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

Purpose: To demonstrate the possibility of achieving plane-wise uniform B₁ 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 B₁ field from each current loop was calculated using the FDTD numerical method at 170 MHz. Analytical target RF field patterns with plane-wise uniform B₁ 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 B₁ 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 B₁ amplitude distribution with a high degree of plane-wise homogeneity can be achieved simultaneously in multiple parallel planes in a 3D volume.