The increased signal‐to‐noise ratio (SNR) and chemical shift dispersion at high magnetic fields (≥7 T) have enabled neuro‐metabolic imaging at high spatial resolutions. To avoid very long acquisition times with conventional magnetic resonance spectroscopic imaging (MRSI) phase‐encoding schemes, solutions such as pulse‐acquire or free induction decay (FID) sequences with short repetition time and inner volume selection methods with acceleration (echo‐planar spectroscopic imaging [EPSI]), have been proposed. With the inner volume selection methods, limited spatial coverage of the brain and long echo times may still impede clinical implementation. FID‐MRSI sequences benefit from a short echo time and have a high SNR per time unit; however, contamination from strong extra‐cranial lipid signals remains a problem that can hinder correct metabolite quantification. L2‐regularization can be applied to remove lipid signals in cases with high spatial resolution and accurate prior knowledge. In this work, we developed an accelerated two‐dimensional (2D) FID‐MRSI sequence using an echo‐planar readout and investigated the performance of lipid suppression by L2‐regularization, an external crusher coil, and the combination of these two methods to compare the resulting spectral quality in three subjects. The reduction factor of lipid suppression using the crusher coil alone varies from 2 to 7 in the lipid region of the brain boundary. For the combination of the two methods, the average lipid area inside the brain was reduced by 2% to 38% compared with that of unsuppressed lipids, depending on the subject's region of interest. 2D FID‐EPSI with external lipid crushing and L2‐regularization provides high in‐plane coverage and is suitable for investigating brain metabolite distributions at high fields.
To demonstrate the feasibility of deuterium echo-planar spectroscopic imaging (EPSI) to accelerate 3D deuterium metabolic imaging in the human liver at 7 T. Methods: A deuterium EPSI sequence, featuring a Hamming-weighted k-space acquisition pattern for the phase-encoding directions, was implemented.Three-dimensional deuterium EPSI and conventional MRSI were performed on a water/acetone phantom and in vivo in the human liver at natural abundance. Moreover, in vivo deuterium EPSI measurements were acquired after oral administration of deuterated glucose. The effect of acquisition time on SNR was evaluated by retrospectively reducing the number of averages. Results:The SNR of natural abundance deuterated water signal in deuterium EPSI was 6.5% and 5.9% lower than that of MRSI in the phantom and in vivo experiments, respectively. In return, the acquisition time of in vivo EPSI data could be reduced retrospectively to 2 min, beyond the minimal acquisition time of conventional MRSI (of 20 min in this case), while still leaving sufficient SNR. Three-dimensional deuterium EPSI, after administration of deuterated glucose, enabled monitoring of hepatic glucose dynamics with full liver coverage, a spatial resolution of 20 mm isotropic, and a temporal resolution of 9 min 50 s, which could retrospectively be shortened to 2 min. Conclusion:In this work, we demonstrate the feasibility of accelerated 3D deuterium metabolic imaging of the human liver using deuterium EPSI. The acceleration obtained with EPSI can be used to increase temporal and/or spatial resolution, which will be valuable to study tissue metabolism of deuterated compounds over time.
Deuterium metabolic imaging (DMI) is an emerging technique to spatially map metabolism in vivo through the intake of deuterium (i.e., 2H or D) labeled substrates such as [6,6′-2H2]-glucose. Although DMI has the potential to become a powerful tool to assess liver metabolism, it has limitations due to its long scan time, and low signal-to-noise ratio (SNR) for high spatial resolution in the human body. In this work, we demonstrated the feasibility of low-rank and subspace modeling (LRSM) reconstruction to increase SNR by reducing spectral noise, allowing high spatiotemporal resolution for 3D DMI of the human liver at 7T.
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