| B1 Mapping: What's Your Angle? |
|
Tuesday 21 April 2009 |
| Room 314 |
16:00-18:00 |
Moderators: |
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
1Center 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
1Philips 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
1Institute 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
1Dept. 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
1Departments 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
1Center 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
1Institute 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
1NYU 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
1Imaging 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
1Radiology 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. |
|
|
|
|
|