The invention relates to an MR arrangement which includes a medical instrument (3), an RF arrangement (17) which is provided thereon and in which at least one microcoil (1) and a capacitor (2) are connected so as to form a resonant circuit, and also an optical signal lead (4) via which a light signal is applied to the RF arrangement. In order to achieve simple and fast determination of the position of the medical instrument (3) within the examination zone of an MR apparatus, the invention proposes to provide a modulator (20) for modulating the light signal applied to the RF arrangement and also an optoelectrical converter (5) which converts the modulated light signal into an electrical signal and is coupled to the resonant circuit in such a manner that the resonant circuit is triggered by the electrical signal from the optoelectrical converter (5) so as to emit an RF field.