Purpose
The aim of this study was to apply compressed sensing to accelerate the acquisition of high resolution metabolite maps of the human brain using a nonlipid suppressed ultra‐short TR and TE 1H ...FID MRSI sequence at 9.4T.
Methods
X‐t sparse compressed sensing reconstruction was optimized for nonlipid suppressed 1H FID MRSI data. Coil‐by‐coil x‐t sparse reconstruction was compared with SENSE x‐t sparse and low rank reconstruction. The effect of matrix size and spatial resolution on the achievable acceleration factor was studied. Finally, in vivo metabolite maps with different acceleration factors of 2, 4, 5, and 10 were acquired and compared.
Results
Coil‐by‐coil x‐t sparse compressed sensing reconstruction was not able to reliably recover the nonlipid suppressed data, rather a combination of parallel and sparse reconstruction was necessary (SENSE x‐t sparse). For acceleration factors of up to 5, both the low‐rank and the compressed sensing methods were able to reconstruct the data comparably well (root mean squared errors RMSEs ≤ 10.5% for Cre). However, the reconstruction time of the low rank algorithm was drastically longer than compressed sensing. Using the optimized compressed sensing reconstruction, acceleration factors of 4 or 5 could be reached for the MRSI data with a matrix size of 64 × 64. For lower spatial resolutions, an acceleration factor of up to R∼4 was successfully achieved.
Conclusion
By tailoring the reconstruction scheme to the nonlipid suppressed data through parameter optimization and performance evaluation, we present high resolution (97 µL voxel size) accelerated in vivo metabolite maps of the human brain acquired at 9.4T within scan times of 3 to 3.75 min.
Deuterium Magnetic Resonance Spectroscopy (DMRS) is a non-invasive technique that allows the detection of deuterated compounds in vivo. DMRS has a large potential to analyze uptake, perfusion, ...washout or metabolism, since deuterium is a stable isotope and therefore does not decay during biologic processing of a deuterium labelled substance. Moreover, DMRS allows the distinction between different deuterated substances. In this work, we performed DMRS of deuterated 3-O-Methylglucose (OMG). OMG is a non-metabolizable glucose analog which is transported similar to D-glucose. DMRS of OMG was performed in phantom and in vivo measurements using a preclinical 7 Tesla MRI system. The chemical shift (3.51 ± 0.1 ppm) and relaxation times were determined. OMG was injected intravenously and spectra were acquired over a period of one hour to monitor the time evolution of the deuterium signal in tumor-bearing rats. The increase and washout of OMG could be observed. Three different exponential functions were compared in terms of how well they describe the OMG washout. A mono-exponential model with offset seems to describe the observed time course best with a time constant of 1910 ± 770 s and an offset of 2.5 ± 1.2 mmol/l (mean ± std, N = 3). Chemical shift imaging could be performed with a voxel size of 7.1 mm x 7.1 mm x 7.9 mm. The feasibility of DMRS with deuterium labelled OMG could be demonstrated. These data might serve as basis for future studies that aim to characterize glucose transport using DMRS.
For the first time, labeling effects after oral intake of 1-13Cglucose are observed in the human brain with pure 1H detection at 9.4 T. Spectral time series were acquired using a short-TE 1H MRS ...MC-semiLASER (Metabolite Cycling semi Localization by Adiabatic SElective Refocusing) sequence in two voxels of 5.4 mL in the frontal cortex and the occipital lobe. High-quality time-courses of 4-13Cglutamate, 4-13Cglutamine, 3-13Cglutamate + glutamine, 2-13C glutamate+glutamine and 3-13Caspartate for individual volunteers and additionally, group-averaged time-courses of labeled and non-labeled brain glucose could be obtained. Using a one-compartment model, mean metabolic rates were calculated for each voxel position: The mean rate of the TCA-cycle (Vtca) value was determined to be 1.36 and 0.93 μmol min−1 g−1, the mean rate of glutamine synthesis (Vgln) was calculated to be 0.23 and 0.45 μmol min−1 g−1, the mean exchange rate between cytosolic amino acids and mitochondrial Krebs cycle intermediates (Vx) rate was found to be 0.57 and 1.21 μmol min−1 g−1 for the occipital lobe and the frontal cortex, respectively. These values were in agreement with previously reported data. Altogether, it can be shown that this most simple technique combining oral administration of 1-13CGlc with pure 1H MRS acquisition is suitable to measure metabolic rates.
Purpose
To present 31P whole brain MRSI with a high spatial resolution to probe quantitative tissue analysis of 31P MRSI at an ultrahigh field strength of 9.4 Tesla.
Methods
The study protocol ...included a 31P MRSI measurement with an effective resolution of 2.47 mL. For SNR optimization, the nuclear Overhauser enhancement at 9.4 Tesla was investigated. A sensitivity correction was achieved by applying a low rank approximation of the γ‐adenosine triphosphate signal. Group analysis and regression on individual volunteers were performed to investigate quantitative concentration differences between different tissue types.
Results
Differences in gray and white matter tissue 31P concentrations could be investigated for 12 different 31P resonances. In addition, the first highly resolved quantitative MRSI images measured at B0 = 9.4 Tesla of 31P detectable metabolites with high SNR could be presented.
Conclusion
With an ultrahigh field strength B0 = 9.4 Tesla, 31P MRSI moves further toward quantitative metabolic imaging, and subtle differences in concentrations between different tissue types can be detected.
Purpose
A 16‐channel multi‐coil shimming setup was developed to mitigate severe B0 field perturbations at ultrahigh field and improve data quality for human brain imaging and spectroscopy.
Methods
...The shimming setup consisted of 16 circular B0 coils that were positioned symmetrically on a cylinder with a diameter of 370 mm. The latter was large enough to house a shielded 18/32‐channel RF transceiver array. The shim performance was assessed via simulations and phantom as well as in vivo measurements at 9.4 T. The global and dynamic shimming performance of the multi‐coil setup was compared with the built‐in scanner shim system for EPI and single voxel spectroscopy.
Results
The presence of the multi‐coil shim did not influence the performance of the RF coil. The performance of the proposed setup was similar to a full third‐order spherical harmonic shim system in the case of global static and dynamic slice‐wise shimming. Dynamic slice‐wise shimming with the multi‐coil setup outperformed global static shimming with the scanner's second‐order spherical‐harmonic shim. The multi‐coil setup allowed mitigating geometric distortions for EPI. The combination of the multi‐coil shim setup with the zeroth and first‐order shim of the scanner further reduced the standard deviation of the B0 field in the brain by 12% compared with the case in which multi‐coil was used exclusively.
Conclusion
The combination of a multi‐coil setup and the linear shim channels of the scanner provides a straightforward solution for implementing dynamic slice‐wise shimming without requiring an additional pre‐emphasis setup.
Purpose
To present
31
P whole brain MRSI with a high spatial resolution to probe quantitative tissue analysis of
31
P MRSI at an ultrahigh field strength of 9.4 Tesla.
Methods
The study protocol ...included a
31
P MRSI measurement with an effective resolution of 2.47 mL. For SNR optimization, the nuclear Overhauser enhancement at 9.4 Tesla was investigated. A sensitivity correction was achieved by applying a low rank approximation of the γ‐adenosine triphosphate signal. Group analysis and regression on individual volunteers were performed to investigate quantitative concentration differences between different tissue types.
Results
Differences in gray and white matter tissue
31
P concentrations could be investigated for 12 different
31
P resonances. In addition, the first highly resolved quantitative MRSI images measured at B
0
= 9.4 Tesla of
31
P detectable metabolites with high SNR could be presented.
Conclusion
With an ultrahigh field strength B
0
= 9.4 Tesla,
31
P MRSI moves further toward quantitative metabolic imaging, and subtle differences in concentrations between different tissue types can be detected.
The aim of this study was to apply compressed sensing to accelerate the acquisition of high resolution metabolite maps of the human brain using a nonlipid suppressed ultra-short TR and TE
H FID MRSI ...sequence at 9.4T.
X-t sparse compressed sensing reconstruction was optimized for nonlipid suppressed
H FID MRSI data. Coil-by-coil x-t sparse reconstruction was compared with SENSE x-t sparse and low rank reconstruction. The effect of matrix size and spatial resolution on the achievable acceleration factor was studied. Finally, in vivo metabolite maps with different acceleration factors of 2, 4, 5, and 10 were acquired and compared.
Coil-by-coil x-t sparse compressed sensing reconstruction was not able to reliably recover the nonlipid suppressed data, rather a combination of parallel and sparse reconstruction was necessary (SENSE x-t sparse). For acceleration factors of up to 5, both the low-rank and the compressed sensing methods were able to reconstruct the data comparably well (root mean squared errors RMSEs ≤ 10.5% for Cre). However, the reconstruction time of the low rank algorithm was drastically longer than compressed sensing. Using the optimized compressed sensing reconstruction, acceleration factors of 4 or 5 could be reached for the MRSI data with a matrix size of 64 × 64. For lower spatial resolutions, an acceleration factor of up to R∼4 was successfully achieved.
By tailoring the reconstruction scheme to the nonlipid suppressed data through parameter optimization and performance evaluation, we present high resolution (97 µL voxel size) accelerated in vivo metabolite maps of the human brain acquired at 9.4T within scan times of 3 to 3.75 min.
Non‐water‐suppressed 1H FID‐MRSI at 3T and 9.4T Chang, Paul; Nassirpour, Sahar; Avdievitch, Nikolai ...
Magnetic resonance in medicine,
August 2018, 20180801, Volume:
80, Issue:
2
Journal Article
A 16-channel multi-coil shimming setup was developed to mitigate severe B
field perturbations at ultrahigh field and improve data quality for human brain imaging and spectroscopy.
The shimming setup ...consisted of 16 circular B
coils that were positioned symmetrically on a cylinder with a diameter of 370 mm. The latter was large enough to house a shielded 18/32-channel RF transceiver array. The shim performance was assessed via simulations and phantom as well as in vivo measurements at 9.4 T. The global and dynamic shimming performance of the multi-coil setup was compared with the built-in scanner shim system for EPI and single voxel spectroscopy.
The presence of the multi-coil shim did not influence the performance of the RF coil. The performance of the proposed setup was similar to a full third-order spherical harmonic shim system in the case of global static and dynamic slice-wise shimming. Dynamic slice-wise shimming with the multi-coil setup outperformed global static shimming with the scanner's second-order spherical-harmonic shim. The multi-coil setup allowed mitigating geometric distortions for EPI. The combination of the multi-coil shim setup with the zeroth and first-order shim of the scanner further reduced the standard deviation of the B
field in the brain by 12% compared with the case in which multi-coil was used exclusively.
The combination of a multi-coil setup and the linear shim channels of the scanner provides a straightforward solution for implementing dynamic slice-wise shimming without requiring an additional pre-emphasis setup.
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