Faculty Bibliography

PURPOSE/OBJECTIVE:-weighted images from a single scan and allows for free-breathing acquisition. THEORY AND METHODS/UNASSIGNED:-weighted gradient-echo (GRE) data. Improved robustness is achieved by extracting a respiratory signal from the GRE data and using it for motion-weighted reconstruction. RESULTS:-weighted Dixon acquisition is possible. CONCLUSION/CONCLUSIONS:-weighted imaging in a single scan. In addition to free-breathing abdominal examination, it promises value for clinical applications that are frequently affected by motion artifacts.

PURPOSE/OBJECTIVE:profiles in the longitudinal (z) direction and allow for next-nearest neighbor decoupling. METHODS:Two cylindrical 8-channel arrays having the same length and diameter, 1 of triangular coils and the other of rectangular coils, were constructed and compared in phantom imaging experiments using measures of excitation distribution for a variety of RF shim settings and geometry factor maps for different accelerations on different planes. RESULTS:Coupling between elements was -20 dB or better for all triangular coil pairs, but worse than -12 dB for several of the rectangular coil pairs. Both coils could produce adequate shims on a central transverse plane, but the same shim produced worse results off center for the triangular coil array than for the rectangular coil array. Compared to the rectangular coil array, the maximum geometry factor for the triangular coil array was reduced by a factor of 13.1 when using a 2-fold acceleration in the z-direction. CONCLUSION/CONCLUSIONS:profiles along the z-direction, although this also means that individual slices must be shimmed separately. This design is well suited for parallel transmit applications while also having high receive sensitivity.

INTRODUCTION/BACKGROUND:-weighted DCE-MRI. MATERIALS AND METHODS/METHODS:) and arterial input function (AIF). In addition, the effect of the conversion method on the diagnostic accuracy was evaluated with 36 breast lesions (19 benign and 17 malignant). RESULTS:. CONCLUSION/CONCLUSIONS:measurement is not available and a low FA is used for DCE-MRI, the uncertainty in the contrast kinetic parameter estimation can be reduced by using the LC method with pAIF, without compromising the diagnostic accuracy.

Owing to a technical error, this Article was originally published with an incorrect published online date of '4 May 2018'; it should have been '7 May 2018'. This has now been corrected.

PURPOSE/OBJECTIVE:Introduce a novel coil design using an electrically long transversely oriented slot in a conductive sheet. THEORY AND METHODS/UNASSIGNED:Theoretical considerations, numerical simulations, and experimental measurements are presented for transverse slot antennas as compared with electric dipole antennas. RESULTS:Simulations show improved central and average transmit and receive efficiency, as well as larger coverage in the transverse plane, for a single slot as compared to a single dipole element. Experiments on a body phantom confirm the simulation results for a slot antenna relative to a dipole, demonstrating a large region of relatively high sensitivity and homogeneity. Images in a human subject also show a large imaging volume for a single slot and six slot antenna array. High central transmit efficiency was observed for slot arrays relative to dipole arrays. CONCLUSION/CONCLUSIONS:Transverse slots can exhibit improved sensitivity and larger field of view compared with traditional conductive dipoles. Simulations and experiments indicate high potential for slot antennas in high field MRI. Magn Reson Med, 2018. © 2018 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

BACKGROUND:There is growing interest in detecting cerebro-cerebellar circuits, which requires adequate blood oxygenation level dependent contrast and signal-to-noise ratio (SNR) throughout the brain. Although 7T scanners offer increased SNR, coverage of commercial head coils is currently limited to the cerebrum. PURPOSE/OBJECTIVE:To improve cerebellar functional MRI (fMRI) at 7T with high permittivity material (HPM) pads extending the sensitivity of a commercial coil. STUDY TYPE/METHODS:Simulations were used to determine HPM pad configuration and assess radiofrequency (RF) safety. In vivo experiments were performed to evaluate RF field distributions and SNR and assess improvements of cerebellar fMRI. SUBJECTS/METHODS:Eight healthy volunteers enrolled in a prospective motor fMRI study with and without HPM. FIELD STRENGTH/SEQUENCE/UNASSIGNED:Gradient echo (GRE) echo planar imaging for fMRI, turbo FLASH for flip angle mapping, GRE sequence for SNR maps, and T1 -weighted MPRAGE were acquired with and without HPM pads at 7T. ASSESSMENT/RESULTS:Field maps, SNR maps, and anatomical images were evaluated for coverage. Simulation results were used to assess SAR levels of the experiment. Activation data from fMRI experiments were compared with and without HPM pads. STATISTICAL TESTS: fMRI data were analyzed using FEAT FSL for each subject followed by group level analysis using paired t-test of acquisitions with and without HPM. RESULTS:Simulations showed 52% improvement in transmit efficiency in cerebellum with HPM and SAR levels well below recommended limits. Experiments showed 27% improvement in SNR in cerebellum and improvement in coverage on T1 -weighted images. fMRI showed greater cerebellar activation in individual subjects with the HPM pad present (Z > = 4), especially in inferior slices of cerebellum, with 59% average increase in number of activated voxels in the cerebellum. Group-level analysis showed improved functional activation (Z > = 2.3) in cerebellar regions with HPM pads without loss of measured activation elsewhere. DATA CONCLUSION/UNASSIGNED:HPM pads can improve cerebellar fMRI at 7T with a commonly-used head coil without compromising RF safety. LEVEL OF EVIDENCE/METHODS:2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2017.

Densely packed resonant structures used for magnetic resonance imaging (MRI), such as nuclear magnetic resonance phased-array detectors, suffer from resonant inductive coupling, which restricts coil design to fixed geometries, imposes performance limitations, and narrows the scope of MRI experiments to motionless subjects. Here, we report the design of high-impedance detectors, and the fabrication and performance of a wearable detector array for MRI of the hand, that cloak themselves from electrodynamic interactions with neighboring elements. We experimentally verified that the detectors do not suffer from signal-to-noise degradation mechanisms typically observed with the use of traditional low-impedance elements. The detectors are adaptive and can accommodate movement, providing access to the imaging of soft-tissue biomechanics with unprecedented flexibility. The design of the wearable detector glove exemplifies the potential of high-impedance detectors in enabling a wide range of applications that are not well suited to traditional coil designs.

PURPOSE: The aim of this study was to evaluate the effect of integrated high-permittivity materials (HPMs) on excitation homogeneity and global specific absorption rate (SAR) for transmit arrays at 7T. METHODS: A rapid electrodynamic simulation framework was used to calculate L-curves associated with excitation of a uniform 2D profile in a dielectric sphere. We used ultimate intrinsic SAR as an absolute performance reference to compare different transmit arrays in the presence and absence of a layer of HPM. We investigated the optimal permittivity for the HPM as a function of its thickness, the sample size, and the number of array elements. RESULTS: Adding a layer of HPM can improve the performance of a 24-element array to match that of a 48-element array without HPM, whereas a 48-element array with HPM can perform as well as a 64-element array without HPM. Optimal relative permittivity values changed based on sample and coil geometry, but were always within a range obtainable with readily available materials (epsilonr = 100-200). CONCLUSION: Integration of HPMs could be a practical method to improve RF shimming performance, alternative to increasing the number of coils. The proposed simulation framework could be used to explore the design of novel transmit arrays for head imaging at ultra-high field strength. Magn Reson Med, 2017. (c) 2017 International Society for Magnetic Resonance in Medicine.

PURPOSE: To develop and evaluate a novel dynamic contrast-enhanced imaging technique called RACER-GRASP (Respiratory-weighted, Aortic Contrast Enhancement-guided and coil-unstReaking Golden-angle RAdial Sparse Parallel) MRI that extends GRASP to include automatic contrast bolus timing, respiratory motion compensation, and coil-weighted unstreaking for improved imaging performance in liver MRI. METHODS: In RACER-GRASP, aortic contrast enhancement (ACE) guided k-space sorting and respiratory-weighted sparse reconstruction are performed using aortic contrast enhancement and respiratory motion signals extracted directly from the acquired data. Coil unstreaking aims to weight multicoil k-space according to streaking artifact level calculated for each individual coil during image reconstruction, so that coil elements containing a high level of streaking artifacts contribute less to the final results. Self-calibrating GRAPPA operator gridding was applied as a pre-reconstruction step to reduce computational burden in the subsequent iterative reconstruction. The RACER-GRASP technique was compared with standard GRASP reconstruction in a group of healthy volunteers and patients referred for clinical liver MR examination. RESULTS: Compared with standard GRASP, RACER-GRASP significantly improved overall image quality (average score: 3.25 versus 3.85) and hepatic vessel sharpness/clarity (average score: 3.58 versus 4.0), and reduced residual streaking artifact level (average score: 3.23 versus 3.94) in different contrast phases. RACER-GRASP also enabled automatic timing of the arterial phases. CONCLUSIONS: The aortic contrast enhancement-guided sorting, respiratory motion suppression and coil unstreaking introduced by RACER-GRASP improve upon the imaging performance of standard GRASP for free-breathing dynamic contrast-enhanced MRI of the liver. Magn Reson Med, 2017. (c) 2017 International Society for Magnetic Resonance in Medicine.

OBJECTIVE: To use high-permittivity materials (HPM) positioned near radiofrequency (RF) surface coils to manipulate transmit/receive field patterns. MATERIALS AND METHODS: A large HPM pad was placed below the RF coil to extend the field of view (FOV). The resulting signal-to-noise ratio (SNR) was compared with that of other coil configurations covering the same FOV in simulations and experiments at 7 T. Transmit/receive efficiency was evaluated when HPM discs with or without a partial shield were positioned at a distance from the coil. Finally, we evaluated the increase in transmit homogeneity for a four-channel array with HPM discs interposed between adjacent coil elements. RESULTS: Various configurations of HPM increased SNR, transmit/receive efficiency, excitation/reception sensitivity overlap, and FOV when positioned near a surface coil. For a four-channel array driven in quadrature, shielded HPM discs enhanced the field below the discs as well as at the center of the sample as compared with other configurations with or without unshielded HPM discs. CONCLUSION: Strategically positioning HPM at a distance from a surface coil or array can increase the overlap between excitation/reception sensitivities, and extend the FOV of a single coil for reduction of the number of channels in an array while minimally affecting the SNR.