Loreen Ruhm1, Johanna Dorst1, Nikolai Avdievich1, and Anke Henning1,2
1Max Planck Institute for Biological Cybernetics, Tuebingen, Germany, 2Advanced Imaging Research Center, UT Southwestern Medical Center, Dallas, TX, United States
Synopsis
Correct calibration of the transmit
field B1+ is crucial to achieve optimal SNR. However,
fast and robust B1+ calibration is difficult for X-nuclei
due to the low signal sensitivity. In this work, we proposed a fast B1+
calibration method based on the Bloch-Siegert shift and single voxel ISIS localization.
With the proposed sequence, the B1+ calibration can be
done in less than 5 min for the human brain at B0 = 9.4 T.
Purpose
Fast
in vivo flip angle calibration for 31P Magnetic Resonance
Spectroscopy in the human brain using Bloch-Siegert pulses with ISIS
localization.Introduction
Flip angle calibration is critical to
achieve optimal signal-to-noise (SNR) in magnetic resonance spectroscopy.
Nevertheless, it is difficult to reliable measure the transmit field
strength B1+ for X-nuclei in vivo due to the low
sensitivity of the X-nuclei signal. In
this work, we present a fast volume selective method to calibrate B1+
and thus flip angles for 31P Magnetic Resonance Spectroscopy (MRS)
in the human brain applying a single voxel MRS method called ISIS
(Image-Selected in vivo Spectroscopy)1 combined with off-resonant
Bloch-Siegert pulses. The Bloch-Siegert shift2 was already used for 31P
flip angle calibration at the human heart in combination with a Chemical shift
imaging (CSI) sequence3. The advantage of this sequence is the
simultaneous acquisition of B1+ values from different
locations. However, the needed measurement time was 25 min. The flip angle
calibration with the proposed sequence can be done in less then 5 min. For 1H
MRS, calibration methods based on single voxel STEAM4 and PRESS5
were presented earlier. However, because of the low T2 values6,7,
echo-based methods are not optimal for 31P spectroscopy.
The sequence is shown in Fig. 1. The
B1+ calculation is based on the phase difference of the
PCr resonance introduced by the Bloch-Siegert shift by applying a Fermi pulse with two different frequency offsets3:
$$\gamma B_1 = \sqrt{\frac{(\Phi_2 - \Phi_1) \omega_{RF}}{2 \pi A}}$$
A is the normalized pulse-envelope
squared-integral of the Bloch-Siegert pulse, ωRF
the
frequency offset and φ1,2
the measured phases from the positive and negative frequency offset.Method
The
data was acquired on a 9.4 T whole body MRI scanner (Siemens Healthineers,
Erlangen, Germany) with an in-house built double tuned 31P/1H
array coil (8TxRx/2Rx for 31P, 10TxRx for 1H)8.
All volunteer measurements were done after informed consent.
First, the sequence was tested in a
phantom measurement by comparing to an established phase sensitive spiral B1+
mapping method9. This method is not suitable for
in vivo B1+ calibration
because of the extended measurement time needed resulting from the low sensitivity of 31P.
The values of the reference B1+ mapping method where
averaged over the area of interest covered by the ISIS BS sequence. The
following parameters were used for the proposed sequence: voxel size (8 cm)3,
TR = 5 s, TE = 0.3 ms, 64 averages, rectangular excitation pulse with TP = 0.35 ms, vector
size = 1024, pulse duration GOIA = 5 ms, voltage BS pulse = 250 V,
duration BS pulse = 5 ms, frequency offset = ±2500 Hz. The measurement of the ISIS BS was repeated ten
times to estimate the variance of the measurement and probe the stability in a phantom.
In a second step, the ISIS BS
sequence was tested in vivo in the human brain. First, the reproducibility of
the B1+ was probed by repeating the measurement for one
volunteer ten times. Second, the B1+ measurement was
repeated for seven different volunteers. The parameters were chosen as in the
phantom measurement except of a reduction of the number of averages to 16 which
reduced the total measurement time to 2.6 min (2x80 sec).
The data evaluation was performed in
Matlab by using a self-implemented fitting routine based on the AMARES
algorithm10.Results and Discussion
Fig.
2 shows the results of the phase sensitive B1+ sequence.
The rectangular area represents the area that was selected with the proposed
ISIS BS single voxel B1+ calibration method. For the
single voxel sequence, an average B1+ value of (106.43 ± 0.16) nT/V was measured. The corresponding
averaged value for the reference method was 101 nT/V. The values only deviate
slightly from each other. A reason for that could be that unlike the reference methode the Bloch Siegert
sequence measures a weighted average over the area of interest due to different
sensitivities of the coil at different positions.
Fig. 3 shows the positioning of the
voxel in an in vivo measurement and the corresponding spectra for both
frequency offsets with the fitted PCr resonance for one volunteer to show
spectral quality. No post-processing was applied and
the relative phase difference was derived from the two fitted spetra. The reproducibility
measurement for ten measurements on a single volunteer resulted in an average
measured B1+ of 72.3 nT/V with a standard deviation of 5.3 nT/V.
Tab. 1 shows the values for different
volunteers. The given standard deviation was calculated from the CRLBs calculated from the
fit of the spectra without any post-processing. As known from the reproducibility
measurement, this error probably does not reflect the full variation in the
measured B1+ values. The higher variance measured in the reproducibility
measurement can arise from different reasons as volunteer motion. Also the CRLBs
only give a lower estimate of the standard deviation of the fit.Conclusion
We
successfully implemented a single voxel B1+ calibration
sequence that can be used for in vivo flip angle calibration for X-nuclei MRS
and MRI in less than 5 min. We verified the sequence in phantom measurements
and probed a high reproducibility by in vivo experiments in the human brain at
9.4 T for 31P.Acknowledgements
Funding by the European
Union (ERC Starting Grant, SYNAPLAST MR, Grant Number: 679927) and the Cancer Prevention and Research Institute of Texas (CPRIT, Grant Number: RR180056) is gratefully
acknowledged. Special thanks to Prof. Wolfgang Bogner for his help regarding the ISIS sequence.References
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