteristics of the signal estimation. We compared short T2 fractions and T2 properties of the intermediate water pool at 3 T and 7 T.<br/><br/>Results<br/>Flip angle mapping confirmed that EPG accurately determined the larger B1+ inhomogeneity at 7 T. Multi‐component T2 analysis demonstrated shortened T2 times at 7 T compared with 3 T. Fit‐to‐noise and signal‐to‐noise ratios were improved at 7 T but depended on B1+ homogeneity. Adjusting the shortest T2 to 8 ms and the T2 threshold that separates different water compartments to 20 ms yielded short T2 fractions at 7 T that conformed to 3 T data. Short T2 fractions in myelin‐rich white matter regions were lower at 7 T than at 3 T, and higher in iron‐rich structures.<br/><br/>Discussion<br/>Adjusting the T2 compartment boundaries was required due to the shorter T2 relaxation times at 7 T. Shorter echo spacing would better sample the fast decaying signal but would increase peripheral nerve stimulation. Multi‐channel transmission will improve T2 measurements at 7 T.<br/><br/>Conclusion<br/>We used a multi‐echo 3D GraSE sequence to characterize the multi‐exponential T2 decay at 7 T. We adapted T2 parameters for evaluation of the short T2 fraction. Obtained 7 T multi‐component T2 maps were in good agreement with 3 T data.},
  author       = {Wiggermann, Vanessa and MacKay, Alexander L and Rauscher, Alexander and Helms, Gunther},
  issn         = {0952-3480},
  language     = {eng},
  number       = {2},
  publisher    = {John Wiley and Sons},
  series       = {NMR in Biomedicine},
  title        = {In vivo investigation of the multi‐exponential T2 decay in human white matter at 7 T: Implications for myelin water imaging at UHF},
  url          = {http://dx.doi.org/10.1002/nbm.4429},
  doi          = {10.1002/nbm.4429},
  volume       = {34},
  year         = {2021},
}