A theoretical comparison of two optimization methods for radiofrequency drive schemes in high frequency MRI resonators

Optimization of a quadrature birdcage coil for functional imaging of squirrel monkey brain at 9.4T

A quadrature transmit/receive birdcage coil was optimized for squirrel monkey functional imaging at the high field of 9.4 T. The coil length was chosen to gain maximum coil efficiency/signal-to-noise ratio (SNR) and meanwhile provide enough homogenous RF field in the whole brain area. Based on the numerical simulation results, a 16-rung high-pass birdcage coil with the optimal length of 9 cm was constructed and evaluated on phantom and in vivo experiments. Compared to a general-purpose non-optimized coil, it exhibits approximately 25% in vivo SNR improvement. In addition to the volume coil, details about how to design and construct the associated animal preparation system were provided.

Inverse field-based approach for simultaneous B₁ mapping at high fields - a phantom based study

Based on computational electromagnetics and multi-level optimization, an inverse approach of attaining accurate mapping of both transmit and receive sensitivity of radiofrequency coils is presented. This paper extends our previous study of inverse methods of receptivity mapping at low fields, to allow accurate mapping of RF magnetic fields (B(1)) for high-field applications. Accurate receive sensitivity mapping is essential to image domain parallel imaging methods, such as sensitivity encoding (SENSE), to reconstruct high quality images. Accurate transmit sensitivity mapping will facilitate RF-shimming and parallel transmission techniques that directly address the RF inhomogeneity issue, arguably the most challenging issue of high-field magnetic resonance imaging (MRI). The inverse field-based approach proposed herein is based on computational electromagnetics and iterative optimization. It fits an experimental image to the numerically calculated signal intensity by iteratively optimizing the coil-subject geometry to better resemble the experiments. Accurate transmit and receive sensitivities are derived as intermediate results of the optimization process. The method is validated by imaging studies using homogeneous saline phantom at 7T. A simulation study at 300MHz demonstrates that the proposed method is able to obtain receptivity mapping with errors an order of magnitude less than that of the conventional method. The more accurate receptivity mapping and simultaneously obtained transmit sensitivity mapping could enable artefact-reduced and intensity-corrected image reconstructions. It is hoped that by providing an approach to the accurate mapping of both transmit and receive sensitivity, the proposed method will facilitate a range of applications in high-field MRI and parallel imaging.

The optimization of an 8-channel transceive volume array for small animal MRI at 9.4T

A shielded, non-overlapped 9.4T 8-element transceive volume array was designed and constructed for small animal MRI studies. The coil configuration was optimized to obtain an appropriate trade-off between coil sensitivity, field penetration, and mutual decoupling. The simulated and experimental results presented herein demonstrate that the new volume array is capable of offering higher RF penetration, efficiency and minimum mutual coupling; the proposed technique can also be implemented for a wide range of ultra high-field applications, including dense coil arrays for human studies.

Understanding and manipulating the RF fields at high field MRI

This paper presents a complete overview of the electromagnetics (radiofrequency aspect) of MRI at low and high fields. Using analytical formulations, numerical modeling (computational electromagnetics), and ultrahigh field imaging experiments, the physics that impacts the electromagnetic quantities associated with MRI, namely (1) the transmit field, (2) receive field, and (3) total electromagnetic power absorption, is analyzed. The physical interpretation of the above-mentioned quantities is investigated by electromagnetic theory, to understand 'What happens, in terms of electromagnetics, when operating at different static field strengths?' Using experimental studies and numerical simulations, this paper also examines the physical and technological feasibilities by which all or any of these specified electromagnetic quantities can be manipulated through techniques such as B(1) shimming (phased array excitation) and signal combination using a receive array in order to advance MRI at high field strengths. Pertinent to this subject and with highly coupled coils operating at 7 T, this paper also presents the first phantom work on B(1) shimming without B(1) measurements.

A ultra high field multi-element transceive volume array for small animal MRI

In this work, a dedicated, 9.4T shielded 8-element transceive volume-array for large rat MRI applications is designed and the prototype constructed. Angularly oriented radiating structured coil elements are used. It is shown that by orienting the radiating conductors of the coil element angularly, the RF penetration depth can be increased and the mutual coupling effect can be regulated to minimum. In addition, experimental results show that the prototype can produce homogenous B(1) field and is well suited for the Transmit SENSE application and the GRAPPA parallel imaging technique.

Insight into RF power requirements and B1 field homogeneity for human MRI via rigorous FDTD approach

PURPOSE

To study the dependence of radiofrequency (RF) power deposition on B(0) field strength for different loads and excitation mechanisms.

MATERIAL AND METHODS

Studies were performed utilizing a finite difference time domain (FDTD) model that treats the transmit array and the load as a single system. Since it was possible to achieve homogenous excitations across the human head model by varying the amplitudes/phases of the voltages driving the transmit array, studies of the RF power/B(0) field strength (frequency) dependence were achievable under well-defined/fixed/homogenous RF excitation.

RESULTS

Analysis illustrating the regime in which the RF power is dependent on the square of the operating frequency is presented. Detailed studies focusing on the RF power requirements as a function of number of excitation ports, driving mechanism, and orientations/positioning within the load are presented.

CONCLUSION

With variable phase/amplitude excitation, as a function of frequency, the peak-then-decrease relation observed in the upper axial slices of brain with quadrature excitation becomes more evident in the lower slices as well. Additionally, homogeneity optimization targeted at minimizing the ratio of maximum/minimum B(1) (+) field intensity within the region of interest, typically results in increased RF power requirements (standard deviation was not considered in this study). Increasing the number of excitation ports, however, can result in significant RF power reduction.