Jan Sedlacik1,2,3,4, Raphael Tomi-Tricot1,2,3,5, Tom Wilkinson1,2,3, Pip Bridgen1,2,3, Franck Mauconduit6, Alexis Amadon6, Sharon Giles1,2,3, Joseph V Hajnal1,2,3, and Shaihan J Malik1,2,3
1Centre for the Developing Brain, School of Biomedical Engineering & Imaging Sciences, King’s College London, London, United Kingdom, 2Biomedical Engineering Department, School of Biomedical Engineering & Imaging Sciences, King’s College London, London, United Kingdom, 3London Collaborative Ultra high field System (LoCUS), London, United Kingdom, 4Radiology Department, Great Ormond Street Hospital for Children, London, United Kingdom, 5MR Research Collaborations, Siemens Healthcare Limited, Frimley, United Kingdom, 6Paris-Saclay University, CEA, CNRS, BAOBAB, NeuroSpin, Gif-sur-yvette, France
Synopsis
A fast and
accurate B1+mapping method is essential for making proper use of a
parallel transmit array with high number of coil elements in a
clinical setting. The saturation-prepared turbo FLASH method offers
this
capability and
is implemented in the fully automated pre-scan adjustments of the
currently only clinical ultra-high magnetic field strength MRI
system.
However, some parallel transmit methods
require
an even higher accuracy. This
can be achieved
by calibrating
the fast
B1+measurements
with
a more accurate but longer B1+mapping method.
INTRODUCTION
A fast and accurate B1+mapping method is essential
for making proper
use of a parallel transmit array with high number of coil elements in
a clinical setting (1). The saturation-prepared turbo FLASH method
(sat.prep.TFL) (2) offers the capability to map the B1+field in a few
seconds, but lacks
high
accuracy required by
many
imaging methods using parallel transmit like the DiSCoVER method (3).
However, it is
still implemented
in the fully automated pre-scan adjustments of a current generation
ultra-high magnetic field strength MRI system. The purpose of this
work is to report means to calibrate the sat.prep.TFL B1+measurements
by a more accurate but slower
B1+mapping method which will enable improved performance while
retaining the workflow benefits of the fully automated and
fast pre-scan adjustments of the scanner.METHODS
12 healthy subjects (5 female, age range 25-43
years) and one spherical phantom filled with water, 1% Agarose, 0.5%
NaCl and 0.03mM MnCl (resulting in T1=1.9s at room temperature) were
scanned on a 7T scanner (MAGNETOM
Terra, Siemens Healthcare, Erlangen, Germany) with a 1Tx-32Rx-channel
head coil (Nova Medical, Wilmington MA, USA) transmitting in
Circular-Polarised (CP) mode. Human subject scanning was approved by
the Institutional Research Ethics Committee (HR-18/19-8700).
The phantom was scanned by the
following B1+mapping techniques: solving the signal equation for the
spoiled gradient echo sequence with 3D
Multiple Flip Angles (MultiFA) (4) with
FA=5-90°, ΔFA=5°, T1=1.9s and acquisition time (TA)=31:10min; 3D
Actual Flip angle Imaging (AFI) (5) with
FA=50°, TR1/TR2=40ms/200ms and TA=3:36min; 3D
SAturation-prepared 2 RApid Gradient Echoes
(SA2RAGE) (6) with TA=1:41min; 2D
sat.prep.TFL (2) with 42 phase encoding
steps, 21 slices, TR=5s,
saturation FA=90° and TA=10s and 3D Dual
Refocusing Echo Acquisition Mode (3DREAM) (7) with TA=5s. Imaging
position, matrix size and field of view (voxel
size=4.5x4.5x5mm^3) were matched for all
mapping methods of the phantom. The B1+maps of all methods were
compared to the B1+map derived from the MultiFA scan. All B1+values
are reported relative to the target B1+value, i.e., the nominal flip
angle of the respective mapping method.
Following assessment of
performance, in vivo B1+maps were acquired by the AFI and
sat.prep.TFL methods only. The sat.prep.TFL method was acquired with
64 phase encoding steps causing a longer readout echo train for
subjects 1-6. The same sat.prep.TFL imaging parameters as for the
phantom were used for subjects 7-12. The AFI imaging parameters were
the same for all subjects and for the phantom. Only voxels within the
brain, extracted using
the FSL brain extraction tool (8), were
taken into account for the calibration analysis.RESULTS
All B1+mapping methods showed high B1+values in the centre of the
phantom which is characteristic for transmitting in CP-mode at 7T.
The B1+values measured by the AFI method most closely match the
values measured by the gold standard MultiFA method. The sat.prep.TFL
method shows a large
difference with much lower B1+values in the centre of the phantom
(Fig.2).
The in vivo B1+maps also show this characteristic
pattern for transmitting in CP-mode at 7T (Fig.3). The sat.prep.TFL
method shows lower B1+values than the AFI method in the centre region
but the mismatch is much smaller as compared to the phantom.
Scatter-plots and linear regression of the B1+values of the
sat.prep.TFL and AFI methods show good linear correlation but
slightly different linear slopes and intercepts for the sat.prep.TFL
method depending on the readout echo train length (Fig.4).
Calibrating the B1+values of the sat.prep.TFL
method with the average linear slope and intercepts lowers the root
mean square error (RMSE) to the AFI method by about 2.4±1.3%
or 6.0±1.9% for the
sat.prep.TFL scan acquired with 64 or 42 phase encoding steps,
respectively.
For brain regions with relative B1+values greater than 1, the RMSE is
lowered by 16±5.6% or
10±3% for the
sat.prep.TFL scan acquired with 64 or 42 phase encoding steps,
respectively.
The difference map between the AFI and sat.prep.TFL B1+maps also show
that the calibration reduces the mismatch between these
methods especially for regions in the centre of the brain with high
B1+values (Fig.5).DISCUSSION
The very similar B1+values measured by the MultiFA
and AFI methods in the phantom show
the high accuracy of the AFI method compared to other B1+mapping
methods. This also demonstrates the soundness of choosing the AFI
method for the in vivo scans. The mismatch between
AFI and sat.prep.TFL is much
less pronounced in vivo than
in the phantom. This is
caused
by the very high B1+values in the centre of the phantom where the
sat.prep.TFL method is inaccurate.
Furthermore, the
calibration depends on the imaging parameters of the sat.prep.TFL
method, such as the readout echo train length, due
to the transient state of the longitudinal magnetisation after the
saturation preparation
(2). The successful application of such a
calibration was already shown in a previous work (3).CONCLUSION
The accuracy of the sat.prep.TFL method, as
currently implemented in the fully automated pre-scan adjustments of
the clinical ultra-high magnetic field MRI scanner, can be increased
by linear calibration from in vivo measurements averaged over
multiple subjects. Regions with high B1+values in the centre of the
brain show the biggest improvement. Such a calibration will
facilitate routine 7T scanning of parallel transmit imaging methods
which require high accuracy B1+maps.Acknowledgements
This work was supported by core funding
from the Wellcome/EPSRC Centre for Medical Engineering
[WT203148/Z/16/Z] and by the National Institute for Health Research
(NIHR) Biomedical Research Centre based at Guy’s and St Thomas’
NHS Foundation Trust and King’s College London and/or the NIHR
Clinical Research Facility. The views expressed are those of the
author(s) and not necessarily those of the NHS, the NIHR or the
Department of Health and Social Care.References
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