An instrument 14 e.g. needle or stent, for use in a magnetic resonance imaging (MRI) or computed tomography (CT) procedure. The instrument 14 comprising an anode 24 and a cathode 26, wherein, in use, current flows between the anode 24 and cathode 26, thereby producing a plurality of bubbles (not shown). The bubbles are detectable by the MRI or CT so the therefore the instrument can be detected. The power source can be integral with the instrument or electrically connected via wires.