(a) at least two conductive elements, each having a first curvature along a direction perpendicular to a longitudinal axis, said at least two conductive elements being spaced along said longitudinal axis to define a volume between said at least two conductive elements, wherein said at least two conductive elements are characterized by an RF shield structure for substantially blocking RF radiation while transmitting low frequency radiation, and wherein an RF current flowing within said at least two conductive elements in a direction of said longitudinal axis induces in said volume a substantially homogenous RF magnetic field directed perpendicular to said longitudinal axis; and

(b) at least one additional conductive element, electrically communicating with said at least two conductive elements in a manner such that a phase of an RF current flowing through said at least one additional conductive element equals a phase of at least one of said RF currents flowing through said at least two conductive elements.