Apparatus and methods are described for use with a blood vessel of a subject. An annular antenna is placed inside the blood vessel such that radial expansion of the antenna is limited by a circumference of the blood vessel. A transmitter generates an inductive current in the antenna, by transmitting RF energy toward the antenna. A control unit measures the inductive current in the antenna, and, in response thereto, determines a physiological parameter of the subject. Other applications are also described.
