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Gradient echo-based quantitative MRI of human brain at 7T : Mapping of T1, MT saturation and local flip angle

Olsson, Hampus LU (2021)
Abstract
Quantitative MRI (qMRI) refers to the process of deriving maps of MR contrast parameters, such as relaxation times, from conventional images. If the qMRI maps have a high degree of precision and a low degree of bias, they can be compared longitudinally, across subjects, and (ideally) between measurement protocols and research sites. They also provide a more direct biophysical interpretation of the pixel intensities.
The increased magnetization of spins at ultra-high field (UHF) strengths of 7T and above could potentially be translated into higher spatial resolution and/or reduced scan time. This thesis tackles UHF-related challenges in qMRI, namely the increased inhomogeneity of the radio frequency (RF) field (B1) and increased... (More)
Quantitative MRI (qMRI) refers to the process of deriving maps of MR contrast parameters, such as relaxation times, from conventional images. If the qMRI maps have a high degree of precision and a low degree of bias, they can be compared longitudinally, across subjects, and (ideally) between measurement protocols and research sites. They also provide a more direct biophysical interpretation of the pixel intensities.
The increased magnetization of spins at ultra-high field (UHF) strengths of 7T and above could potentially be translated into higher spatial resolution and/or reduced scan time. This thesis tackles UHF-related challenges in qMRI, namely the increased inhomogeneity of the radio frequency (RF) field (B1) and increased specific absorption rate (SAR). The changing relaxation times (i.e. prolonged T1 and shortened T2) also needs to be accounted for.
Here, spoiled gradient-recalled echo (GRE) techniques are employed to map (primarily) two structural MR parameters, i.e. the longitudinal relaxation time (T1) and the magnetization transfer (MT) saturation (MTsat). Because of its influence at UHF, emphasis is also put on mapping of the local flip angle. Primarily, qMRI is performed by the inversion of analytical signal equations, as opposed to numerical approaches.
The process of implementing and modifying the dual flip angle (DFA) technique in conjunction with an MT-weighted GRE for 7T is described. Implementation is performed within the well-established multi-parameter mapping (MPM) framework and special attention is afforded to the reduction of biases as well as overcoming saftey restrictions imposed by SAR. An approach to obtain high-SNR low-bias flip angle maps at 7T, using the dual refocusing echo acquisition mode (DREAM) technique is also presented. This is important since high fidelity flip angle maps are a prerequisite in DFA-based T1-mapping and recommended for correcting MTsat at UHF. Furthermore, MPRAGE-based techniques are discussed. Firstly, it is demonstrated how to most effectively obtain B1-corrected MPRAGE images of “pure” T1 contrast using a sequential protocol This is followed by a description of T1-mapping using MP2RAGE. Finally, an innovative technique for simultaneous mapping of T1 and the local flip angle is introduced, dubbed “MP3RAGE”. (Less)
Abstract (Swedish)
I denna avhandling beskrivs processesen att implementera och optimisera metoder inom kvantitativ magnetresonansomografi (MRT) på en fältstyrka av 7 T. Metoderna är baserade på gradientekon med påtvingad transversell urfasning (spoiling) och applicerade på den mänskliga hjärnan i syfte att avbilda i huvudsak tre olika MR-parametrar: Den longitudinella relaxationstiden (T1), magnetization transfer saturation (MTsat) samt den lokala flippvinkeln.
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author
supervisor
opponent
  • Fil. Dr. Hagberg, Gisela, Max-Planck-Institute for Biological Cybernetics, Tübingen, Germany
organization
publishing date
type
Thesis
publication status
published
subject
keywords
MR studies; magnetic resonance (MR), 7 Tesla, T1 mapping, magnetization transfer saturation, Flip angle mapping, spoiled gradient echo, MR studies; magnetic resonance (MR), 7T, T1 mapping, magnetization transfer saturation, Flip angle mapping, spoiled gradient echo
pages
192 pages
publisher
Media-Tryck, Lund University, Sweden
defense location
Lundmarksalen, Astronomihuset. Join via zoom: https://www.msf.lu.se/evenemang/disputation-hampus-olsson
defense date
2021-10-29 09:00:00
ISBN
978-91-7895-966-2
978-91-7895-965-5
project
Gradient echo-based quantitative MRI of human brain at 7T
language
English
LU publication?
yes
id
984ff642-e1e3-4dc5-8111-11672461596f
date added to LUP
2021-10-04 12:39:27
date last changed
2021-10-05 08:48:19
@phdthesis{984ff642-e1e3-4dc5-8111-11672461596f,
  abstract     = {{Quantitative MRI (qMRI) refers to the process of deriving maps of MR contrast parameters, such as relaxation times, from conventional images. If the qMRI maps have a high degree of precision and a low degree of bias, they can be compared longitudinally, across subjects, and (ideally) between measurement protocols and research sites. They also provide a more direct biophysical interpretation of the pixel intensities. <br/>The increased magnetization of spins at ultra-high field (UHF) strengths of 7T and above could potentially be translated into higher spatial resolution and/or reduced scan time. This thesis tackles UHF-related challenges in qMRI, namely the increased inhomogeneity of the radio frequency (RF) field (B1) and increased specific absorption rate (SAR). The changing relaxation times (i.e. prolonged T1 and shortened T2) also needs to be accounted for.<br/>Here, spoiled gradient-recalled echo (GRE) techniques are employed to map (primarily) two structural MR parameters, i.e. the longitudinal relaxation time (T1) and the magnetization transfer (MT) saturation (MTsat). Because of its influence at UHF, emphasis is also put on mapping of the local flip angle. Primarily, qMRI is performed by the inversion of analytical signal equations, as opposed to numerical approaches.<br/>The process of implementing and modifying the dual flip angle (DFA) technique in conjunction with an MT-weighted GRE for 7T is described. Implementation is performed within the well-established multi-parameter mapping (MPM) framework and special attention is afforded to the reduction of biases as well as overcoming saftey restrictions imposed by SAR. An approach to obtain high-SNR low-bias flip angle maps at 7T, using the dual refocusing echo acquisition mode (DREAM) technique is also presented. This is important since high fidelity flip angle maps are a prerequisite in DFA-based T1-mapping and recommended for correcting MTsat at UHF. Furthermore, MPRAGE-based techniques are discussed. Firstly, it is demonstrated how to most effectively obtain B1-corrected MPRAGE images of “pure” T1 contrast using a sequential protocol This is followed by a description of T1-mapping using MP2RAGE. Finally, an innovative technique for simultaneous mapping of T1 and the local flip angle is introduced, dubbed “MP3RAGE”.}},
  author       = {{Olsson, Hampus}},
  isbn         = {{978-91-7895-966-2}},
  keywords     = {{MR studies; magnetic resonance (MR); 7 Tesla; T1 mapping; magnetization transfer saturation; Flip angle mapping; spoiled gradient echo; MR studies; magnetic resonance (MR); 7T; T1 mapping; magnetization transfer saturation; Flip angle mapping; spoiled gradient echo}},
  language     = {{eng}},
  month        = {{10}},
  publisher    = {{Media-Tryck, Lund University, Sweden}},
  school       = {{Lund University}},
  title        = {{Gradient echo-based quantitative MRI of human brain at 7T : Mapping of T1, MT saturation and local flip angle}},
  url          = {{https://lup.lub.lu.se/search/files/103310653/Hampus_Olsson_summary_thesis_nopapers.pdf}},
  year         = {{2021}},
}