Recent studies at high field (7Tesla) have reported small metabolite changes, in particular lactate and glutamate (below 0.3μmol/g) during visual stimulation. These studies have been limited to the ...visual cortex because of its high energy metabolism and good magnetic resonance spectroscopy (MRS) sensitivity using surface coil. The aim of this study was to extend functional MRS (fMRS) to investigate for the first time the metabolite changes during motor activation at 7T. Small but sustained increases in lactate (0.17μmol/g±0.05μmol/g, p<0.001) and glutamate (0.17μmol/g±0.09μmol/g, p<0.005) were detected during motor activation followed by a return to the baseline after the end of activation. The present study demonstrates that increases in lactate and glutamate during motor stimulation are small, but similar to those observed during visual stimulation. From the observed glutamate and lactate increase, we inferred that these metabolite changes may be a general manifestation of the increased neuronal activity. In addition, we propose that the measured metabolite concentration increases imply an increase in ΔCMRO2 that is transiently below that of ΔCMRGlc during the first 1 to 2min of the stimulation.
Purpose
To improve whole‐brain SNR at 7 Tesla, a novel 32‐element hybrid human head array coil was developed, constructed, and tested.
Methods
Our general design strategy is based on 2 major ideas: ...Firstly, following suggestions of previous works based on the ultimate intrinsic SNR theory, we combined loops and dipoles for improvement of SNR near the head center. Secondly, we minimized the total number of array elements by using a hybrid combination of transceive (TxRx) and receive (Rx) elements. The new hybrid array consisted of 8 folded‐end TxRx‐dipole antennas and 3 rows of 24 Rx‐loops all placed in a single layer on the surface of a tight‐fit helmet.
Results
The developed array significantly improved SNR in vivo both near the center (∼20%) and at the periphery (∼20% to 80%) in comparison to a common commercial array coil with 8 transmit (Tx) and 32 Rx‐elements. Whereas 24 loops alone delivered central SNR very similar to that of the commercial coil, the addition of complementary dipole structures provided further improvement. The new array also provided ∼15% higher Tx efficiency and better longitudinal coverage than that of the commercial array.
Conclusion
The developed array coil demonstrated advantages in combining complementary TxRx and Rx resonant structures, that is, TxRx‐dipoles and Rx‐loops all placed in a single layer at the same distance to the head. This strategy improved both SNR and Tx‐performance, as well as simplified the total head coil design, making it more robust.
Display omitted
•A metamaterial inspired wireless coil for targeted breast MRI at 1.5T is presented.•Wireless coil is based on electromagnetically coupled split-loop resonators.•Improvements of local ...transmit and SAR efficiencies of the body coil were achieved.•The phantom test showed a tenfold increase of SNR when compared to the body coil.
In this work, we propose an application of a metamaterial inspired volumetric wireless coil (WLC) based on coupled split-loop resonators for targeted breast MRI at 1.5 T. Due to strong electromagnetic coupling with the body coil, the metamaterial inspired WLC locally focuses radiofrequency (RF) magnetic flux in the target region, thus improving both transmit and receive performance of the external body coil. This leads to substantial enhancement in local transmit efficiency and improvement of RF safety. Phantom images showed a tenfold increase of signal-to-noise ratio (SNR) in the region-of-interest (ROI) and, at the same time, an almost 50-fold reduction in transmit power relative to the same body coil used alone.
Purpose
To develop a framework for 3D sodium (23Na) MR fingerprinting (MRF), based on irreducible spherical tensor operators with tailored flip angle (FA) pattern and time‐efficient data acquisition ...for simultaneous quantification of T1, T2l∗, T2s∗, and T2∗ in addition to ΔB0.
Methods
23Na‐MRF was implemented in a 3D sequence and irreducible spherical tensor operators were exploited in the simulations. Furthermore, the Cramér Rao lower bound was used to optimize the flip angle pattern. A combination of single and double echo readouts was implemented to increase the readout efficiency. A study was conducted to compare results in a multicompartment phantom acquired with MRF and reference methods. Finally, the relaxation times in the human brain were measured in four healthy volunteers.
Results
Phantom experiments revealed a mean difference of 1.0% between relaxation times acquired with MRF and results determined with the reference methods. Simultaneous quantification of the longitudinal and transverse relaxation times in the human brain was possible within 32 min using 3D 23Na‐MRF with a nominal resolution of (5 mm)3. In vivo measurements in four volunteers yielded average relaxation times of: T1,brain = (35.0 ± 3.2) ms, T2l,brain∗ = (29.3 ± 3.8) ms and T2s,brain∗ = (5.5 ± 1.3) ms in brain tissue, whereas T1,CSF = (61.9 ± 2.8) ms and T2,CSF∗ = (46.3 ± 4.5) ms was found in cerebrospinal fluid.
Conclusion
The feasibility of in vivo 3D relaxometric sodium mapping within roughly ½ h is demonstrated using MRF in the human brain, moving sodium relaxometric mapping toward clinically relevant measurement times.
SA2RAGE: A new sequence for fast B1+-mapping Eggenschwiler, Florent; Kober, Tobias; Magill, Arthur W. ...
Magnetic resonance in medicine,
June 2012, Letnik:
67, Številka:
6
Journal Article
The advancement of clinical applications of ultrahigh field (UHF) MRI depends heavily on advances in technology, including the development of new radiofrequency (RF) coil designs. Currently, the ...number of commercially available 7 T head RF coils is rather limited, implying a need to develop novel RF head coil designs that offer superior transmit and receive performance. RF coils to be used for clinical applications must be robust and reliable. In particular, for transmit arrays, if a transmit channel fails the local specific absorption rate may increase, significantly increasing local tissue heating. Recently, dipole antennas have been proposed and used to design UHF head transmit and receive arrays. The dipole provides a unique simplicity while offering comparable transmit efficiency and signal‐to‐noise ratio with the conventional loop design. Recently, we developed a novel array design in our laboratory using a folded‐end dipole antenna. In this work, we developed, constructed and evaluated an eight‐element transceiver bent folded‐end dipole array for human head imaging at 7 T. Driven in the quadrature circularly polarized mode, the array demonstrated more than 20% higher transmit efficiency and significantly better whole‐brain coverage than that provided by a widely used commercial array. In addition, we evaluated passive dipole antennas for decoupling the proposed array. We demonstrated that in contrast to the common unfolded dipole array, the passive dipoles moved away from the sample not only minimize coupling between the adjacent folded‐end active dipoles but also produce practically no destructive interference with the quadrature mode of the array.
An eight‐element transceiver bent folded‐end dipole array for human head imaging at 7 T was developed. Driven in the quadrature mode, the array demonstrated more than 20% higher transmit efficiency and significantly better whole‐brain coverage than that provided by a widely used commercial array. We demonstrated that in contrast to the common dipole array, the passive decoupling dipoles not only minimize coupling between the adjacent active dipoles but also produce minimal destructive interference with the quadrature mode of the array.
Purpose
To investigate the correlation between electrical conductivity and sodium concentration, both measured in vivo, in the human brain.
Methods
Conductivity measurements were performed on samples ...with different sodium (Na+) and agarose concentrations using a dielectric probe, and the correlation between conductivity and Na+ content was evaluated. Subsequently, brain conductivity and total Na+ content maps were measured in 8 healthy subjects using phase‐based MREPT and sodium MRI, respectively. After co‐registration and spatial normalization to the 1 mm 152 MNI brain atlas, the relationship between conductivity and tissue sodium concentration (TSC) was examined within different brain regions.
Results
The conductivities of agarose gels increased linearly with NaCl concentration, while remaining almost independent of agarose content. When measured in healthy subjects, conductivities showed positive correlation with total tissue sodium concentration (R = 0.39, P < 0.005). The same trend was found in gray matter (R = 0.36, P < 0.005) and in white matter (R = 0.28, P < 0.05).
Conclusion
Tissue conductivity shows a positive correlation with total sodium concentration. Conductivity might serve as a novel technique to visualize the total tissue electrolyte concentration, although refinements in the consideration of e.g., tissue water content, would be necessary to improve the quantitative value.
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