An embodiment of the invention relates to an apparatus for determining a location of a surface of an internal organ of a body, the apparatus comprising: a marker support (12) having proximal and distal ends, an axis, a collapsed state and a deployed state, the marker support (12) comprising: at least three flexible filaments, which in the collapsed state lie within a relatively small radial distance from the axis of the marker support (12), and in the deployed state, each filament forms a loop, the loops each configured to abut a surface of an internal body tissue at an abutment region of each filament and at least three tissue markers (28), each tissue marker (28) located at an abutment region of a filament wherein projections of each of the abutment regions on a plane perpendicular to the axis of the marker support (12) each lie along a side of a polygon.
