FIELD : medicine.SUBSTANCE : invention relates to medical equipment. Cavity determination apparatus for determining a cavity within an object comprises an introduction element for being introduced into the cavity, curve determination unit for determining contact curves and a cavity reconstruction unit for reconstructing the cavity based on the determined curves. Bendable segment of an introduction element for arranging within the cavity in different arrangements is configured for determining a curve thereof in different arrangements. Contact curves are defined by the bendable segment that is in contact with the inner wall of the cavity in different arrangements. Cavity reconstruction unit is configured to reconstruct the cavity depending on the contact curves.EFFECT : increased speed and accuracy of determining the cavity in the object is achieved due to the fact that much less catheter movements and measurements are required, since the determined curve of the catheter segment contains more geometric information than one point.13 cl, 3 dwg