New Angles on B1 Mapping
Tuesday 4 May 2010
Room A8 10:30-12:30 Moderators: Ulrich Katscher and Yudong Zhu

10:30   Introduction
Hans-Peter Fautz
10:54 236

Fast 2D B1 Mapping by K-Space Processing of Tagging Patterns
Wayne R. Dannels1, Andrew J. Wheaton1

1Toshiba Medical Research Institute, Mayfield Village, OH, United States

Measuring B1 transmit fields in vivo has importance in areas such as high field imaging, parallel transmission design, and quantitative imaging. A new method of acquisition and data analysis is presented for generating 2D B1 maps in vivo in as little as one TR. In this method saturation tag lines are applied before rapid imaging, tag lines are separated from the underlying image with k-space processing, and RF angles are computed from the tagging efficiency ratio.

11:06 237.  

Improved Phase-Based Adiabatic B1 Mapping
Franciszek Hennel1, Sascha Köhler1
Bruker BioSpin MRI, Ettlingen, Germany

A method for the mapping of the radio-frequency transmission field is proposed that derives B1 values from the phase of the signal. The sequence consists of a block pulse to produce a B1-dependent nutation, followed by an inverse adiabatic half passage (IAFP) that converts the nutation phase to signal phase. Two ways to compensate the undesired dephasing caused by the IAFP are proposed: a rewinder RF pulse, or a matched adiabatic echo. The method provides an increased dynamic range compared to known phase-based B1-mapping sequences.

11:18 238.

Flip Angle Taxonomy: Measuring Transmit (B1) Profile Distribution Without Imaging
Roman Fleysher1, Lazar Fleysher1, Joel A. Tang2, Daniel Sodickson1
1Radiology, New York University, School of Medicine, New York, United States; 2Chemistry, New York University, New York, United States

A method of measuring transmit (micro-) coil profile (B1) distribution is presented. In as much as it does not use spatial encoding, it reaches fine resolution in B1 at very high signal-to-noise ratios. The procedure can be used to alleviate systematic errors in spectroscopic data analysis caused by transmit field non-uniformity or can be employed for a quick evaluation of transmit (micro-) coil performance.

11:30 239

Permittivity Determination Via Phantom and in Vivo B1 Mapping
Ulrich Katscher1, Philipp Karkowski1, Christian Findeklee1, Tobias Voigt2
1Philips Research Europe, Hamburg, Germany; 2Institute of Biomedical Engineering, University of Karlsruhe, Karlsruhe, Germany

Tissue permittivity might serve as diagnostic parameter, e.g., for oncology. However, the diagnostic use of the permittivity is significantly hampered by the lack of suitable methods to determine the permittivity in vivo. A possible approach for the determination of permittivity in vivo is given by analyzing the B1 map in the framework of standard MRI, called "Electric Properties Tomography" (EPT). Hitherto, studies were focussed on the ability of EPT to reconstruct the electric conductivity and local SAR. This study demonstrates the ability of EPT to determine the permittivity via numerous phantom and in vivo experiments.

11:42 240

Simultaneous 3D B1 and T1 Mapping Using the New Method of Slopes (MoS)
Sofia Chavez1, Greg Stanisz1,2
1Imaging Research, Sunnybrook Health Sciences Centre, Toronto, Ontario, Canada; 2Medical Biophysics, University of Toronto, Toronto, Ontario, Canada

A new 3D method for simultaneous B1 and T1 mapping is presented. It relies on the quasi-linear relationship between the measured SPGR signal and nominal flip angle near the origin and near the signal null. The B1 mapping estimation is similar to that already existing in the literature with a more practical implementation requiring flip angles < 180° which are readily available on most scanners. The B1 mapping data with an additional acquisition of  the SPGR signal  at a low flip angle allows for the proposed T1 mapping. MoS yields accurate T1 values (within 10% of  IR esimates)  for an entire brain volume in ~12 min.

11:54 241

Comparison Between RF Spoiling Schemes in the Actual Flip-Angle Imaging (AFI) Sequence for Fast B1 Mapping
Vasily L. Yarnykh

1Department of Radiology, University of Washington, Seattle, WA, United States

The Actual Flip-angle Imaging (AFI) method allows fast B1 mapping based on the spoiled steady-state principle. The combination of diffusion-based gradient and RF spoiling mechanisms was recently shown to considerably improve accuracy of this method. Two RF spoiling techniques were proposed for AFI in the literature: a standard phase incrementing scheme with a constant value of the phase increment and a modified scheme with two intermittently applied phase increments dependent on the ratio n=TR2/TR1. This study compares the spoiling behavior of the AFI sequence and accuracy of B1 measurements between the above RF spoiling schemes.

12:06 242.

SVD Based Calibration of Transmit Arrays
David Otto Brunner1, Klaas Paul Pruessmann1
Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland

Using transmit-receive arrays the acquisition of transmit and receive sensitivities are both of crucial importance but there are also great difficulties involved in bootstrapping such a process. In regions of low excitation, the receive sensitivities cannot be estimated correctly, leading to strong noise enhancement in the reconstructed images as well as in the transmit calibration data. This noise then propagates into the calculated transmit profiles hindering transmit calibration. In this work we present an acquisition and reconstruction technique that solves this entangled problem and allows finding concomitantly the signal optimal global RF shims and local receive channel combinations.

12:18 243

RF Field Profiling Through Element Design for High Field Volume Coils
Can Akgun1, Lance DelaBarre1, Carl J. Snyder1, Gregor Adriany1, Anand Gopinath2, Kamil Ugurbil1, John Thomas Vaughan1
1University of Minnesota-Center for Magnetic Resonance Research, Minneapolis, MN, United States; 2University of Minnesota-Department of Electrical and Computer Engineering, Minneapolis, MN, United States

Multi-channel volume coils can be comprised of an array of transmission line elements operated as independent coils in multiple-channel transmit and receive configurations. In these designs, microstrip transmission elements have been implemented as magnetic field propagating elements. However, at high fields, RF in-homogeneities and inefficiencies require the optimization of these elements.  In this study, two different microstrip designs with varying impedance lines; one producing peak B1+ in the center and the other extending usable B1+ along the coil are investigated.  Simulation and image results for 8-channel volume coils incorporating these element designs were obtained using a phantom at 7T.



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