Using radiofrequency (rf) transmission system to find opening in tissue wall

A system, comprising:

• an interface, configured to receive, from a probe that comprises electrodes and is positioned inside a cavity in an organ of a patient, (i) proximity signals indicative of proximity of the electrodes to a wall of the cavity, and (ii) position signals indicative of positions of the electrodes within the cavity;
• and
• a processor, configured to:
• based on the proximity signals and the position signals, represent at least a portion of a volume of the cavity by a sphere model comprising multiple spheres;
• identify a direction along which one or more spheres are larger than one or more surrounding spheres by at least a given factor;
• based on the indicated direction, estimate a location of an opening in the wall of the cavity;
• and
• present the location of the opening to a user.