A mechanically tuned 8-channel microstrip array for parallel transmission at 7T (297MHz)

Introduction: Microstrip have been developed due to specific features: high Q factor, low mutual coupling (often used at high field MRI for parallel imaging), and radiation losses are significantly reduced (which enhances SNR). Balanced feeding of the microstrip preserves the electrical symmetry with respect to the strip center, making the probe invariant under different loading conditions (different subjects). The aim of this study is to present a novel 8-channel microstrip array designed for RF shimming and parallel transmission. The new design is tuned by adjusting the gap between ground plane and strip, and introduces a symmetric feed via a lattice balun [3] removing the need to match the probe to each subject. Materials and Methods: Each strip element consists of a 135×15mm microstrip over an 80×190mm ground plane. The strip is electrically shortened by capacitors at each end. An air dielectric is used to maximize the magnetic field above the strip. Tuning is achieved by adjusting the gap between strip and ground plane (fig. 2). The strip is symmetrically fed from each end using a lattice balun (fig.1) [1]. The lattice is tuned to 290 MHz, which is close to the strip resonance (297.2MHz) but far enough away to avoid strong coupling to the strip. The strip is matched to the balun with fixed capacitors. All components were mounted on the back of the ground plane to reduce interaction with the strip and minimize SAR. Eight strip elements were placed on an octagonal holder (inner Ø235mm, outer Ø285mm). Full wave simulations (Microwave Studio, CST) were used to optimize the strip dimensions using a single strip, loaded with a spherical saline phantom (Ø170mm, εr=60, σ=0.5S/m); coupling between strips was then investigated by simulating the full eight strip array. S11 and S21 were measured with a network analyzer (E5071C, Agilent) and images acquired with a Siemens 7T human scanner using standard Gradient Echo sequence (TE=1000ms, TR=9ms). B1 maps were acquired using a Sa2rage sequence (TE=1.38ms, TR=2s) [2], transmitting and receiving with one strip in the presence of two neighboring elements (fig. 3.E and F).