A local multi-transmit coil combined with a high-density receive array for cerebellar fMRI at 7 T.

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.

Pushing functional MRI spatial and temporal resolution further: High-density receive arrays combined with shot-selective 2D CAIPIRINHA for 3D echo-planar imaging at 7 T.

To be able to examine dynamic and detailed brain functions, the spatial and temporal resolution of 7 T MRI needs to improve. In this study, it was investigated whether submillimeter multishot 3D EPI fMRI scans, acquired with high-density receive arrays, can benefit from a 2D CAIPIRINHA sampling pattern, in terms of noise amplification (g-factor), temporal SNR and fMRI sensitivity. High-density receive arrays were combined with a shot-selective 2D CAIPIRINHA implementation for multishot 3D EPI sequences at 7 T. In this implementation, in contrast to conventional inclusion of extra kz gradient blips, specific EPI shots are left out to create a CAIPIRINHA shift and reduction of scan time. First, the implementation of the CAIPIRINHA sequence was evaluated with a standard receive setup by acquiring submillimeter whole brain T2 *-weighted anatomy images. Second, the CAIPIRINHA sequence was combined with high-density receive arrays to push the temporal resolution of submillimeter 3D EPI fMRI scans of the visual cortex. Results show that the shot-selective 2D CAIPIRINHA sequence enables a reduction in scan time for 0.5 mm isotropic 3D EPI T2 *-weighted anatomy scans by a factor of 4 compared with earlier reports. The use of the 2D CAIPIRINHA implementation in combination with high-density receive arrays, enhances the image quality of submillimeter 3D EPI scans of the visual cortex at high acceleration as compared to conventional SENSE. Both the g-factor and temporal SNR improved, resulting in a method that is more sensitive to the fMRI signal. Using this method, it is possible to acquire submillimeter single volume 3D EPI scans of the visual cortex in a subsecond timeframe. Overall, high-density receive arrays in combination with shot-selective 2D CAIPIRINHA for 3D EPI scans prove to be valuable for reducing the scan time of submillimeter MRI acquisitions.

Evaluation of short folded dipole antennas as receive elements of ultra-high-field human head array.

PURPOSE: To improve the receive (Rx) performance of a human head transceiver (TxRx) array at 9.4T without compromising its transmit (Tx) performance, a novel 16-element array was developed, constructed, and tested. METHODS: We designed and constructed a phased array, which consists of 8 TxRx surface loops placed in a single row and circumscribing a head, and 8 Rx-only short folded dipole antennas. Dipoles were positioned along the central axis of each transceiver loop perpendicular to its surface. We evaluated the effect of Rx dipoles on the Tx efficiency of the array and maximum local specific absorption rate (SAR) as compared to the array of 8 surface loops only. We also compared the new array to a 16-channel array of the same size consisting of 8 TxRx surface loops and 8 Rx-only vertical loops in terms of Tx efficiency, SAR, and signal-to-noise ratio (SNR). RESULTS: The new array improves both peripheral (up to 2 times) and central (1.17 times) SNR as compared to the 16-element array of the same geometry consisting of 8 TxRx surface loops and 8 Rx-only vertical loops. We demonstrated that an addition of actively detuned Rx-only dipole elements produces only a small decrease (~7%) of the B1+ transmit field and a small increase (<7%) of the maximum local SAR. CONCLUSION: As a proof of concept, we developed and constructed a prototype of a 9.4T (400 MHz) head array consisting of 8 TxRx surface loops and 8 Rx-only short optimized folded dipoles. We demonstrated that at ultra-high field, dipoles outperformed Rx-only vertical loops in vivo.

Comparision of new element designs for combined RF-Shim arrays at 7 T.

PURPOSE: To identify novel concepts for RF-shim loop architectures suitable for 7T made of two concentric conducting loops fulfilling RF and DC functions, respectively, and to determine their relative SNR performance. The goal is to minimize interference between the two systems while making efficient use of the space closest to the body. THEORY: We show by means of theoretical derivation of the frequency spectrum that the proposed two-loop structure exhibits an anti-resonant null and a resonant peak in the frequency domain. METHODS: The proposed structure is comprised of two concentric wire loops either arranged as nested loops or in the form of a coaxial cable, in which the two conductors carry the RF and shim signals, respectively. We use theory, simulation, and phantom measurements to obtain frequency spectra and SNR maps for the proposed structures. RESULTS: Retained SNR is found to be 75% in the coaxial loop and ranges from 57% to 67% in three different coaxial configurations. We have found both implementations to be a viable concept for the use in RF-shim devices if remaining SNR limitations can be overcome. CONCLUSIONS: We have investigated two new design modalities in 7T RF-shim coil design that separate the RF and shim conductors such that the previously proposed toroidal chokes are eliminated - thereby removing undesired additional loss, bulk, and design complexity.

A 32-channel combined RF and B0 shim array for 3T brain imaging.

PURPOSE: We add user-controllable direct currents (DC) to the individual elements of a 32-channel radio-frequency (RF) receive array to provide B0 shimming ability while preserving the array's reception sensitivity and parallel imaging performance. METHODS: Shim performance using constrained DC current (± 2.5A) is simulated for brain arrays ranging from 8 to 128 elements. A 32-channel 3-tesla brain array is realized using inductive chokes to bridge the tuning capacitors on each RF loop. The RF and B0 shimming performance is assessed in bench and imaging measurements. RESULTS: The addition of DC currents to the 32-channel RF array is achieved with minimal disruption of the RF performance and/or negative side effects such as conductor heating or mechanical torques. The shimming results agree well with simulations and show performance superior to third-order spherical harmonic (SH) shimming. Imaging tests show the ability to reduce the standard frontal lobe susceptibility-induced fields and improve echo planar imaging geometric distortion. The simulation of 64- and 128-channel brain arrays suggest that even further shimming improvement is possible (equivalent to up to 6th-order SH shim coils). CONCLUSION: Including user-controlled shim currents on the loops of a conventional highly parallel brain array coil is feasible with modest current levels and produces improved B0 shimming performance over standard second-order SH shimming.

Improving multi-channel compressed sensing MRI with reweighted l 1 minimization.

Integrating compressed sensing (CS) and parallel imaging (PI) with multi-channel receiver has proven to be an effective technology to speed up magnetic resonance imaging (MRI). In this paper, we propose a method that extends the reweighted l 1 minimization to the CS-MRI with multi-channel data. The method applies a reweighted l 1 minimization algorithm to reconstruct each channel image, and then generates the final image by a sum-of-squares method. Computer simulations based on synthetic data and in vivo MRI imaging data show that the new method can improve the reconstruction quality at a slightly increased computation cost.

Effects of receive-only inserts on specific absorption rate, B1 (+) field, and Tx coil performance.

PURPOSE: To evaluate the effect of different cylindrical and close-conforming receive-only array designs on spin excitation and specific absorption rate (SAR) of a 7 T transmit-only head coil. MATERIALS AND METHODS: We developed finite difference time domain (FDTD) models of different receive-only array geometries. Cylindrical and close-fitting helmet arrays with varying copper trace widths, a TEM Tx coil model, and two head models were used in numerical simulations. Tx coil coupling was experimentally measured and validated with FDTD modeling. RESULTS: Changing copper trace width of loops in array models caused subtle changes in radiofrequency (RF) absorption (<5%). Changes in SAR distribution were observed in the head models with Rx-only inserts. Peak SAR increased (-1 to +15%) in different tissues for a mean B1 (+) in the brain of 2 µT. Total absorption in the head models for 1 W forward power increased (5%-21%) in the heads with Rx-only inserts. Changes in RF absorption with different Rx inserts indicate a change in RF radiation of the Tx coil even when changes in B1 (+) and coupling between ports of Tx coil were minimal. CONCLUSION: Changes in local/global SAR and subtle changes in B1 (+) field distributions were observed in the presence of Rx-only inserts. Thus, incorporation of the receive-only array effects are needed when evaluating SAR and designing RF transmit pulse waveform parameters for shimming and/or Tx-SENSE for 7 T MRI.