New Mechanisms & Sequences for Contrast Generation
Friday 24 April 2009
Room 313BC 10:30-12:30

Moderators:

 Natalia Petridou and Fritz Schick
 
10:30 862. Contribution of Protein-Induced Magnetic Susceptibility and 1H Exchange Effects to Water MR Frequency Shifts
    Jie Luo1, Xiang He2, D. Andre' d'Avignon1, Joseph J. H. Ackerman1, Dmitriy A. Yablonskiy2
1Department of Chemistry, Washington University, Saint Louis, MO, USA; 2Department of Radiology, Washington University, Saint Louis, MO, USA
    High field, conventional GRE-MR phase images show superior contrast between gray and white matter in human brain. Possible origins of this contrast, such as magnetic susceptibility due to tissue lipids, non-heme iron, deoxyhemoglobin, and water-protein 1H exchange have been suggested, yet the genesis remains to be elucidated. Herein, separation of susceptibility and 1H exchange effects in protein solutions is described. The frequency shift due to protein susceptibility is about three times greater (and with opposite sign) compared to that due to 1H exchange. Further, significant protein content is cross-linked in vivo, possibly reducing frequency shift contributions from water-protein 1H exchange.
     
10:42 863. On the Nature of Phase Contrast in Gradient Echo MRI:A Generalized Lorentzian Approach
    Xiang He1, Dmitriy A. Yablonskiy1,2
1Mallinckrodt Institute of Radiology, Washington University in St. Louis, St. Louis, MO, USA; 2Department of Physics, Washington University in St. Louis, St. Louis, MO, USA
    Herein we propose a theoretical framework based on the concept of Lorentzian ellipsoid that allows quantitative evaluation of brain tissue phase contrast in Gradient Echo MRI. Our approach takes into account the specific geometric properties of the magnetic susceptibility inclusions (mostly proteins, lipids, deoxyhemoglobin and non-heme iron) in the brain tissue. We demonstrated that not just the amount, but, more importantly, spatial distribution of the susceptibility inclusions at the sub-cellular level, as well as global cellular organization and its relative orientation with respect to the external B0 field are the dominant factors in the observed phase contrast.
     
10:54 864. Molecular Size Dependency of Water Macromolecule Exchange Induced Frequency Shift
    Kai Zhong1, Karl-Heinz Smalla2, Alexander Brensing1, Oliver Speck1
1Otto-von-Guericke University, Magdeburg, Germany; 2Leibniz Institute for Neurobiology, Magdeburg, Germany
    Previous study suggested that water macromolecule exchange (WME) processes could contribute significantly to the in vivo GM/WM contrast. In this study, we studied systematically the WME shift abilities of several proteins with different molecular weight (Mw) under neutral pH. It is found that the WME shift ability of macromolecules depends strongly on their size and can be quantitatively determined. A strong relationship between the macromolecule distribution and the corresponding in vivo frequency shift for phase imaging is thus predicted. This finding paves the way for phase imaging applications in pathologies with altered in vivo macromolecule content.
     
11:06 865. The Role of Intravascular Effects in Phase Contrast Between Gray and White Matter
    Natalia Petridou1, Samuel J. Wharton1, Ashley Lotfipour1, Penny Gowland1, Richard Bowtell1
1SPMMRC, University of Nottingham, Nottingham, UK
    MR phase images exhibit exceptional contrast between Gray and White Matter. Here we examine the contribution of blood-related sources to this contrast by modulating the blood susceptibility with a gadolinium-based contrast agent during time-series phase imaging. We found that the GM/WM phase difference increased during the bolus pass but returned approximately to pre-contrast values after recirculation, while the GM/vasculature phase difference remained enhanced. Thus the significantly elevated susceptibility of blood due to the “steady-state” concentration of gadolinium had no significant effect on the GM/WM susceptibility difference, indicating that the latter is not explained by deoxyhaemoglobin in blood.
     
11:18 866. Balanced SSFP Profile Asymmetries Are Sensitive to White Matter Tract Structure
    Karla L. Miller1, Peter Jezzard1, Stephen M. Smith1
1FMRIB Centre, Oxford University, Oxford, Oxon, UK
    A new contrast mechanism based on balanced SSFP is presented, which is hypothesized to be sensitive to tissue microstructure. The balanced SSFP profile is theoretically symmetric if only T1, T2 and diffusion effects are considered. However, in the presence of an asymmetric lineshape, the profile can become asymmetric. Strong asymmetries are observed in white matter, and maps of these asymmetries are robust and reproducible across subjects. White matter fibres parallel to B0 have significantly reduced asymmetry, indicating that the largest asymmetries may relate to tract geometry. This novel form of contrast may be a sensitive marker to white matter integrity.
     
11:30 867. On the Cause of Transient Off-Resonance Stopbands in TIDE BSSFP Imaging
    Yin-Cheng Kris Huang1,2, Teng-Yi Huang3, Wen-Chau Wu4, Ying-Po Liao5, Hsiao-Wen Chung1,2
1Department of Electrical Engineering, National Taiwan University, Taipei City, Taiwan; 2Department of Radiology, Tri-Service General Hospital, Taipei City, Taiwan; 3Department of Electrical Engineering, National Taiwan University of Scienece and Technology, Taipei City, Taiwan; 4Graduate Institute of Clinical Medicine, National Taiwan University, Taipei City, Taiwan; 5Department of Civil Engineering, National Taiwan University, Taipei City, Taiwan
    Transition into Driven Equilibrium balanced steady-state free precession sequence (TIDE bSSFP) has been shown to carry an intrinsic fat suppression. This phenomenon was described to come from the appearance of an off-resonance signal null or transient stopbands. Fat suppression remained although TIDE preparations of various TIDE-specific parameters were used. In this study, we devoted ourselves to the analysis of the mechanism why the signal null comes to existence, using simulation and analytical derivation. The results show that the magnetization of a dephasing angle £c = 360¢X evolves in a fashion analogous to inversion recovery (IR), which might be informally dubbed as ¡§frequency-selective IR-like evolution.¡¨
     
11:42 868. High Positive Contrast Generation of a Subvoxel Susceptibility Deviation in Ultrashort TE (UTE) Radial Center-Out Imaging at 3T
    Peter Roland Seevinck1, Hendrik de Leeuw1, Clemens Bos2, Chris J.G. Bakker1
1Image Sciences Institute, Department of Radiology , University Medical Center, Utrecht, Netherlands; 2Philips Healthcare Best, Best, Netherlands
    In this work the artefact of a spherical subvoxel susceptibility deviation in ultrashort TE (UTE) center-out radial imaging was characterized. UTE imaging can be used to generate high positive contrast from susceptibility inhomogeneities with long T2 suppresion. Increased CNR compared to conventional imaging methods with longer TE was observed, related to the lack of dephasing of the signal pile up created by the Ä÷ inclusion. Simulations have indicated that this imaging strategy may be well suitable for the detection of small amounts of (super)-paramagnetic substances.
     
11:54 869. Adiabatic Pulse Preparation for Imaging Iron Oxide Nanoparticles
    Steven Harris1, Xiaoping Hu1
11Department of Biomedical Engineering, Georgia Institute of Technology / Emory University, Atlanta, GA, USA
    A method for producing contrast based on the failure of the adiabatic condition in regions surrounding iron oxide nanoparticles is presented. Numerical simulations and experimental data show that the microscopic changes in the magnetic field around the particles produce frequency offsets that can be detected using an adiabatic preparation pulse. SPIO sensitive images generated using the prepared and non-prepared images show increasing image intensity with increasing iron concentration. This contrast appears linear and shows only a small increase in intensity with particles in agar versus water, suggesting insensitivity to magnetization transfer.
     
12:06 870. Cyclic Motion Encoding for Enhanced MR Visualization of Slip Interfaces
    Yogesh Kannan Mariappan11, Kevin J. Glaser1, Armando Manduca2, Richard L. Ehman1
1Department of Radiology, Mayo clinic, Rochester, MN, USA; 2Biomathematics resource, Mayo clinic, Rochester, MN, USA
    The presence of low-friction “slip interfaces” between some tissue surfaces is very important and when lost, can lead to serious conditions, such as bowel adhesions in the abdomen or flexor muscle adhesions of the forearm. Hence, there is potential diagnostic value for improved methods for noninvasively assessing the degree of mechanical connectedness across tissue interfaces. We developed and tested a “shear-line” imaging technique, employing cyclic motion-encoding gradients and synchronized vibrational motion, to generate intravoxel phase dispersion and signal loss at slip interfaces between tissues in MR magnitude images, thereby providing a sensitive method for evaluating the functionality of tissue slip interfaces.
     
12:18 871. Improved Transverse Relaxometry: A New Fitting Model with Stimulated Echo Compensation
    Robert Marc Lebel1, Alan H. Wilman1
1Biomedical Engineering, University of Alberta, Edmonton, Alberta, Canada
    We present a robust fitting model for transverse relaxometry with multi-echo spin-echo data. This model compensates for stimulated echo contributions arising from non-ideal RF pulse shape, width, and amplitude. We integrate echo amplitudes over the slice profile to obtain an aggregate decay curve for non-linear fitting to experimental data. This routine provides accurate relaxation times and an estimate of the transmit B1 field. This approach is validated in phantom models and in-vivo at 4.7 T. Our model improves relaxometry reliability at all field strengths and permits efficient data acquisition with thin refocusing widths.