Instrument guidance in interventional MRI is often performed on the basis of signal voids generated by the interventional device, either as a result of different magnetic susceptibility compared to the tissue (e.g., titanium needles) or the displacement of hydrogen protons (e.g., catheters). These guidance techniques suffer from difficult controllability of the signal void size especially for small plastic devices (limited conspicuity) or larger metallic devices (overwhelming artifacts). Recently, the visualization of a conductor by the use of a spin-echo phase imaging was reported.¹ However, spin echo sequences are time consuming, making instrument tracking during interventional treatment impracticable. To overcome these problems, a triggered direct current based technique in combination with a bSSFP sequence was developed.²Transient, local magnetic field alterations generate unique phase variations in spin-echo phase images.¹ This principle can be transferred to bSSFP sequences, because of similar rephasing characteristics for TE = TR/2 and TR << T₁, T₂.³ Magnetically homogeneous image areas or such with static magnetic field alterations are displayed in the bSSFP phase image either as values of zero or +/- pi. By use of a simple linearly rising mask function, phase variations in a certain range uniquely originating from transient magnetic field alterations (e.g., from 0.2 to 1.57 rad) are translated in an RGB color space. The colored mask image is superimposed to the magnitude image displaying the position of the device. If needed, a k-space low-pass filter can be used to remove false positive pixels in areas with low signal intensity. A bSSFP sequence was modified to obtain a trigger signal at the output of the scanner. To avoid disturbed spatial encoding, the trigger signal was set between the end of RF excitation and the start of signal acquisition (Fig. 1a). The trigger signal switched a relay that let a direct current flow through a conductor with a double-helical geometry wound on the surface of a catheter (Fig. 1b). This geometry was capable to generate an effective magnetic field component in every orientation to B₀.⁴ The modified catheter was placed in a water basin with a gimbal.The double-helical wound wire generates strong magnetic field gradients in its vicinity resulting in characteristic phase offset pattern (Fig. 2). In all three orientations (perpendicular (a), 45° (b), and parallel (c) toward B₀) the catheter is clearly visible. The dimension of the phase offset pattern can be increased by setting the trigger duration to 1.5 ms (d). In the magnitude image of the porcine liver, the catheter is visible because of its water filled lumen but cannot be distinguished from other bright structures (Fig. 3). The characteristic pattern is visible in the phase image. By combination of the post-processed phase image with the magnitude image, the superimposed phase pattern reveals unambiguously the position of the catheter in the resulting image. Application of the k-space low pass filter removes colored pixels in areas with low signal intensity (white arrows point on air cavities).The described technique allows distinct localization of interventional devices with a fast sequence commonly used in interventional magnetic resonance imaging. The choice of sequence parameters is not restricted and no additional acquisition time is needed. Care has to be taken that the device or the connecting cables are not coupling to the radiofrequency pulses and that safety regulations are satisfied. Further, the material of the metallic devices must have a water equivalent magnetic susceptibility (e.g., brass).The visualization of devices by transient, local magnetic field alterations was successfully adapted to a bSSFP sequence, allowing short acquisition times mandatory for MR guided interventions. Further investigations have to be performed to optimize and minimize the setup, possibly by use of a battery and optical relay switching.