Nadège Corbin1 and Martina F Callaghan1
1Wellcome Centre for Human Neuroimaging, UCL Institute of Neurology, London, United Kingdom
Synopsis
High reproducibility of longitudinal relaxation rate (R1) estimation
using the variable flip angle approach requires unbiased and precise estimation
of the transmit field inhomogeneity. In this context, the Bloch-Siegert
(BSS)-based B1+ mapping approaches were evaluated at 3T and 7T and compared to
the SE/STE approach. R1 values in grey and white matter were slightly higher
with the BSS-based techniques. At 3T, the highest reproducibility was observed
with the single-echo BSS approach, using an RF spoiling increment of 90°. At
7T, the reproducibility of the techniques were equivalent in well B0-shimmed
regions but diverged in regions of high B0 inhomogenity.
Introduction
The longitudinal relaxation rate
(R1) is a useful myelin marker for neuroscience research.
The Multi-Parameter Mapping (MPM) protocol1 estimates R1 from two
multi-echo-spoiled-gradient-echo sequences and a third acquisition mapping the
transmit field (B1+). Unbiased and high precision B1+ mapping is essential for reproducible
R1 estimates2 and becomes even more
important at ultra-high field due to the increased inhomogeneity of the B1+
field.
Here we evaluate R1
reproducibility, at 3T and 7T, using recently optimised Bloch-Siegert-shift(BSS)-based
B1+ mapping techniques in comparison to a Spin-Echo-Stimulated-Echo (SE/STE) approach commonly used in the MPM
protocol. Methods
- 3T Prisma: MPM data
were
acquired on the same participant on 4 consecutive days using a 32 channel head coil.
- 7T Terra: MPM data were acquired with the 8Tx/32Rx channels Nova head coil on
2 different days on 2 participants. The system was operated in the normal
mode for SAR management for the first participant and in the first level mode for the second, allowing higher
flip angle for the off-resonance pulse of the BSS technique.
T1/PD weighted images Two FLASH volumes were acquired
for each session with different excitation flip angle. At 3T, PD-weighted images from each day were coregistered to an
independently acquired R
1 map from which grey (GM) and white (WM) matter probabilities were computed. At 7T, data from the second session was
coregistered to the first for each participant.
B1+ mapping techniques
BSS B
1+ mapping uses an off-resonance RF pulse after the excitation to induce a phase accrual proportional
to the square of the B
1+ amplitude. Two acquisitions with opposite
off-resonance frequency are acquired to isolate the phase of
interest. Previous work
3 has demonstrated that this “classic”
approach is sensitive to the RF spoiling increment such that a 90° increment
(ɸ
inc) should
be used to estimate bias-free B
1+ maps. Alternatively, it is possible to use a
multi-echo approach combined with a General Linear Model
(GLM)-based-reconstruction to create bias-free maps regardless of ɸ
inc. At 3T, 3
protocols were tested:
-
SE90: BSS-Single-echo (SE) with ɸ
inc=90°
-
ME90: BSS-Multi-echo (ME) with ɸ
inc=90°
-
ME120: BSS-Multi-echo (ME) with ɸ
inc=120°
At 7T, only protocols with ɸ
inc=90° were applied.
Two B
1+ maps
were reconstructed from the ME acquisitions at 3T, one with the “classic”
approach based on the difference between the two echoes acquired immediately after
the off-resonance pulses with opposite off-resonance frequencies, the other one based
on the GLM parameters as described in
3 .
For each session, the SE/STE approach, typical of the MPM protocol and previously been optimized for
both 3T
4 and 7T
5, was also used to map B
1+. Since SE/STE uses an EPI readout, a B
0 field map was additionally acquired
for distortion correction.
Acquisition parameters are
shown in Fig.1.
Image processing:
The hMRI toolbox
6 (hMRI.info) was used to compute 6 or 3 R
1 maps per session at 3T and 7T respectively,
with the same T
1 and PD weighted images but different B
1+ maps. Correction for imperfect spoiling was applied
7. At 3T, the coefficient of variation (CoV) of
each technique, across sessions, was computed for each voxel as the ratio of
the difference between the maximum and the minimum R
1 value over the mean
across the 4 sessions. As such it is a
very conservative estimate of the reproducibility sensitive to outliers. At 7T, the CoV was
computed as the difference between the two sessions over the mean value.
Results
3T
The average R1 estimates in GM
and WM were similar across the BSS-based techniques (less than 0.4% difference
on average to the SE90Classic approach). The exception was ME120Classic which
resulted in higher R1 estimates (1.5% greater (WM) and 3% (GM)). The SE/STE approach
resulted in lower estimates than SE90Classic (1.5% smaller (WM) and 1% (GM))
(Fig.2)
Across the whole brain, the lowest
CoV for the R1 estimates was observed with SE90Classic (Fig.3). However, the
CoV was highly dependent on the B0 inhomogeneity. In well-shimmed regions, ME120Classic
and ME120GLM showed the highest CoV. In regions with high B0 inhomogeneity, GLM-based B1+ maps led to the highest CoV. SE/STE provided marginally higher CoV than SE90Classic
in general, with much larger divergence seen only for very high off-resonance.
7T
Similar reproducibility was achieved for all approaches in
well-shimmed regions but was reduced in areas of high B0 inhomogeneity (Fig.4),
especially in orbito-frontal cortex (OFC) and cerebellum (Fig.3). Reproducibility
was generally highest for SE90Classic in regions of negative off-resonance
frequencies and highest for SE/STE in regions of positive off-resonance
frequency.
No substantial difference was observed between normal and first
level modes.Discussion
The BSS-based approaches provided
similar R1 estimates except for ME120Classic, likely due to the biased B1+ maps obtained with Φinc≠90° when a GLM-based reconstruction
is not used3.
Although the GLM approach removed the bias of the 120° case, it did
not improve the reproducibility of ME120 and ME90, and even accentuated its
sensitivity to B0 inhomogeneity.
SE/STE produced lower R1 estimates
and slightly lower reproducibility than the SE-BSS approach at 3T.
At 7T, the
B0 sensitivity was more pronounced and the three techniques showed high CoV in
off-resonance regions.
SAR is an anticipated limitation
of BSS-based techniques at UHF8.
However, high
SAR regime was not necessary to match the SE/STE reproducibility of
SE/STE. Acknowledgements
The Wellcome Centre for Human Neuroimaging is supported by core funding from the Wellcome [203147/Z/16/Z].References
1.
Weiskopf N, Suckling J, Williams G, et al. Quantitative multi-parameter mapping
of R1, PD*, MT, and R2* at 3T: a multi-center validation. Front Neurosci 2013;7
doi: 10.3389/fnins.2013.00095.
2. Lee Y, Callaghan MF, Nagy Z. Analysis of the
Precision of Variable Flip Angle T1 Mapping with Emphasis on the Noise
Propagated from RF Transmit Field Maps. Front Neurosci 2017;11 doi:
10.3389/fnins.2017.00106.
3. Corbin N, Acosta‐Cabronero J, Malik SJ, Callaghan
MF. Robust 3D Bloch-Siegert based mapping using multi-echo general linear
modeling. Magnetic Resonance in Medicine 2019;82:2003–2015 doi:
10.1002/mrm.27851.
4. Lutti A, Hutton C, Finsterbusch J, Helms G,
Weiskopf N. Optimization and Validation of Methods for Mapping of the
Radiofrequency Transmit Field at 3T. Magn Reson Med 2010;64:229–238 doi:
10.1002/mrm.22421.
5. Lutti A, Stadler J, Josephs O, et al. Robust and
Fast Whole Brain Mapping of the RF Transmit Field B1 at 7T. PLoS One 2012;7
doi: 10.1371/journal.pone.0032379.
6. Tabelow K, Balteau E, Ashburner J, et al. hMRI - A
toolbox for quantitative MRI in neuroscience and clinical research. Neuroimage
2019;194:191–210 doi: 10.1016/j.neuroimage.2019.01.029.
7. Preibisch C, Deichmann R. Influence of RF spoiling
on the stability and accuracy of T1 mapping based on spoiled FLASH with varying
flip angles. Magnetic Resonance in Medicine 2009;61:125–135 doi:
10.1002/mrm.21776.
8. Pohmann R, Scheffler K. A theoretical and
experimental comparison of different techniques for B1 mapping at very high
fields. NMR in Biomedicine 2013;26:265–275 doi: 10.1002/nbm.2844.