A Coaxial RF Applicator for Ultra-High Field Human MRI

High dynamic range B 1 + mapping for the evaluation of parallel transmit arrays

PURPOSE

Demonstration of a high dynamic-range and high SNR method for acquiring absoluteB 1 + $$ {\mathrm{B}}_1^{+} $$ maps from a combination of gradient echo and actual-flip-angle measurements that is especially useful during the construction of parallel-transmit arrays.

METHODS

Low flip angle gradient echo images, acquired when transmitting with each channel individually, are used to compute relativeB 1 + $$ {\mathrm{B}}_1^{+} $$ maps. Instead of computing these in a conventional manner, the equivalence of the problem to the ESPIRiT parallel image reconstruction method is used to computeB 1 + $$ {\mathrm{B}}_1^{+} $$ maps with a higher SNR. Absolute maps are generated by calibration against a single actual flip-angle acquisition when transmitting on all channels simultaneously.

RESULTS

Depending on the number of receiver channels and the location of the receive elements with respect to the subject being investigated, moderate to high gains in the SNR of the acquiredB 1 + $$ {\mathrm{B}}_1^{+} $$ maps can be achieved.

CONCLUSIONS

The proposed method is especially suited for the acquisition ofB 1 + $$ {\mathrm{B}}_1^{+} $$ maps during the construction of transceiver arrays. Compared to the original method, maps with higher SNR can be computed without the need for additional measurements, and maps can also be generated using previously acquired data. Furthermore, easy adoption and fast estimation of receiver channels is possible because of existing highly optimized open-source implementations of ESPIRiT, such as in the BART toolbox.

External Waveguide Magnetic Resonance Imaging for lower limbs at 3 T

INTRODUCTION

We report a method based on the travelling-wave MRI approach, in order to acquire images of human lower limbs with an external waveguide at 3 T.

METHOD

We use a parallel-plate waveguide and an RF surface coil for reception, while a whole-body birdcage is used for transmission. The waveguide and the surface coil are located right outside the magnet, in the MR conditional devices zone. We ran numerical simulations to investigate the B₁ field generated by the surface coil located at one of the waveguides, as well as a saline-solution phantom positioned on the opposite side (150 cm away) inside the magnet.

RESULTS

We obtained phantom images by varying the distance between the coil and the phantom, in order to investigate the signal-to-noise ratio and to validate our numerical simulations. Lower limb images of a healthy volunteer were also acquired, demonstrating the viability of this approach. Standard pulse sequences were used and no physical modifications were made to the MR imager.

CONCLUSIONS

These numerical and experimental results show that travelling-wave MRI can produce high-quality images with only a simple waveguide and an RF coil located outside the magnet. This can be particularly favorable when acquiring images of lower limbs requiring a larger field of view.