Investigation of the Transition from Travelling Wave to Near Field Excitation Using Crossed Dipole Antennas

Karthik Lakshmanan¹, Martijn A. Cloos², Graham C. Wiggins¹

Crossed Dipoles have been used to create travelling waves in the bore of the magnet which can be used for MR imaging. Travelling wave excitation is relatively inefficient compared to conventional volume resonators. If the crossed dipole is placed close to the object, however, efficiency increases while excitation pattern changes. In this study we compared the excitation profile and efficiency of a crossed dipole antenna pair as a function of distance between the antenna and the object imaged. The efficiency of the antenna configuration showed an inverse proportionality to the distance between the antenna and the Imaging object. When the distance between the antenna pair and the imaging object was less than quarter wavelength the excitation pattern changed from a travelling pattern with peak low in the head and with neck excitation to more of a near field excitation with the excitation primarily confined to the top of the head.