Double-row 18-loop transceive-32-loop receive tight-fit array provides for whole-brain coverage, high transmit performance, and SNR improvement near the brain center at 9.4T.

PURPOSE: To improve the transmit (Tx) and receive (Rx) performance of a human head array and provide whole-brain coverage at 9.4T, a novel 32-element array design was developed, constructed, and tested. METHODS: The array consists of 18 transceiver (TxRx) surface loops and 14 Rx-only vertical loops all placed in a single layer. The new design combines benefits of both TxRx and transmit-only-receive-only (ToRo) designs. The general idea of the design is that the total number of array elements (both TxRx and Rx) should not exceed the number of required Rx elements. First, the necessary number of TxRx loops is placed around the object tightly to optimize the Tx performance. The rest of the elements are loops, which are used only for reception. We also compared the performance of the new array with that of a state-of-the-art ToRo array consisting of 16 Tx-only loops and 31 Rx-only loops. RESULTS: The new array provides whole-brain coverage, ~1.5 times greater Tx efficiency and 1.3 times higher SNR near the brain center as compared to the ToRo array, while the latter delivers higher (up to 1.5 times) peripheral SNR. CONCLUSION: In general, the new approach of constructing a single-layer array consisting of both TxRx- and Rx-only elements simplifies the array construction by minimizing the total number of elements and makes the entire design more robust and, therefore, safe. Overall, our work provides a recipe for a Tx- and Rx-efficient head array coil suitable for parallel transmission and reception as well as whole-brain imaging at UHF.

Decoupling of a double-row 16-element tight-fit transceiver phased array for human whole-brain imaging at 9.4 T.

One of the major challenges in constructing multi-channel and multi-row transmit (Tx) or transceiver (TxRx) arrays is the decoupling of the array's loop elements. Overlapping of the surface loops allows the decoupling of adjacent elements and also helps to improve the radiofrequency field profile by increasing the penetration depth and eliminating voids between the loops. This also simplifies the design by reducing the number of decoupling circuits. At the same time, overlapping may compromise decoupling by generating high resistive (electric) coupling near the overlap, which cannot be compensated for by common decoupling techniques. Previously, based on analytical modeling, we demonstrated that electric coupling has strong frequency and loading dependence, and, at 9.4 T, both the magnetic and electric coupling between two heavily loaded loops can be compensated at the same time simply by overlapping the loops. As a result, excellent decoupling was obtained between adjacent loops of an eight-loop single-row (1 × 8) human head tight-fit TxRx array. In this work, we designed and constructed a 9.4-T (400-MHz) 16-loop double-row (2 × 8) overlapped TxRx head array based on the results of the analytical and numerical electromagnetic modeling. We demonstrated that, simply by the optimal overlap of array loops, a very good decoupling can be obtained without additional decoupling strategies. The constructed TxRx array provides whole-brain coverage and approximately 1.5 times greater Tx efficiency relative to a transmit-only/receive-only (ToRo) array, which consists of a larger Tx-only array and a nested tight-fit 31-loop receive (Rx)-only array. At the same time, the ToRo array provides greater peripheral signal-to-noise ratio (SNR) and better Rx parallel performance in the head-feet direction. Overall, our work provides a recipe for a simple, robust and very Tx-efficient design suitable for parallel transmission and whole-brain imaging at ultra-high fields.

Fast and efficient free induction decay MR spectroscopic imaging of the human brain at 9.4 Tesla.

PURPOSE: The purpose of this work was to develop a fast and efficient MRSI-FID acquisition scheme and test its performance in vivo. The aim was to find a trade-off between the minimal total acquisition time and signal-to-noise ratio of the acquired spectra. METHODS: Measurements were performed on a 9.4 Tesla system. Sequence optimization included redesign of water suppression, optimization of the sequence gradients, and improvement of the sampling efficiency by minimizing the read-out time. This resulted in an acquisition time of 2:47 and 22:13 minutes for 2D (TR = 57 ms; 3-mm in-plane resolution) and 3D MRSI (TR = 57 ms; 16 slices; 3-mm isotropic resolution), respectively. RESULTS: Despite strong T1 weighting and first-order phase problems, it was possible to obtain spectra of an acceptable quality. The average line width calculated for the tCr peak across the entire field of view was 26.9 ± 9.6 Hz for 2D and 30.0 ± 11.3 Hz for 3D MRSI. In 3D measurements, the percent fraction of voxels fitted with Cramer-Rao lower bounds below 10% was 53.3 ± 4.1%, 63.4 ± 8.4%, and 81.0 ± 2.9% for Glu, tCr, and tNAA, respectively. CONCLUSION: Considering the typically long duration of high-resolution MRSI, the proposed technique may be of interest for clinical applications and/or studies that focus on following the biochemistry of dynamic processes. Magn Reson Med 78:1281-1295, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

(31)P CSI of the human brain in healthy subjects and tumor patients at 9.4 T with a three-layered multi-nuclear coil: initial results.

OBJECTIVE: Investigation of the feasibility and performance of phosphorus ((31)P) magnetic resonance spectroscopic imaging (MRSI) at 9.4 T with a three-layered phosphorus/proton coil in human normal brain tissue and tumor. MATERIALS AND METHODS: A multi-channel (31)P coil was designed to enable MRSI of the entire human brain. The performance of the coil was evaluated by means of electromagnetic field simulations and actual measurements. A 3D chemical shift imaging approach with a variable repetition time and flip angle was used to increase the achievable signal-to-noise ratio of the acquired (31)P spectra. The impact of the resulting k-space modulation was investigated by simulations. Three tumor patients and three healthy volunteers were scanned and differences between spectra from healthy and cancerous tissue were evaluated qualitatively. RESULTS: The high sensitivity provided by the 27-channel (31)P coil allowed acquiring CSI data in 22 min with a nominal voxel size of 15 × 15 × 15 mm(3). Shimming and anatomical localization could be performed with the integrated four-channel proton dipole array. The amplitudes of the phosphodiesters and phosphoethanolamine appeared reduced in tumorous tissue for all three patients. A neutral or slightly alkaline pH was measured within the brain lesions. CONCLUSION: These initial results demonstrate that (31)P 3D CSI is feasible at 9.4 T and could be performed successfully in healthy subjects and tumor patients in under 30 min.

Volumetric imaging with homogenised excitation and static field at 9.4 T.

OBJECTIVES: To overcome the challenges of B0 and RF excitation inhomogeneity at ultra-high field MRI, a workflow for volumetric B0 and flip-angle homogenisation was implemented on a human 9.4 T scanner. MATERIALS AND METHODS: Imaging was performed with a 9.4 T human MR scanner (Siemens Medical Solutions, Erlangen, Germany) using a 16-channel parallel transmission system. B0- and B1-mapping were done using a dual-echo GRE and transmit phase-encoded DREAM, respectively. B0 shims and a small-tip-angle-approximation kT-points pulse were calculated with an off-line routine and applied to acquire T1- and T 2 (*) -weighted images with MPRAGE and 3D EPI, respectively. RESULTS: Over six in vivo acquisitions, the B0-distribution in a region-of-interest defined by a brain mask was reduced down to a full-width-half-maximum of 0.10 ± 0.01 ppm (39 ± 2 Hz). Utilising the kT-points pulses, the normalised RMSE of the excitation was decreased from CP-mode's 30.5 ± 0.9 to 9.2 ± 0.7 % with all B 1 (+)  voids eliminated. The SNR inhomogeneities and contrast variations in the T1- and T 2 (*) -weighted volumetric images were greatly reduced which led to successful tissue segmentation of the T1-weighted image. CONCLUSION: A 15-minute B0- and flip-angle homogenisation workflow, including the B0- and B1-map acquisitions, was successfully implemented and enabled us to reduce intensity and contrast variations as well as echo-planar image distortions in 9.4 T images.

In vivo proton magnetic resonance spectroscopic imaging of the healthy human brain at 9.4 T: initial experience.

OBJECT: In this study, the feasibility of in vivo proton magnetic resonance spectroscopic imaging ((1)H MRSI) of the healthy human brain at a field strength of 9.4 T, using conventional acquisition techniques, is examined and the initial experience is summarized. MATERIALS AND METHODS: MRSI measurements were performed on a 9.4 T MR scanner (Siemens, Erlangen, Germany) equipped with head-only gradient insert (AC84, Siemens) and custom-developed, 8-channel transmit/24-channel receive, and 16-channel transmit/31-channel receive coils. Spectra were acquired from the superior part of the human brain with a modified STEAM sequence. Spectral quantification was done with LCModel software. RESULTS: Reasonable quality and signal-to-noise ratio of the acquired spectra allowed reliable quantification of 12 metabolites (Cramer-Rao lower bounds < 20 %), some of which may be difficult to quantify at field strengths below 7 T due to overlapping resonances or low concentrations. CONCLUSION: While further developments are necessary to minimize chemical shift displacement and homogeneity of the transmit field, it is demonstrated that in vivo (1)H MRSI at a field strength of 9.4 T is possible. However, further studies applying up-to-date techniques to overcome high-field specific problems are needed in order to assess the potential gain in sensitivity that may be offered by MRSI at 9.4 T.

Numerical and experimental evaluation of RF shimming in the human brain at 9.4 T using a dual-row transmit array.

OBJECTIVE: To provide a numerical and experimental investigation of the static RF shimming capabilities in the human brain at 9.4 T using a dual-row transmit array. MATERIALS AND METHODS: A detailed numerical model of an existing 16-channel, inductively decoupled dual-row array was constructed using time-domain software together with circuit co-simulation. Experiments were conducted on a 9.4 T scanner. Investigation of RF shimming focused on B1(+) homogeneity, efficiency and local specific absorption rate (SAR) when applied to large brain volumes and on a slice-by-slice basis. RESULTS: Numerical results were consistent with experiments regarding component values, S-parameters and B1(+) pattern, though the B1(+) field was about 25% weaker in measurements than simulations. Global shim settings were able to prevent B1(+) field voids across the entire brain but the capability to simultaneously reduce inhomogeneities was limited. On a slice-by-slice basis, B1(+) standard deviations of below 10% without field dropouts could be achieved in axial, sagittal and coronal orientations across the brain, even with phase-only shimming, but decreased B1(+) efficiency and SAR limitations must be considered. CONCLUSION: Dual-row transmit arrays facilitate flexible 3D RF management across the entire brain at 9.4 T in order to trade off B1(+) homogeneity against power-efficiency and local SAR.