A Simulational Study on the Homogeneity of Dual-Tuned Birdcage Coils

Introduction Dual-tuned (H/P) birdcage coils have become a well known and widely used tool for magnetic resonance spectroscopy (MRS). Oxidative metabolism indices, such as phosphocreatine [PCr], are important indicators of biochemical processes. MRS of phosphorous (P MRS) is well suited to monitor these processes in both skeletal muscles and neuronal synaptic development in human brains. However, the P MRS signal sensitivity is usually low due to low concentrations of the nucleus and magnetic field inhomogeneity from currently available commercial surface coils. The goal is to create a very homogeneous B1 field inside the coil. The practicality of building a dual-tuned coil, however, can be troublesome due to many factors such as coil structure (e.g., # of legs/rings, dimensions) and biological objects to be imaged. In this abstract, the homogeneity comparisons for the 8-leg, 16-leg, and the 24-leg, 4-ring low-pass birdcage coils are presented with the presence of simulated human head and thigh modules. Methods The dual-tuned low-pass birdcage coil uses 4 conducting rings connected with conducting legs that form three individual sections, an inner section and two outer sections. The outer section is for H imaging (127.72MHz for Hydrogen at 3T) and the inner section is for P MRS (51.7MHz for Phosphorus at 3T). The optimization of the physical parameters of a dual-tuned coil has been toyed with in the past (1-4). However, the coil for this experiment is intended for imaging of the adult human thigh and head. The diameter, length of the inner coil, and length of the outer coil were set as 23.45cm, 19.54cm, and 3.26cm respectively to fit both anatomies, with consideration of the optimum dimensions found by Duan et al. (1). Using the above dimensions, simulations were run with a finite difference time domain program, xFDTD (Remcom, State College, PA.), to calculate and map the magnetic field. Rather than tuning capacitors at each leg of the inner birdcage coil, the magnetic field is generated with sinusoidal voltage sources placed in series with each leg, at a frequency of 51.7MHz and phase shifted to form a sinusoidal current distribution. For example, the shift of each leg of an 8-leg coil would be: 0, 45, 90, 135, 180, 225, 270, and 315 degrees. Capacitances placed on the outer coils were estimated according to the Penn State NMR birdcage builder software, which were 6.19pF, 3.39pF, and 2.32pF for the 8-leg, 16-leg, and 24-leg coils, respectively. One full cycle of a 51.7MHz excitation wave was simulated and the magnetic field generated around the mid-point of the cycle is analyzed for homogeneity in the xy-plane (transverse) and the yz-plane (longitudinal) as shown in Fig.1. The analysis was performed in free space and presence of biological objects, a human head and the thigh modules (Virtual family models, IT’IS Foundation, Switzerland), respectively.