A transformer line (46) extends through a catheter or other interventional instrument (30) that is to be used in the examination region (14) of a magnetic resonance imaging apparatus (10). The transformer line includes pairs of transformer windings (28) which are tuned in order to adjust the operating frequency and the maximum attenuation frequencies. Eccentric cams or other tuning elements (50, 64) are disposed in the transformer windings. Rotating the eccentric cams mechanically changes the geometry of the transformer windings, changing their inductive properties, and thus the frequency to which each is tuned.