GABA spectroscopy edited GABA 1H MEGA-PRESS spectra GABA-edited In this study, we have developed and demonstrated a non-water suppressed GABA editing Magnetic Resonance Spectroscopic Imaging technique using density-weighted concentric rings k-space trajectory that performs robustly within a clinically feasible acquisition time at 3T. The method has been validated in a series of phantom experiments and its feasibility assessed in a healthy volunteer with a high in-plane resolution of 7.5 × 7.5 mm. Experiments qualitatively demonstrate the advantage of the proposed method in terms of its improved resolution and reduced contamination of spectra from neighboring voxels.

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Non-water Suppressed GABA Edited Magnetic Resonance Spectroscopic Imaging
using Density Weighted Concentric Rings k-space Trajectory
Conference Paper · June 2018
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Non-water Suppressed GABA Edited Magnetic Resonance Spectroscopic Imaging using
Density Weighted Concentric Rings k-space Trajectory
Uzay E Emir , Pingyu Xia , Xiaopeng Zhou , Mark Chiew , Adam Steel , M Albert Thomas , and Ulrike Dydak
School of Health Sciences, Purdue University, West Lafayette, IN, United States, Wellcome Centre for Integrative Neuroimaging, FMRIB Division, Nuffield
Department of Clinical Neurosciences, University of Oxford, Oxford, United Kingdom, Nuffield Department of Medicine, University of Oxford, Oxford, United
Kingdom, National Institute of Mental Health, National Institutes of Health, Bethesda, MD, United States, Department of Radiological Sciences, University of
California, Los Angeles, CA, United States, Department of Radiology and Imaging Sciences, Indiana University School of Medicine, Indianapolis, IN, United
States
Synopsis
In this study, we have developed and demonstrated a non-water suppressed GABA editing Magnetic Resonance Spectroscopic Imaging technique
using density-weighted concentric rings k-space trajectory that performs robustly within a clinically feasible acquisition time at 3T. The method has
been validated in a series of phantom experiments and its feasibility assessed in a healthy volunteer with a high in-plane resolution of 7.5 × 7.5 mm .
Experiments qualitatively demonstrate the advantage of the proposed method in terms of its improved resolution and reduced contamination of
spectra from neighboring voxels.
Introduction
Water-suppressed MRS acquisition techniques have been the standard approach used in research and clinical MRI scanners to date. The acquisition of a non-
water-suppressed spectrum is used for artifact correction, reconstruction of phased-array coil data, and metabolite quantification. A non-water suppressed
metabolite-cycling (1) MRSI acquisition technique has recently been proposed for simultaneous detection of the metabolite and water signals. Potential
improvements of voxel-wise frequency, phase, and eddy current correction have been demonstrated (1). In this study, we propose using a MEGA semi-LASER
sequence with non-water suppressed metabolite-cycling as 2D MRSI data acquisition method at 3T for high-resolution GABA detection throughout a whole
axial slice. To improve the point-spread function of the 2D-MRSI acquisition without any loss of SNR and time efficiency, a density-weighted concentric rings k-
space trajectory (DW-CRT) sampling approach is used for the k-space acquisition trajectory of MRSI (2).
Methods
All scans were acquired using a Siemens Prisma 3-Tesla (Siemens, Erlangen, Germany) whole-body MRI scanner and a 20-channel head array receive coil. B0
shimming was achieved using GRESHIM (3). The non-water-suppressed metabolite-cycling MRSI acquisition was achieved by utilizing two asymmetric narrow-
transition-band adiabatic RF pulses with mirrored inversion profiles applied in alternate scans for the inversion of the upfield and downfield (relative to water)
spectral ranges (4) before the semi-LASER localization with a gap of 9.6 ms. The semi-LASER (5) localization (TR = 1.5 s, TE = 68 ms), was used to excite
whole brain slice with a 125 mm x 105 mm x 20 mm region centrally within the field of view (FOV= 240 mm x 240 mm). The Gaussian editing pulses of 65 Hz
width were placed on the H3’ protons at 1.9 ppm in the “ON” acquisition, and at 9.0 ppm in the “OFF” acquisition. For DW-CRT, 64 points per ring were
collected with an ADC bandwidth of 80 kHz, resulting in 512 temporal samples in an effective spectral bandwidth of 1250 Hz (Figure 1). To cover the 32x32
grid, 16 rings resulting in an individual voxel size of 1.125 mL (7.5 mm x 7.5 mm x 20 mm) were acquired for DW-CRT. The Hanning filter weighting function with
an alpha value of 1 was used to compute radial sampling locations for the DW-CRT (2). For DW-CRT, four spatial interleaves (64 unique rings) were used to
ensure that minimum field of view requirement was met, resulting in an acquisition duration of 384 s with metabolite cycling and mega editing. The number of
averages was 2, corresponding to a total acquisition duration of 12.8 minutes. The feasibility of the whole brain slice with the proposed methods was tested on
a healthy volunteer (male; 31 years old) after giving informed consent under an institutionally approved technical development protocol. All reconstructions
were implemented in MATLAB using the non-uniform FFT (NUFFT) toolbox (6). Lipid contamination was removed during post‐processing using a lipid‐basis
penalty algorithm (7).
Results
Figure 2 shows the non-water suppressed image of a whole brain slice as well as in-vivo edited spectra from the same slice of a healthy volunteer using
metabolite cycling. After subtraction of the even MR spectra from the odd MR spectra, the edited in vivo MR spectra from 1.125 mL voxels showed well-
defined GABA peaks at 3.0 ppm both in a central brain region (Figure 2) and a cortical brain region close to the lipid layer (Figure 3).
Conclusion
We have developed and demonstrated a non-water suppressed MEGA semi-LASER MRSI sequence with DW-CRT that performs robustly at 3 Tesla, within a
clinically feasible acquisition time of 13 min. The feasibility of the detection of GABA with the proposed method was shown in a healthy volunteer with a high in-
plane resolution of 7.5 × 7.5 mm2. Experiments qualitatively demonstrate the advantage of DW-CRT in terms of its improved resolution and reduced
contamination of spectra from neighboring voxels. Future work will focus on implementing the method for whole brain coverage with a reduced TR.
Acknowledgements
This work was supported by NIH R01 ES020529
References
1. Emir UE, Burns B, Chiew M, Jezzard P, Thomas MA. Non-water-suppressed short-echo-time magnetic resonance spectroscopic imaging using a concentric
ring k-space trajectory. NMR Biomed 2017;30(7).
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2. Chiew M, Jiang W, Burns B, Larson P, Steel A, Jezzard P, Albert Thomas M, Emir UE. Density-weighted concentric rings k-space trajectory for 1H magnetic
resonance spectroscopic imaging at 7 T. NMR in Biomedicine:e3838-n/a.
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International Society for Magnetic Resonance in Medicine. Honolulu 2009. p 565.
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2):95-101.
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Figures
Figure 1 Pulse sequence diagram of the proposed MEGA semi-LASER method without water suppression, allowing simultaneous detection of GABA and water
signals. Prior to a MEGA semi-LASER editing sequence, two asymmetric narrow-transition-band adiabatic RF pulses invert the spectral range where metabolite
signals are expected either upfield or downfield with respect to water.
Figure 2 Edit “OFF”, EDIT “ON” and difference spectra from 3 x 3 voxels in the center of the whole brain slice semi-LASER localization acquired using non-
water suppressed metabolite cycling with MEGA editing for GABA detection.
Edit “OFF”, EDIT “ON” and difference spectra from 3 x 3 voxels close to the lipid layer of the whole brain slice semi-LASER localization acquired using non-
water suppressed metabolite cycling with MEGA editing for GABA detection.
Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)
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