Achieving plane‐wise uniform B₁ amplitude in a 3D volume for high‐field MRI: A computer simulation study

Abstract

Purpose:To demonstrate the possibility of achieving plane-wise uniform B 1 field amplitude for human head imaging with an array coil configuration by using computer simulations. Materials and Methods:We considered the use of an excitation array coil that employed composite excitation elements. Each composite excitation element consisted of three small current loops centered close to each other with axes along the x, y, and z directions, respectively. The excitation elements were distributed to surround a model human head. The vector B 1 field from each current loop was calculated using the FDTD numerical method at 170 MHz. Analytical target RF field patterns with plane-wise uniform B 1 field amplitude were derived and approximately constructed from the fields of individual current loops through a least-squares procedure. Results:The RF field patterns generated by the computer simulations closely followed the target field patterns. Highly uniform B 1 field amplitude was obtained within parallel sagittal planes or parallel axial-to-coronal oblique planes in the brain with the expected plane-to-plane variations. Conclusion:In principle, patterns of B 1 amplitude distribution with a high degree of plane-wise homogeneity can be achieved simultaneously in multiple parallel planes in a 3D volume.

Towards a complete coil array

“…How such a coil system may be constructed has not been demonstrated previously [7,8]. Recently, our work based on computer simulations has shown that an array coil consisting of "composite elements" offers exceptional flexibility in shaping the RF field and allowed us to construct analytical target RF field patterns [12,13]. Here, with computer simulations, we demonstrate that reconstruction of the basis field modes up to certain specific orders with a magnetic dipole array in uniform media is possible [14].…”

“…The key feature of our new array system is the use of "3D" current loops ( Fig. 1), where one planar coil element in the traditional flat array design is replaced by a "composite element" that consists of three independent current loops with their axes along three orthogonal spatial directions [12,13].…”

Design of microstrip‐based surface coils for low‐field small‐bore MR applications

“…Finite‐difference time‐domain (FDTD) method (13–17) was employed for numerical computation of RF field penetration vs. depth using software written in MATLAB Release 2009b (The Mathworks, Natick, MA) running on a workstation (Dual‐core Intel® Xeon 5200 processor with 1.86 GHz). The computer simulation was performed on the same water phantom used above with a dielectric constant = 80 and an electric conductivity = 0.5 S/m using the G10 ( H = 2 mm) as the substrate and the same dimensions of the physical coil which was used for building the two‐turn microstrip coil.…”

Improving SNR of RF coils using composite coil elements

“…The finite-difference time-domain (FDTD) method (14) was employed to calculate the RF electromagnetic fields when an RF electric current flows through the coil loops, using an in-house developed software written in IDL 5.6 (Research System, CO). The software, used previously for a human head and shoulder model (7), was adapted to calculate the field in the half-space model. The simulation was confined to a cubic volume of 80 Â 80 Â 80 cm 3 using the perfectly matched layer boundary condition (15) at the surface of this volume, with the size of the Yee cell equal to 0.5 cm.…”

“…In this report, we explore the potential improvement in SNR brought about by replacing the conventional single-loop surface coil with a composite coil element, both in a single-element coil and array coils. A composite coil element consists of up to three independent coil loops that are orthogonal to each other with their centers located close to each other (6,7). A composite element offers additional degrees of freedom compared to a single coil loop in shaping the RF field in its vicinity.…”