Sparse MRI has been introduced to reduce the acquisition time and raw data size by undersampling the k-space data. However, the image quality, particularly the contrast to noise ratio (CNR), decreases with the undersampling rate. In this work, we proposed

Background

Radiative coil arrays, e.g., dipole or monopole arrays, are increasingly used in MR signal excitation and reception for ultrahigh field MRI. Technically, it is challenging to suppress the electromagnetic (EM) coupling of radiative array elements due to their unique structures.

Methods

In this study, we proposed a combined decoupling and matching network (DMN) for monopole arrays for MRI applications. Compared with separate decoupling network and matching network, the combined network proposed here needs less components and rather suitable for decoupling radiative arrays in MRI.

Results

Our study shows that the transmission coefficient between two coupled monopoles can be reduced from -5 dB to -24.8 dB by using the combined DMN. It is also clearly demonstrated in this study that this decoupling method is a port decoupling method rather than an element decoupling method.

Conclusions

With the proposed DMN, the monopole coil provides locally strong and spatially diverse B1 fields, which is essential to the improvement of MR sensitivity and parallel imaging performance.

Quadrature coils are often desired in MR applications because they can improve MR sensitivity and also reduce excitation power. In this work, we propose, for the first time, a quadrature array design strategy for parallel transmission at 298 MHz using single-feed circularly polarized (CP) patch antenna technique. Each array element is a nearly square ring microstrip antenna and is fed at a point on the diagonal of the antenna to generate quadrature magnetic fields. Compared with conventional quadrature coils, the single-feed structure is much simple and compact, making the quadrature coil array design practical. Numerical simulations demonstrate that the decoupling between elements is better than -35 dB for all the elements and the RF fields are homogeneous with deep penetration and quadrature behavior in the area of interest. Bloch equation simulation is also performed to simulate the excitation procedure by using an 8-element quadrature planar patch array to demonstrate its feasibility in parallel transmission at the ultrahigh field of 7 Tesla.

In this work, we developed and tested a multi-channel radio frequency (RF) transmission system with compact metal-oxide semiconductor field effect transistor (MOSFET) amplifiers for parallel excitation in 7 T animal MRI scanner. The system is composed of a multi-channel RF controller and four independent RF power amplifiers. Each power amplifier contains two amplification stages. The design was validated by simulation and bench test. The power gain for the amplifier is 18.7 dB at 300 MHz, demonstrating the sufficient amplification capability of the transmission system for small animal parallel excitation applications at 7 T. This compact RF power amplifier can be potentially used for on-coil amplification in multichannel RF array system.

Copyright © 2014 Wolters Kluwer Health / Lippincott Williams & Wilkins.One of the technical challenges in designing a dedicated transceiver radio frequency (RF) array for MR imaging in humans at ultrahigh magnetic fields is how to effectively decouple the

A multi-reception strategy with extended GRAPPA is proposed in this work to improve MR imaging performance at ultra-high field MR systems with limited receiver channels. In this method, coil elements are separated to two or more groups under appropriate gr

Radiofrequency (RF) field (B 1) inhomogeneity due to shortened wavelength at high field is a major cause of magnetic resonance imaging (MRI) nonuniformity in high dielectric biological samples (e.g., human body). In this work, we propose a method to improve the B 1 and MRI homogeneity by using pre-emphasized non-uniform B 1 distribution. The intrinsic B 1 distribution that could be generated by a RF volume coil, specifically a microstrip transmission line (MTL) coil used in this work, was pre-emphasized in the sample's periphery region of interest to compensate for the central brightness induced by high frequency interference effect due to shortened wave length. This pre-emphasized non-uniform B 1 can be realized by varying the parameters of microstrip elements, such as the substrate thickness of MTL volume coil. Both numerical simulation and phantom MR imaging studies were carried out to investigate the feasibility and merit of the proposed method in achieving homogeneous MR images. The simulation results demonstrate that by using a pre-emphasized B 1 distribution generated by the MTL volume coil, relatively uniform B 1 distribution and homogeneous MR image (98% homogeneity) within the spherical phantom (15 cm diameter) were achieved with 4.5 mm thickness. The B 1 and MRI intensity distributions of a 16-element MTL volume coil with fixed substrate thickness and five varied saline loads were modeled and experimentally tested. Similar results from both simulation and experiments were obtained, suggesting substantial improvements of B 1 and MRI homogeneities within the phantom containing 125 mM saline. The overall results demonstrate an efficient B 1 shimming approach for improving high field MRI.

The structures and stabilities of the high-pressure phases of titania (TiO2) are of great interest in the earth sciences, as these phases are the accessible analogs of component minerals in the earth's mantle. Brookite is a natural titania mineral, whose bulk phase is hard to synthesize in the laboratory, and its phase behavior at very high pressure remains unknown. Thus, in this work, using phase-pure natural brookite as the sample, we studied the phase transition of bulk brookite under compression up to ∼60 GPa in three different pressure transmitting media. Results show that, at room temperature and in near hydrostatic conditions, brookite undergoes a series of transitions, i.e., brookite (∼0-12 GPa) → TiO2-II and minor rutile (∼1-21 GPa) → baddeleyite (∼12-60 GPa) → TiO2-OI (∼33-60 GPa). Taking into account that all transitions occur at finite rates and that solidification of a pressure medium can influence transition kinetics, we show that the observed transition sequence is consistent with the expected high-pressure phase stability of TiO2 calculated from density functional theory. The knowledge obtained from this work may be used to infer the geological fate of brookite in nature.

Purpose

To develop a lump-element double-tuned common-mode-differential-mode (CMDM) radiofrequency (RF) surface coil with independent frequency tuning capacity for MRS and MRI applications.

Methods

The presented design has two modes that can operate with different current paths, allowing independent frequency adjustment. The coil prototype was tested on the bench and then examined in phantom and in vivo experiments.

Results

Standard deviations of frequency and impedance fluctuations measured in one resonator, while changing the tuning capacitor of another resonator, were less than 13 kHz and 0.55 Ω. The unloaded S21 was -36 dB and -41 dB, while the unloaded Q factor was 260 and 287, for ¹³ C and ¹ H, respectively. In vivo hyperpolarized ¹³ C MR spectroscopy data demonstrated the feasibility of using the CMDM coil to measure the dynamics of lactate, alanine, pyruvate and bicarbonate signal in a normal rat head along with acquiring ¹ H anatomical reference images.

Conclusion

Independent frequency tuning capacity was demonstrated in the presented lump-element double-tuned CMDM coil. This CMDM coil maintained intrinsically decoupled magnetic fields, which provided sufficient isolation between the two resonators. The results from in vivo experiments demonstrated high sensitivity of both the ¹ H and ¹³ C resonators. Magn Reson Med 76:1612-1620, 2016. © 2015 International Society for Magnetic Resonance in Medicine.