Purpose
The coax dipole antenna, a flexible antenna for body imaging at 7T is presented. Similar to the high impedance coil, this coaxial cable antenna is fed on the central conductor and through ...gaps in the shield, the current passes to the outside of the antenna to generate B1 field. This could achieve more favorable current distributions and better adaptation to the body curvature.
Methods
Finite difference time domain (FDTD) simulations are performed to optimize the positions of the gaps in the shield for a flat current profile. Lumped inductors are added to each end to reduce losses. The performance of a single antenna is compared to a fractionated dipole using B1 maps and MR thermometry. Finally, an array of eight coax dipoles is evaluated in simulations and used for in‐vivo scanning.
Results
An optimal configuration is found with gaps located at 10 cm from the center and inductor values of 28 nH. In comparison to the fractionated dipole antenna, in single antenna phantom measurements the coax dipole achieves similar B1 amplitude with 18% lower peak temperature. In simulations, the eight‐channel array of coax dipoles improved B1 homogeneity by 18%, along with small improvements in transmit efficiency and specific absorption rate (SAR). MRI measurements on three volunteers show more consistent performance for the coax dipoles.
Conclusion
The coax dipole is a novel antenna design with a flattened current distribution resulting in beneficial properties. Also, the flexible design of the coax dipoles allows better adaptation to the body curvature and can potentially be used for a wide range of imaging targets.
Objective
To demonstrate imaging performance for cardiac MR imaging at 7 T using a coil array of 8 transmit/receive dipole antennas and 16 receive loops.
Materials and methods
An 8-channel dipole ...array was extended by adding 16 receive-only loops. Average power constraints were determined by electromagnetic simulations. Cine imaging was performed on eight healthy subjects. Geometrical factor (g-factor) maps were calculated to assess acceleration performance. Signal-to-noise ratio (SNR)-scaled images were reconstructed for different combinations of receive channels, to demonstrate the SNR benefits of combining loops and dipoles.
Results
The overall image quality of the cardiac functional images was rated a 2.6 on a 4-point scale by two experienced radiologists. Imaging results at different acceleration factors demonstrate that acceleration factors up to 6 could be obtained while keeping the average g-factor below 1.27. SNR maps demonstrate that combining loops and dipoles provides a more than 50% enhancement of the SNR in the heart, compared to a situation where only loops or dipoles are used.
Conclusion
This work demonstrates the performance of a combined loop/dipole array for cardiac imaging at 7 T. With this array, acceleration factors of 6 are possible without increasing the average g-factor in the heart beyond 1.27. Combining loops and dipoles in receive mode enhances the SNR compared to receiving with loops or dipoles only.
Abstract
Loss-of-function mutations in glutaminase (GLS), the enzyme converting glutamine into glutamate, and the counteracting enzyme glutamine synthetase (GS) cause disturbed glutamate homeostasis ...and severe neonatal encephalopathy. We report a de novo Ser482Cys gain-of-function variant in GLS encoding GLS associated with profound developmental delay and infantile cataract. Functional analysis demonstrated that this variant causes hyperactivity and compensatory downregulation of GLS expression combined with upregulation of the counteracting enzyme GS, supporting pathogenicity. Ser482Cys-GLS likely improves the electrostatic environment of the GLS catalytic site, thereby intrinsically inducing hyperactivity. Alignment of +/−12.000 GLS protein sequences from >1000 genera revealed extreme conservation of Ser482 to the same degree as catalytic residues. Together with the hyperactivity, this indicates that Ser482 is evolutionarily preserved to achieve optimal-but submaximal-GLS activity. In line with GLS hyperactivity, increased glutamate and decreased glutamine concentrations were measured in urine and fibroblasts. In the brain (both grey and white matter), glutamate was also extremely high and glutamine was almost undetectable, demonstrated with magnetic resonance spectroscopic imaging at clinical field strength and subsequently supported at ultra-high field strength. Considering the neurotoxicity of glutamate when present in excess, the strikingly high glutamate concentrations measured in the brain provide an explanation for the developmental delay. Cataract, a known consequence of oxidative stress, was evoked in zebrafish expressing the hypermorphic Ser482Cys-GLS and could be alleviated by inhibition of GLS. The capacity to detoxify reactive oxygen species was reduced upon Ser482Cys-GLS expression, providing an explanation for cataract formation. In conclusion, we describe an inborn error of glutamate metabolism caused by a GLS hyperactivity variant, illustrating the importance of balanced GLS activity.
Proton MRS (1H MRS) provides noninvasive, quantitative metabolite profiles of tissue and has been shown to aid the clinical management of several brain diseases. Although most modern clinical MR ...scanners support MRS capabilities, routine use is largely restricted to specialized centers with good access to MR research support. Widespread adoption has been slow for several reasons, and technical challenges toward obtaining reliable good‐quality results have been identified as a contributing factor. Considerable progress has been made by the research community to address many of these challenges, and in this paper a consensus is presented on deficiencies in widely available MRS methodology and validated improvements that are currently in routine use at several clinical research institutions. In particular, the localization error for the PRESS localization sequence was found to be unacceptably high at 3 T, and use of the semi‐adiabatic localization by adiabatic selective refocusing sequence is a recommended solution. Incorporation of simulated metabolite basis sets into analysis routines is recommended for reliably capturing the full spectral detail available from short TE acquisitions. In addition, the importance of achieving a highly homogenous static magnetic field (B0) in the acquisition region is emphasized, and the limitations of current methods and hardware are discussed. Most recommendations require only software improvements, greatly enhancing the capabilities of clinical MRS on existing hardware. Implementation of these recommendations should strengthen current clinical applications and advance progress toward developing and validating new MRS biomarkers for clinical use.
The human cerebellum is involved in a wide array of functions, ranging from motor control to cognitive control, and as such is of great neuroscientific interest. However, its function is ...underexplored in vivo, due to its small size, its dense structure and its placement at the bottom of the brain, where transmit and receive fields are suboptimal. In this study, we combined two dense coil arrays of 16 small surface receive elements each with a transmit array of three antenna elements to improve BOLD sensitivity in the human cerebellum at 7 T. Our results showed improved B1+ and SNR close to the surface as well as g‐factor gains compared with a commercial coil designed for whole‐head imaging. This resulted in improved signal stability and large gains in the spatial extent of the activation close to the surface (<3.5 cm), while good performance was retained deeper in the cerebellum. Modulating the phase of the transmit elements of the head coil to constructively interfere in the cerebellum improved the B1+, resulting in a temporal SNR gain. Overall, our results show that a dedicated transmit array along with the SNR gains of surface coil arrays can improve cerebellar imaging, at the cost of a decreased field of view and increased signal inhomogeneity.
Human cerebellar fMRI is hindered by B1+ cancellation in ultra‐high fields and the need for high resolution. Using a dedicated 3Tx array improved the transmit field close to the cerebellar surface compared with a commercial 8Tx coil. The signal‐to‐noise and parallel imaging performance were further increased by the usage of a high‐density surface receive array. The combination of improved transmit and receive fields resulted in increased BOLD sensitivity close to the surface of the cerebellum.
Objective
Sodium concentration is responsible for (at least part of) the stiffness of articular cartilage due to the osmotic pressure it generates. Therefore, we hypothesized that we could use sodium ...MRI to approximate the stiffness of cartilage to assess early cartilage degeneration.
Methods
Four human tibial plateaus were retrieved from patients undergoing total knee replacement (TKR), and their cartilage stiffness mapped with indentation testing, after which samples were scanned in a 7 T MRI to determine sodium concentration. The relation of biomechanical parameters to MRI sodium and glycosaminoglycan (GAG) concentration was explored by a linear mixed model.
Results
Weak correlations of GAG concentration with apparent peak modulus (
p
= 0.0057) and apparent equilibrium modulus (
p
= 0.0181) were observed and lack of correlation of GAG concentration versus MRI sodium concentration was observed. MRI sodium concentration was not correlated with apparent peak modulus, though a moderate correlation of MRI sodium concentration with permeability was shown (
p
= 0.0014).
Discussion and conclusion
Although there was correlation between GAG concentration and cartilage stiffness, this was not similar with sodium concentration as measured by MRI. Thus, if the correlation between MRI sodium imaging and GAG concentration could be resolved, this strategy for assessing cartilage functional quality still holds promise.
Objectives
Acoustic noise in magnetic resonance imaging (MRI) negatively impacts patients. We assessed a silent gradient coil switched at 20 kHz combined with a T
1
-weighted magnetisation prepared ...rapid gradient-echo (MPRAGE) sequence at 7 T.
Methods
Five healthy subjects (21–29 years; three females) without previous 7-T MRI experience underwent both a quiet MPRAGE (Q-MPRAGE) and conventional MPRAGE (C-MPRAGE) sequence twice. Image quality was assessed quantitatively, and qualitatively by two neuroradiologists. Sound level was measured objectively and rated subjectively on a 0 to 10 scale by all subjects immediately following each sequence and after the whole examination (delayed). All subjects also reported comfort level, overall experience and willingness to undergo the sequence again.
Results
Compared to C-MPRAGE, Q-MPRAGE showed higher signal-to-noise ratio (10%;
p
= 0.012) and lower contrast-to-noise ratio (20%;
p
< 0.001) as well as acceptable to good image quality. Q-MPRAGE produced 27 dB lower sound level (76
versus
103 dB). Subjects reported lower sound level for Q-MPRAGE both immediate (4.4 ± 1.4
versus
6.4 ± 1.3;
p
= 0.007) and delayed (4.6 ± 1.4
versus
6.3 ± 1.3;
p
= 0.005), while they rated comfort level (7.4 ± 1.0
versus
6.1 ± 1.7;
p
= 0.016) and overall experience (7.6 ± 1.0
versus
6.0 ± 0.9;
p
= 0.005) higher. Willingness to undergo the sequence again was also higher, however not significantly (8.1 ± 1.0
versus
7.2 ± 1.3;
p
= 0.066).
Conclusion
Q-MPRAGE using a silent gradient coil reduced sound level by 27 dB compared to C-MPRAGE at 7 T while featuring acceptable-to-good image quality and a quieter and more pleasant subject experience.
Purpose
The design and performance of a novel head coil setup for 31P spectroscopy at ultra‐high field strengths (7T) is presented. The described system supports measurements at both the 1H and 31P ...resonance frequencies.
Methods
The novel coil consists of 2, actively detunable, coaxial birdcage coils to give homogeneous transmit, combined with a double resonant 30 channel receive array. This allows for anatomical imaging combined with 31P acquisitions over the whole head, without changing coils or disturbing the subject. A phosphate buffer phantom and 3 healthy volunteers were scanned with a pulse acquire CSI sequence using both the novel array coil and a conventional transceiver birdcage. Four different methods of combining the array channels were compared at 3 different levels of SNR.
Results
The novel coil setup delivers significantly increased 31P SNR in the peripheral regions of the brain, reaching up to factor 8, while maintaining comparable performance relative to the birdcage in the center.
Conclusions
The new system offers the potential to acquire whole brain 31P MRSI with superior signal relative to the standard options.
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