Koch Snowflake Fractal RF Surface Coils to Improve ~(23)Na-Magnetic Resonance Imaging at 3T

摘要

The significantly lower signal-to-noise ratio (SNR) of sodium (~(23)Na) MRI, compared to proton (~1H)-MRI [1] justifies interest in developing improvements to create higher quality ~(23)Na-MR images. While developments have been made in pulse sequence design, image reconstruction, and approaches to image filtering [1], RF hardware is not often considered as many believe it to be already optimized. However, the aim of this project is hardware, based on newer approaches to RF engineering. Fractal antennas have been used in far-field systems including cellphones for years [2] and so we propose that a fractal geometry surface coil design will produce improved images compared to that of a standard circular surface coil. It is hypothesized that a fractal design will produce a higher amplitude B+1~+ field with improved homogeneity [3, 4] and therefore higher SNR. Subjects and Methods: Two coils were simulated using HFSS [5]: a simple circular surface (SCS) coil, and a third generation Koch snowflake fractal surface (KSFS) coil. A Koch snowflake geometry was chosen because: (1) it closely resembles the geometry of a circular loop; (2) preliminary simulations were performed and it showed potential for ~(129)Xe imaging [2], whose gyromagnetic ratio is very close to ~(23)Na; and (3) it has been shown that a KSFS coil design can provide a small increase in SNR in ~1H-MRI [3]. Both coils were simulated as thin copper strips mounted on a RO4350 substrate with an air radiation box above and loaded box of saline below (Fig. 1). The coils were then compared on the magnitude and homogeneity of the H field produced and the magnitude of the E field.