Shortened Quarter Lambda antenna for traveling wave excitation in high field MRI

Introduction Recently the concept of traveling wave excitation was demonstrated for high field MRI (1). In these experiments RF excitation and reception was done by using a patch antenna, suited for this task. The disadvantage of this antenna, however, is its size. A proper patch antenna for 7 T MRI is almost as large as the diameter of the bore. This limits the location of the antenna to the edge of the bore, relatively far from anatomical structures to be imaged in, for instance, the abdominal area. In the latter case, tissue between the object to be imaged and the actual antenna will act as an attenuator, reducing image quality and SN ratio. Moreover, a large patch antenna will block any access to the patient at the backend of the magnet, which may jeopardize access to visual stimuli as well. We therefore propose the use of a mechanically less constrained Shortened Quarter Lambda (SQL) antenna for setting up travelling waves, and demonstrate 8 fold improved efficiency in the abdomen compared to a patch antenna Methods A quarter lambda antenna mounted on a ground plane is a good and efficient radiator (2). At 298.2 MHz such an antenna has a length of 250 mm. To further reduce the length of the antenna, an inductor is introduced at the base of the antenna. The inductor consists of 3 turns of the antenna wire (1.8 mm) with an inner diameter of 12 mm. A groundplane of 160*360 mm was employed. By varying the length of the antenna wire and the length of the inductor, the SQL antenna can easily by tuned and matched. To enable quadrature excitation, two SQL antennas were placed on a 90 folded groundplane (figure 1). To compare the performance of the SQL antenna to a circular quadrature patch antenna, FFE images were obtained from a cylindrical phantom and a healthy male volunteer with both antennas at equal RF power settings.