Abstract
When applying RF saturation to tissue, MRI signal reductions occur due to magnetization transfer (MT) and direct saturation (DS) effects on water protons. It is shown that the direct effects, often considered a nuisance, can be used to distinguish gray matter (GM) regions with different iron content. DS effects were selected by reducing the magnitude and duration of RF irradiation to minimize confounding MT effects. Contrary to MT saturation spectra, direct water saturation spectra are characterized by a symmetric Lorentzian-shaped frequency dependence that can be described by an exact analytical solution of the Bloch equations. The effect of increased transverse relaxation, e.g., due to the presence of iron, will broaden this saturation spectrum. As a first application, DS ratio (DSR) images were acquired to visualize GM structures in the human brain. Similar to T2*-weighted images, the quality of DSR images was affected by local field inhomogeneity, but this could be easily corrected for by centering the saturation spectrum on a voxel-by-voxel basis. The results show that, contrary to commonly used T2*-weighted and absolute R2 images, the DSR images visualize all GM structures, including cortex. A direct correlation between DSR and iron content was confirmed for these structures.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 384-393 |
| Number of pages | 10 |
| Journal | Magnetic Resonance in Medicine |
| Volume | 62 |
| Issue number | 2 |
| DOIs | |
| State | Published - Aug 2009 |
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Keywords
- Brain iron
- Direct saturation
- Magnetization transfer
- Off resonance
- Transverse relaxation
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging
Cite this
Direct saturation MRI : Theory and application to imaging brain iron. / Smith, Seth A.; Bulte, Jeff W; Van Zijl, Peter C.
In: Magnetic Resonance in Medicine, Vol. 62, No. 2, 08.2009, p. 384-393.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - Direct saturation MRI
T2 - Theory and application to imaging brain iron
AU - Smith, Seth A.
AU - Bulte, Jeff W
AU - Van Zijl, Peter C
PY - 2009/8
Y1 - 2009/8
N2 - When applying RF saturation to tissue, MRI signal reductions occur due to magnetization transfer (MT) and direct saturation (DS) effects on water protons. It is shown that the direct effects, often considered a nuisance, can be used to distinguish gray matter (GM) regions with different iron content. DS effects were selected by reducing the magnitude and duration of RF irradiation to minimize confounding MT effects. Contrary to MT saturation spectra, direct water saturation spectra are characterized by a symmetric Lorentzian-shaped frequency dependence that can be described by an exact analytical solution of the Bloch equations. The effect of increased transverse relaxation, e.g., due to the presence of iron, will broaden this saturation spectrum. As a first application, DS ratio (DSR) images were acquired to visualize GM structures in the human brain. Similar to T2*-weighted images, the quality of DSR images was affected by local field inhomogeneity, but this could be easily corrected for by centering the saturation spectrum on a voxel-by-voxel basis. The results show that, contrary to commonly used T2*-weighted and absolute R2 images, the DSR images visualize all GM structures, including cortex. A direct correlation between DSR and iron content was confirmed for these structures.
AB - When applying RF saturation to tissue, MRI signal reductions occur due to magnetization transfer (MT) and direct saturation (DS) effects on water protons. It is shown that the direct effects, often considered a nuisance, can be used to distinguish gray matter (GM) regions with different iron content. DS effects were selected by reducing the magnitude and duration of RF irradiation to minimize confounding MT effects. Contrary to MT saturation spectra, direct water saturation spectra are characterized by a symmetric Lorentzian-shaped frequency dependence that can be described by an exact analytical solution of the Bloch equations. The effect of increased transverse relaxation, e.g., due to the presence of iron, will broaden this saturation spectrum. As a first application, DS ratio (DSR) images were acquired to visualize GM structures in the human brain. Similar to T2*-weighted images, the quality of DSR images was affected by local field inhomogeneity, but this could be easily corrected for by centering the saturation spectrum on a voxel-by-voxel basis. The results show that, contrary to commonly used T2*-weighted and absolute R2 images, the DSR images visualize all GM structures, including cortex. A direct correlation between DSR and iron content was confirmed for these structures.
KW - Brain iron
KW - Direct saturation
KW - Magnetization transfer
KW - Off resonance
KW - Transverse relaxation
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U2 - 10.1002/mrm.21980
DO - 10.1002/mrm.21980
M3 - Article
C2 - 19526497
AN - SCOPUS:67749096074
VL - 62
SP - 384
EP - 393
JO - Magnetic Resonance in Medicine
JF - Magnetic Resonance in Medicine
SN - 0740-3194
IS - 2
ER -