Mid-Bore Excitation of Traveling Waves with an Annular Ladder Resonator for 7T Body Imaging with Reduced SAR

Introduction: The dimensions of the RF shield and the RF frequency for proton imaging at 7T allow the excitation of a propagating TE11 waveguide mode, which has given rise to the concept of traveling wave imaging [1]. RF excitation is typically achieved by using a patch antenna or crossed dipole antenna placed at one end of the bore [1, 2]. A nearly uniform B1 field is produced along the entire length of the bore when it is empty. With a human body in the bore, dielectric boundaries and tissue conductivity give rise to standing wave behavior and attenuation of the B1 field, resulting in a highly non-uniform excitation [3]. The strongest B1 field and maximum SAR occur in the parts of the body closest to the antenna such as the head and shoulders or legs, with very little excitation in the torso. To create a useful excitation in the torso is it necessary to deposit large amounts of energy in the head or legs. Some methods have been proposed for creating structures which guide the traveling wave to deliver the power more efficiently to more distant regions [4]. It would be desirable though to excite the TE11 traveling wave mode at or near the mid section of the bore, close to the imaging volume of the scanner. We propose a novel resonator design, the annular ladder resonator (ALR), which is tailored to couple to the TE11 mode and which can be mounted behind the covers of the 7T scanner in the space normally occupied by the body coil in a conventional scanner. The coil behavior and performance are evaluated through full-wave simulation and the construction of prototype coils.