An apparatus comprising: a flexible probe (14, 65) having a proximal portion and a distal portion adapted for insertion into a heart of a patient; a position sensor and an ablation electrode in the distal portion; a magnetic resonance imager; a processor linked to the position sensor and configured for sending control signals to the magnetic resonance imager, the processor cooperative with the magnetic resonance imager for: obtaining a first reading of the position sensor to obtain a first position when the distal portion is in a contacting relationship with a target tissue of the heart; acquiring a first magnetic resonance image of the heart at the first position; and thereafter iteratively performing the steps of: taking new readings of the position sensor to obtain second positions; wherein: the first magnetic resonance image is a magnetic resonance thermometry image; and the processor is further cooperative with the magnetic resonance imager for: determining for each second position whether a distance between the first position and the second position is less than a predetermined distance; triggering the acquisition of a new magnetic resonance thermometry image of the target tissue upon determination that the distance between the first position and the second position is less than the predetermined distance; and for analyzing the first magnetic resonance thermometry image and the new magnetic resonance thermometry image to determine a temperature of the target tissue. a flexible probe (14, 65) having a proximal portion and a distal portion adapted for insertion into a heart of a patient; a position sensor and an ablation electrode in the distal portion; a magnetic resonance imager; a processor linked to the position sensor and configured for sending control signals to the magnetic resonance imager, the processor cooperative with the magnetic resonance imager for: obtaining a first reading of the position sensor to obtain a first position when the distal portion is in a contacting relationship with a target tissue of the heart; acquiring a first magnetic resonance image of the heart at the first position; and thereafter iteratively performing the steps of: taking new readings of the position sensor to obtain second positions; obtaining a first reading of the position sensor to obtain a first position when the distal portion is in a contacting relationship with a target tissue of the heart; acquiring a first magnetic resonance image of the heart at the first position; and thereafter iteratively performing the steps of: taking new readings of the position sensor to obtain second positions; the first magnetic resonance image is a magnetic resonance thermometry image; and the processor is further cooperative with the magnetic resonance imager for: determining for each second position whether a distance between the first position and the second position is less than a predetermined distance; triggering the acquisition of a new magnetic resonance thermometry image of the target tissue upon determination that the distance between the first position and the second position is less than the predetermined distance; and for analyzing the first magnetic resonance thermometry image and the new magnetic resonance thermometry image to determine a temperature of the target tissue. determining for each second position whether a distance between the first position and the second position is less than a predetermined distance; triggering the acquisition of a new magnetic resonance thermometry image of the target tissue upon determination that the distance between the first position and the second position is less than the predetermined distance; and for analyzing the first magnetic resonance thermometry image and the new magnetic resonance thermometry image to determine a temperature of the target tissue.