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 Alternating Current (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.
