Direct saturation MRI: Theory and application to imaging brain iron

Research output: Contribution to journalArticle

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 languageEnglish (US)
Pages (from-to)384-393
Number of pages10
JournalMagnetic Resonance in Medicine
Volume62
Issue number2
DOIs
StatePublished - Aug 2009

Fingerprint

Neuroimaging
Iron
Water
Protons
Brain
Gray Matter

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 journalArticle

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