An apparatus for determining a position of a medical instrument within a patient receiving zone of a magnetic resonance tomography device is provided. The apparatus includes a sensor and a computing and control device. The sensor is arranged on or in the medical instrument, includes at least one magnetic field sensor for obtaining measured values of a magnetic flux density, and may be connected to the computing and control device for data transfer purposes. The computing and control device is embodied to control magnetic fields of the magnetic resonance tomography device and to determine the position of the medical instrument. The measured values of the at least one magnetic field sensor and control signals for controlling the magnetic fields of the magnetic resonance tomography device are included in the determination of the position of the medical instrument.