Common mode volume coil design for in vivo MR imaging at 7T

Introduction The volume coil is a well-established coil which creates homogeneous B1 field and MR images (1). In order to yield homogenous B1 field distribution, sufficient coupling among the resonant elements (rungs) of a volume coil is necessary. Although the coupling could be improved by reducing the distances between the coil elements, increasing the dimension and number of the coil elements, in some circumstances these methods are not easy to implement or unpractical. In this work, we present a new volume coil design using common mode resonators to increase coupling and improve the homogeneity of B1 field without increasing the number of elements and changing the size of the coil. The common mode (CM) exists within two coupled parallel transmission lines, yielding two identical current along the two legs. Fig.1 shows that the coupling between the two CM resonators is adjustable by changing the gap between the legs. Due to symmetry of the proposed volume coil, the CM currents were obtained within each resonator. The extra flexibility of the current distributions, compared with traditional volume coils, will be helpful for improvements of B1 homogeneity. As a prototype, an 8element volume coil, with the quadrature drive and detection, was implemented for 7T MR imaging. Preliminary results from a water phantom and a kiwi fruit were acquired using the proposed coil. Theoretical comparison between the proposed design and traditional birdcage design using the numerical simulation was also presented. Methods To evaluate the effects of the adjustable distance between the CM elements for B1 homogeneity and coil sensitivity, the magnetic field of the 8-element CM volume coil with different gaps were simulated by using Biot-Savart law. The 2D field calculations were performed on a theoretical model of volume coils (2). The common mode currents are similar to that of the split currents on a traditional volume coil, but the locations of the currents are different. Fig. 3 demonstrates the homogeneity of B1 fields, the radio to the magnetic field of the coil isocenter. The gaps between CM resonators are adjusted from 1/2” to 1/16”. Fig. 3a shows B1 homogeneity for largest gap, 1/2”, same as traditional birdcage, the medium gap, 1/4”, having uniform distributed legs are shown in Fig. 3b and the smallest gap, 1/16” in Fig. 3c. The computed area is 3.75” in plane. Based on the simulation results, A quadrature common mode volume coil was built on an acrylic cylinder with dimensions of 4” O.D, 3.75” I.D and 4” in length. The acrylic cylinder served as both dielectric material and mechanical support. Fig.2 shows a picture of the prototype coil. The CMDM resonator elements had 1/16” gaps between two of them. Each of its 8 resonant elements was a common mode resonator with capacitive termination on both ends. Two quadrature proton ports were electrical driven. Bench tests on coil resonant modes and isolation between two quadrature ports were implemented on a network analyzer (Agilent E5070B). The termination capacitance measurement was conducted on a RCL meter (Fluke PM6303A). The proton MR experiments were performed on a GE 7T magnet (GE Healthcare, Waukesha, WI). A cylindrical water phantom and a kiwi fruit were used in this preliminary study. A set of fast spin echo images in sagittal orientation and axial images were acquired with TR= 2sec, 10mm in plane and 3mm thick, number of excitation (NEX) = 1.