A catheter-based tracking system includes one or more field generators and a processor. The one or more field generators, located adjacent to a Magnetic Resonance Imagining (MRI) system, are configured to apply AlternatingCurrent (AC) magnetic fields. The processor is configured to receive signals, which are produced responsively to the AC magnetic fields in a single axis sensor (SAS) that is fitted at a distal end of a catheter inserted into an organ of a patient, to calculate, based on the signals received from the SAS, a direction of the distal end, to receive from a Hall effect sensor that is fitted at the distal end of the catheter a sensed component of a Direct Current (DC) magnetic field of the magnetic resonance imagining (MRI) system, and, based on the calculated direction and the sensed component of the DC magnetic field, to calculate a roll-angle of the distal end inside the organ.
