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Assessment of spatial BOLD sensitivity variations in fMRI using gradient-echo field maps.

Mannfolk, Peter LU ; Wirestam, Ronnie LU ; Nilsson, Markus LU ; van Westen, Danielle LU ; Ståhlberg, Freddy LU and Olsrud, Johan LU (2010) In Magnetic Resonance Imaging 28(7). p.947-956
Abstract
Clinical blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) is becoming increasingly valuable in, e.g., presurgical planning, but the commonly used gradient-echo echo-planar imaging (GE-EPI) technique is sometimes hampered by macroscopic field inhomogeneities. This can affect the degree of signal change that will occur in the GE-EPI images as a response to neural activation and the subsequent blood oxygenation changes, i.e., the BOLD sensitivity (BS). In this study, quantitative BS maps were calculated directly from gradient-echo field maps obtainable on most clinical scanners. In order to validate the accuracy of the calculated BS-maps, known shim gradients were applied and field maps and GE-EPI images... (More)
Clinical blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) is becoming increasingly valuable in, e.g., presurgical planning, but the commonly used gradient-echo echo-planar imaging (GE-EPI) technique is sometimes hampered by macroscopic field inhomogeneities. This can affect the degree of signal change that will occur in the GE-EPI images as a response to neural activation and the subsequent blood oxygenation changes, i.e., the BOLD sensitivity (BS). In this study, quantitative BS maps were calculated directly from gradient-echo field maps obtainable on most clinical scanners. In order to validate the accuracy of the calculated BS-maps, known shim gradients were applied and field maps and GE-EPI images of a phantom were acquired. Measured GE-EPI image intensity was then compared with the calculated (predicted) image intensity (pII) which was obtained from the field maps using theoretical expressions for image-intensity loss. The validated expressions for pII were used to calculate the corresponding predicted BOLD sensitivity (pBS) maps in healthy volunteers. Since the field map is assumed to be valid throughout an entire fMRI experiment, the influence of subject motion on the pBS maps was also assessed. To demonstrate the usefulness of such maps, pBS was investigated for clinically important functional areas including hippocampus, Broca's area and primary motor cortex. A systematic left/right pBS difference was observed in Broca's area and in the hippocampus, most likely due to magnetic field inhomogeneity of the particular MRI-system used in this study. For all subjects, the hippocampus showed pBS values above unity with a clear anterior-posterior gradient and with an abrupt drop to zero pBS in the anterior parts of hippocampus. It is concluded that GE field maps can be used to accurately predict BOLD sensitivity and that this parameter is useful to assess spatial variations which will influence fMRI experiments. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Magnetic Resonance Imaging
volume
28
issue
7
pages
947 - 956
publisher
Elsevier
external identifiers
  • wos:000281046100005
  • pmid:20573463
  • scopus:77955423161
ISSN
1873-5894
DOI
10.1016/j.mri.2010.05.003
language
English
LU publication?
yes
id
2a8042f4-828e-44d4-a9fa-db1063ca09d7 (old id 1625756)
alternative location
http://www.ncbi.nlm.nih.gov/pubmed/20573463?dopt=Abstract
date added to LUP
2010-07-05 21:17:47
date last changed
2018-05-29 11:06:20
@article{2a8042f4-828e-44d4-a9fa-db1063ca09d7,
  abstract     = {Clinical blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) is becoming increasingly valuable in, e.g., presurgical planning, but the commonly used gradient-echo echo-planar imaging (GE-EPI) technique is sometimes hampered by macroscopic field inhomogeneities. This can affect the degree of signal change that will occur in the GE-EPI images as a response to neural activation and the subsequent blood oxygenation changes, i.e., the BOLD sensitivity (BS). In this study, quantitative BS maps were calculated directly from gradient-echo field maps obtainable on most clinical scanners. In order to validate the accuracy of the calculated BS-maps, known shim gradients were applied and field maps and GE-EPI images of a phantom were acquired. Measured GE-EPI image intensity was then compared with the calculated (predicted) image intensity (pII) which was obtained from the field maps using theoretical expressions for image-intensity loss. The validated expressions for pII were used to calculate the corresponding predicted BOLD sensitivity (pBS) maps in healthy volunteers. Since the field map is assumed to be valid throughout an entire fMRI experiment, the influence of subject motion on the pBS maps was also assessed. To demonstrate the usefulness of such maps, pBS was investigated for clinically important functional areas including hippocampus, Broca's area and primary motor cortex. A systematic left/right pBS difference was observed in Broca's area and in the hippocampus, most likely due to magnetic field inhomogeneity of the particular MRI-system used in this study. For all subjects, the hippocampus showed pBS values above unity with a clear anterior-posterior gradient and with an abrupt drop to zero pBS in the anterior parts of hippocampus. It is concluded that GE field maps can be used to accurately predict BOLD sensitivity and that this parameter is useful to assess spatial variations which will influence fMRI experiments.},
  author       = {Mannfolk, Peter and Wirestam, Ronnie and Nilsson, Markus and van Westen, Danielle and Ståhlberg, Freddy and Olsrud, Johan},
  issn         = {1873-5894},
  language     = {eng},
  number       = {7},
  pages        = {947--956},
  publisher    = {Elsevier},
  series       = {Magnetic Resonance Imaging},
  title        = {Assessment of spatial BOLD sensitivity variations in fMRI using gradient-echo field maps.},
  url          = {http://dx.doi.org/10.1016/j.mri.2010.05.003},
  volume       = {28},
  year         = {2010},
}