B1 Mapping: What's Your Angle?
Tuesday 21 April 2009
Room 314 16:00-18:00


Adam B. Kerr and Rudolf Stollberger

16:00  367.

Very Fast Multi Channel B1 Calibration at High Field in the Small Flip Angle Regime

    Pierre-Francois Van de Moortele1, Kamil Ugurbil1
Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, USA

We describe a fast method to estimate transmit B1 profiles in multi channel transceiver arrays in the small flip angle regime within about 1 minute of data acquisition. Despite of expected residual biases, it was possible to obtain excellent B1 Shim results based on these estimated B1 maps. Further investigation will help determining if this fast B1 estimation, which can cover the whole brain in less than 3 minutes with 40 slices, could become part of common scanner calibration routines, such as B0 mapping, for integrating transmit B1 adjustment in standard MR sessions at high field.

16:12 368. Eigenmode Analysis of Transmit Coil Array for SAR-Reduced B1 Mapping and RF Shimming
    Kay Nehrke1, Peter Börnert1
Philips Research Europe, Hamburg, Germany
    The B1 transmit field inhomogeneity represents a serious problem in whole-body high field MRI (>3T). B1 shimming based on measured B1 maps is a promising approach to cope with this problem and represents the primary application for parallel transmission at this point in time. However, B1 mapping is still an error-prone and time consuming process, potentially resulting in a SAR issue caused by the shimmed RF pulse and the mapping scan itself. In the present work, an eigenmode analysis of the transmit sensitivities is employed to accelerate the B1 mapping process and reduce the SAR of the shimmed RF pulses.


16:24 369. Precise and Robust B1+ Characterization of Transmit Coil Arrays
    Martin Janich1,2, Olaf Dössel1, Sascha Köhler2, Johannes Schneider2, Peter Ullmann2
Institute of Biomedical Engineering, University of Karlsruhe, Karlsruhe, Germany; 2Bruker BioSpin MRI GmbH, Ettlingen, Germany

A common approach to B1+ characterization of TX-arrays involves transmitting with different combinations of array elements in order to improve SNR. TX-element combination can be employed in a more sophisticated manner when using a B1+ mapping method based on saturation and excitation pulses. The saturation can be performed with a single transmit element and the excitation with a B1+-shimmed combination. The present study proposes improvements to the common combined transmission approach for TX-array B1+ mapping as well as for the saturation-based technique. The performance of the improved techniques was experimentally compared to the classical single-element B1+ mapping approach.



16:36 370. Extended Multi-Flip-Angle Approach: A 3D B1unit+ Mapping Method for Inhomogeneous Fields
    Hans Weber1, Dominik Paul1, Maxim Zaitsev1, Jürgen Hennig1, Dominik von Elverfeldt1
Dept. of Diagnostic Radiology, Medical Physics, University Hospital Freiburg, Freiburg, Germany
    Comprehensive characterization of an RF resonator is given by a quantitative and spatial analysis of its magnetic field produced per unit current (B1unit+). In this study, we present an extension of the multi-flip-angle approach which allows mapping of inhomogeneous B1unit+ fields in three dimensions over a large dynamic range. To demonstrate its superiority over the commonly-used conventional double-angle method, a comparison between both is given.
16:48 371.

Targeted B1+ Mapping Using 3D Reduced Field-Of-View Catalyzed Double-Angle Method

    Dingxin Wang1, Sven Zuehlsdorff2, Reed Omary1, Andrew Larson1
Departments of Radiology and Biomedical Engineering, Northwestern University, Chicago, IL, USA; 2Siemens Medical Solutions USA, Inc., Chicago, IL, USA
    This study proposes a targeted B1+ mapping technique using 3D reduced FOV catalyzed double angle method (DAM). This method is based on 3D catalyzed DAM which allows a short TR for fast B1+ mapping by introducing catalyzation pulses at the end of each repetition cycle of DAM to drive the ratio of the ending longitudinal magnetizations (for the two different flip angle excitations) to unity. This method employs an inner volume 3D turbo spin echo (TSE) sequence to limit the FOV and thereby to shorten imaging time.
17:00 372.

Actual Flip Angle Imaging: From 3D to 2D

    Xiaoping Wu1, Dinesh Kumar Deelchand1, Vasily L. Yarnykh2, Kâmil Ugurbil1, Pierre-François Van de Moortele1
Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, USA; 2Department of Radiology, University of Washington, Seattle, WA, USA
    Recently, actual flip angle imaging (AFI) has been introduced as an efficient, fast 3D flip angle (FA) mapping technique. In some circumstance, a 2D version would be preferable (e.g., 2D Parallel Transmission) since it would require a significantly shorter acquisition time. Although in the original 3D version the FA calculation in AFI does not need to take into consideration the impact of slice profiles, this is however not the case when a 2D slice selective version of the same approach is considered, especially with regard to T1 sensitivity. Therefore, the purpose of this study was to evaluate the properties and feasibility of 2D AFI FA mapping where 2D (instead of 3D) image signals are used for FA calculations, using the equation that was derived for the 3D AFI FA mapping. For this purpose, we performed phantom experiments at 9.4 T, together with simulations, to study the relationship between 2D and 3D AFI FA values for different T1's.


17:12 373. Quantitative Comparison of B1+ Mapping Methods for 7T Human Imaging
    Jason E. Moore1, Marcin Jankiewicz1,2, Huairen Zeng1,2, Adam W. Anderson1,3, Malcolm J. Avison1,3, E Brian Welch1,4, John C. Gore1,3
Institute of Imaging Science, Vanderbilt University, Nashville, TN, USA; 2Department of Radiology, Vanderbilt University; 3Department of Biomedical Engineering, Vanderbilt University; 4Philips Healthcare, Cleveland, OH, USA
    B1+ / flip angle mapping results from double-angle, pulsed steady state, and gradient echo series techniques are compared using a dielectric phantom in a 7T human MR system. Under such conditions, B1+ maps are found to vary significantly (~50%) across protocols.
17:24 374. Optimization of a Low-Flip-Angle Phase-Based 3D B1 Mapping Technique for High Field Applications
    Pippa Storey1, Graham C. Wiggins1, Davide Santoro1, Daniel K. Sodickson1
NYU School of Medicine, New York, NY , USA
    A rapid 3D low-flip-angle phase-based method for B1 measurement was originally proposed by Mugler for transmit calibration in hyperpolarized helium studies and also tested as a B1 mapping technique in protons at 1.5T. We explore ways to optimize the sensitivity and accuracy of the technique for high field applications, and present B1 maps of the thighs at 3T and the brain at 7T.
17:36 375. A Simple and Fast Flip Angle Calibration Method
    Sofia Chavez1, Greg Stanisz2
Imaging Research, Sunnybrook Health Sciences Centre, Toronto, Ontario, Canada; 2Sunnybrook Health Sciences Centre, Canada
    A simple flip angle calibration method is introduced. It relies on the quasi-linear characteristics of the signal vs flip angle curve for a spoiled gradient recalled echo at large flip angles and for short repetition time. A straight-line extrapolation is used to determine the signal null point, occurring for a true flip angle of 180°. The data at each pixel is fit to yield a map of the flip angle calibration factor (k). The resulting k map is shown to be in good agreement with that resulting from the standard double angle method in a much shorter acquisition time.
17:48 376. A Noise Analysis of Flip Angle Mapping Methods
    Glen Morrell1,2, Matthias Schabel2
Radiology Department, University of Utah Health Sciences Center, Salt Lake City, UT, USA; 2Utah Center for Advanced Imaging Research, University of Utah, Salt Lake City, UT, USA
    Error analysis of four methods of flip angle mapping was performed by explicit calculation of the probability density functions of the flip angle estimates given corruption of the measured MR signals by Gaussian white noise. The methods investigated were double angle gradient recalled echo, double angle spin echo, “actual flip angle” imaging, and a phase sensitive technique. The phase sensitive technique is shown to be superior to other methods, with lower mean bias and lower variance of the flip angle estimate.