A novel matching strategy to increase power efficiency of the travelling wave MR imaging

Introduction: Travelling wave MR imaging exploits the waveguide action of the RF shield. The transmitting and receiving patch antenna is placed at the beginning of the bore of the scanner. Different sections of the bore will have different wave impedances due to different loading conditions. This can lead to abrupt changes in impedance causing large reflections and inefficient power transfer to the target region [1,2]. Here we present a matching methodology for travelling wave MR imaging based on transmission line theory. We propose to introduce a coaxial waveguide section [3] between the antenna and load to achieve a gradual transformation of the impedance of the antenna to the impedance of the load (Figure 1). Theory: The patient region will generally have much lower impedance than the hollow waveguide due to a lower cut-off frequency caused by dielectric loading effect of the patient. As a result, a large part of the wave will be reflected when it impinges upon the patient, especially at the shoulder region. [2, 4]The coaxial waveguide inset transforms impedance at