Abstract
Water‐suppressed MRS acquisition techniques have been the standard MRS approach used in research and for clinical scanning to date. The acquisition of a non‐water‐suppressed MRS spectrum is used for artefact correction, reconstruction of phased‐array coil data and metabolite quantification. Here, a two‐scan metabolite‐cycling magnetic resonance spectroscopic imaging (MRSI) scheme that does not use water suppression is demonstrated and evaluated. Specifically, the feasibility of acquiring and quantifying short‐echo (T E = 14 ms), two‐dimensional stimulated echo acquisition mode (STEAM) MRSI spectra in the motor cortex is demonstrated on a 3 T MRI system. The increase in measurement time from the metabolite‐cycling is counterbalanced by a time‐efficient concentric ring k‐space trajectory. To validate the technique, water‐suppressed MRSI acquisitions were also performed for comparison. The proposed non‐water‐suppressed metabolite‐cycling MRSI technique was tested for detection and correction of resonance frequency drifts due to subject motion and/or hardware instability, and the feasibility of high‐resolution metabolic mapping over a whole brain slice was assessed. Our results show that the metabolite spectra and estimated concentrations are in agreement between non‐water‐suppressed and water‐suppressed techniques. The achieved spectral quality, signal‐to‐noise ratio (SNR) > 20 and linewidth <7 Hz allowed reliable metabolic mapping of five major brain metabolites in the motor cortex with an in‐plane resolution of 10 × 10 mm2 in 8 min and with a Cramér‐Rao lower bound of less than 20% using LCModel analysis. In addition, the high SNR of the water peak of the non‐water‐suppressed technique enabled voxel‐wise single‐scan frequency, phase and eddy current correction. These findings demonstrate that our non‐water‐suppressed metabolite‐cycling MRSI technique can perform robustly on 3 T MRI systems and within a clinically feasible acquisition time.
Highlights
Compared with single voxel (SV)‐MRS, the use of magnetic resonance spectroscopic imaging (MRSI) has been limited by several challenges, such as inhomogeneity of the main (B0) and RF magnetic fields, long acquisition times, insufficient water suppression, eddy‐current‐induced gradient errors, and line broadening artefacts caused by subject motion and scanner instability.[1]
In order to demonstrate the potential advantages of the non‐ water‐suppressed acquisition scheme for voxel‐wise single‐scan frequency alignment, as well as phase and eddy current correction, we conducted in vivo non‐water‐suppressed and water‐suppressed MRSI measurements from a specific region of interest in the motor cortex with a high in‐plane resolution of 5 mm × 5 mm × 20 mm (Nrings = 24, Np_ring = 64, field of view (FOV) = 240 mm × 240 mm, stimulated echo acquisition mode (STEAM) localization =80 mm × 100 mm × 20 mm, TR = 1.5 s, TE = 14 ms, TM = 32 ms, Navg = 20, ADC bandwidth =80 kHz, maximum slew rate = 168.2 mT/m/ms, Nring × Navg × TR = 720 s) in a healthy subject
This study demonstrates that short‐TE two dimensional (2D) MRSI data can be obtained using both non‐water‐suppressed metabolite‐cycling and water‐suppressed techniques in the same measurement time as SV‐MRS
Summary
In order to separate metabolite signals from abundant water signal robustly, in vivo MRS methods require techniques for suppression of the water signal during acquisition[8,9,10,11] and/or post‐processing.[12] The vendor‐provided MRS packages on clinical scanners offer water‐suppressed spectroscopic acquisition techniques as the standard approach.[13] the acquisition of a non‐water‐suppressed MRS spectrum is generally required to act as an internal reference for metabolite quantification This information can be required for optimal reconstruction of spectra from different phased‐array coils, the correction of gradient‐induced sideband modulations, eddy‐current‐induced artefacts and tracking B0 drifts due to subject motion or scanner drift.
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