Direct saturation MRI: Theory and application to imaging brain iron

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. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.

[1]  Xavier Golay,et al.  Routine clinical brain MRI sequences for use at 3.0 Tesla , 2005, Journal of magnetic resonance imaging : JMRI.

[2]  Jinyuan Zhou,et al.  The interaction between magnetization transfer and blood‐oxygen‐level‐dependent effects , 2005, Magnetic resonance in medicine.

[3]  Peter Andersen,et al.  In vivo 1H2O T  †2 measurement in the human occipital lobe at 4T and 7T by Carr‐Purcell MRI: Detection of microscopic susceptibility contrast , 2002, Magnetic resonance in medicine.

[4]  Weili Lin,et al.  Quantitative regional brain water measurement with magnetic resonance imaging in a focal ischemia model , 1997, Magnetic resonance in medicine.

[5]  Jinyuan Zhou,et al.  Quantitative description of proton exchange processes between water and endogenous and exogenous agents for WEX, CEST, and APT experiments , 2004, Magnetic resonance in medicine.

[6]  N. Nighoghossian,et al.  Contribution of Susceptibility-Weighted Imaging to Acute Stroke Assessment , 2004, Stroke.

[7]  R A Knight,et al.  MR imaging of human brain at 3.0 T: preliminary report on transverse relaxation rates and relation to estimated iron content. , 1999, Radiology.

[8]  V. Mathews,et al.  Cranial tissues: appearance at gadolinium-enhanced and nonenhanced MR imaging with magnetization transfer contrast. , 1994, Radiology.

[9]  Xavier Golay,et al.  Determining the longitudinal relaxation time (T1) of blood at 3.0 Tesla , 2004, Magnetic resonance in medicine.

[10]  N. Gelman,et al.  Interregional variation of longitudinal relaxation rates in human brain at 3.0 T: Relation to estimated iron and water contents , 2001, Magnetic resonance in medicine.

[11]  John F Schenck,et al.  Magnetic resonance imaging of brain iron , 2003, Journal of the Neurological Sciences.

[12]  Craig K. Jones,et al.  Pulsed magnetization transfer imaging with body coil transmission at 3 Tesla: Feasibility and application , 2006, Magnetic resonance in medicine.

[13]  M. Bronskill,et al.  T1, T2 relaxation and magnetization transfer in tissue at 3T , 2005, Magnetic resonance in medicine.

[14]  R M Henkelman,et al.  Quantitative interpretation of magnetization transfer , 1993, Magnetic resonance in medicine.

[15]  A Dean Sherry,et al.  Chemical exchange saturation transfer contrast agents for magnetic resonance imaging. , 2008, Annual review of biomedical engineering.

[16]  R V Mulkern,et al.  The general solution to the Bloch equation with constant rf and relaxation terms: application to saturation and slice selection. , 1993, Medical physics.

[17]  X Golay,et al.  MR imaging of the human brain at 1.5 T: regional variations in transverse relaxation rates in the cerebral cortex. , 2001, AJNR. American journal of neuroradiology.

[18]  R. Bryant,et al.  The dynamics of water-protein interactions. , 1996, Annual review of biophysics and biomolecular structure.

[19]  M. Bernstein,et al.  Measurements of T 1 Relaxation times at 3 . 0 T : Implications for clinical MRA , 2001 .

[20]  Nickolas Papanikolaou,et al.  Non-invasive myocardial iron assessment in thalassaemic patients. T2 relaxometry and magnetization transfer ratio measurements. , 2000, Acta radiologica.

[21]  Xiaoping Hu,et al.  Off‐resonance saturation as a means of generating contrast with superparamagnetic nanoparticles , 2006, Magnetic resonance in medicine.

[22]  J A Frank,et al.  Perfusion imaging with compensation for asymmetric magnetization transfer effects , 1996, Magnetic resonance in medicine.

[23]  G. Pike,et al.  Quantitative interpretation of magnetization transfer in spoiled gradient echo MRI sequences. , 2000, Journal of magnetic resonance.

[24]  R. Balaban,et al.  Magnetization transfer contrast (MTC) and tissue water proton relaxation in vivo , 1989, Magnetic resonance in medicine.

[25]  B. Hallgren,et al.  THE EFFECT OF AGE ON THE NON‐HAEMIN IRON IN THE HUMAN BRAIN , 1958, Journal of neurochemistry.

[26]  R S Balaban,et al.  The effect of off‐resonance radio frequency pulse saturation on fMRI contrast , 1997, NMR in biomedicine.

[27]  Yu-Chung N. Cheng,et al.  Susceptibility weighted imaging (SWI) , 2004, Zeitschrift fur medizinische Physik.

[28]  Jinyuan Zhou,et al.  Quantitative description of the asymmetry in magnetization transfer effects around the water resonance in the human brain , 2007, Magnetic resonance in medicine.

[29]  R. Kauppinen,et al.  Inverse T2 contrast at 1.5 Tesla between gray matter and white matter in the occipital lobe of normal adult human brain , 2001 .