High-field imaging at low SAR: Tx/Rx prostate coil array using radiative elements for efficient antenna-patient power transfer

Introduction Several institutes have presented results on prostate imaging using a surface array of resonant stripline elements [1-3]. In the perspective of antenna theory, resonant stripline elements are near-field antennas with a high magnetic field in the near field zone around their conductors. This is an advantageous concept for MRI as long as the target region can be located in the near field. The near-field, however, decays quickly with distance (near field extends approx. λ/4 = 4 cm @ 298 MHz). Since most interesting sites in the abdomen are far beyond near-field regime, a radiative antenna would be the element of choice for a coil array in MR imaging [4]. A radiative antenna is designed to couple an electromagnetic wave effectively into the patient. In this way the power flow to the target region is maximized while reducing peaks in the electric and magnetic fields at the antenna-body interface. We present an antenna design that is known originally from hyperthermia. It generates Eand H-fields in such a way that the Poynting vector is pointing into the patient (figure 1). Extensive simulations and measurements on phantoms (presented elsewhere) show that radiative elements will enable imaging with a lower maximum SAR level and a wider and more homogeneous field of view. In this work we present imaging and simulation results on a volunteer with a prototype coil array of radiative elements. Advantages of this coil array are a lower maximum SAR level and a wider and more homogeneous field of view.