Methods, apparatuses and computer program products implement embodiments of the present invention that include receiving, from an imaging system operating in an image coordinate system, a three-dimensional image of a body cavity including open space and body tissue, and receiving, from a medical probe having a location sensor and inserted into the body cavity, a signal indicating a location of a distal tip of the medical probe in a sensor coordinate system. The image coordinate system is registered with the sensor coordinate system so as to identify one or more voxels in the three-dimensional image at the indicated location, and when the identified voxels have density values in the received three-dimensional image that do not correspond to the open space, the density values of the identified voxels are updated to correspond to the open space.
