Eva Heckova1, Stephan Gruber1, Bernhard Strasser1, Michal Povazan1, Gilbert Hangel1, Siegfried Trattnig1,2, and Wolfgang Bogner1
1High Field MR Center, Department of Biomedical Imaging and Image-guided Therapy, Medical University of Vienna, Vienna, Austria, 2Christian Doppler Laboratory for Clinical Molecular MR Imaging, Vienna, Austria
Synopsis
Magnetic
resonance spectroscopic imaging (MRSI) allows to measure different
metabolites in the brain. SNR and spectral resolution increases at
higher magnetic fields. We compared FID-MRSI with ultra short
acquisition delay (1.5 ms) and a very high spatial resolution in the
same group of healthy subjects at 3T and 7T. We found 1.87-fold
increased SNR and decreased CRLBs at 7T in comparison with 3T. The
higher spectral resolution at 7T allows to distinguish between NAA
and NAAG and reliable detect other metabolites like Glx or Tau.
Accelerating the acquisition techniques leads to lower SNR, however
not to substantially decreased quantification precision.Purpose
Magnetic
resonance spectroscopic imaging (MRSI) of the brain allows to detect
several metabolites to achieve complementary information to be added
to the conventional MR imaging methods
1.
High field systems (7T)
offer
increased SNR
and spectral resolution with the prospect to increase the spatial
resolution of MRSI and to better quantify adjacent metabolites which
are overlapping at lower field strengths (e.g. NAA and NAAG). In
addition FID-MRSI will add additional SNR, in particular for J-coupled
resonances
2.
For this reason
we compared MRSI at 3 and 7 Tesla in 10 healthy subjects using a
sequence with ultra short acquisition delay (TE*) and high spatial
resolution.
Methods
Ten
healthy subjects (8m/2f; age: 30.6±5.52)
were measured at 3T and 7T scanners (3T Trio, 7T Magnetom; Siemens
Healthcare, Erlangen, Germany) using a 32-channel head coil. A
FID-MRSI sequence
2
with 64×64 phase encoding steps, 2048 spectral points, 6000 Hz
bandwidth, FOV=220×220×10 mm
2,
voxel size 3.4×3.4×10 mm
3,TR=600 ms,
TE*=1.5 ms resulting in an acquisition time (TAQ) of 30 min was used.
Moreover we obtained data with the same sequence and the same
parameters accelerated with (2+1)D Caipirinha
3
by a factor of 5 (TAQ=6 min). Spectra were processed using LCModel
based on a simulated basis set. Metabolic maps were created using a
fully automated script based on Matlab
4
and MINC (Minc tools; v2.0; McConnell Brain Imaging Center, Montreal,
Canada). This included also the removal of lipid contamination from
the spectra using L2-regularisation
5.
Furthermore the SNR was computed using the pseudo-replica method in
frequency domain.
Results
Good
data quality was achieved from all subjects measured at 3T and 7T.
Representative spectra of 3T and 7T are displayed in Figure 1. SNR was
1.87/1.74 times higher at 7T compared to that at 3T for
non-accelerated/accelerated data. CRLBs (Table 1) were below
11% / 5% at 3T / 7T for the main metabolites (tNAA, tCr, tCho, Ins). The
faster acquisition technique led to slightly increased CRLB values at
both field strength, however the estimation of metabolites was still
reliable. Furthermore NAAG, Glx, Glu and Tau could be quantified at
7T with CRLBs < 20 (except of Tau using R=5), which was not
possible at 3T (Table 2, Figure 3). The increased spectral resolution
at 7T allows to distinguish between NAA and NAAG (Figure 2).
Conclusion
FID-MRSI
allows to acquire whole slices without fat contamination from the
scalp, because of the high matrix size which was used together with
hamming filtering and lipid decontamination. With the high in-plane
resolution of 3.4×3.4
mm
2
metabolic
maps showing anatomical details could be constructed at both field
strengths. The increased SNR at 7T compared to that of 3T results in
satisfactory CRLBs of (j-coupled) metabolites such as Glx, Glu and
Tau and high-resolution metabolic maps. NAAG
could be separated from NAA at 7T but not at 3T. Therefore MRSI
improves substantially at 7T allowing to obtain data with high
spatial resolution and high quantification accuracy. MRSI at 7T will
be further improved using fast acquisition techniques and a larger
brain coverage (i.e. 3D-MRSI).
Acknowledgements
This
study was supported by the Austrian Science Fund (FWF): KLI-61 and
the FFG Bridge Early Stage Grant #846505.References
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et al., Radiology
2014; 270(3):658-79.
2. Bogner
et al., NMR in Biomed. 2012; 25(6):873-82.
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et al., Proc. Intl. Soc. MRM
2015; 23: 1973.
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et al., JMRI 2014; 40(1):181-91.