The aim of this paper is to present an extension of the original non-intrusive radioactive particle tracking method (RPT) to any geometries with irregular moving boundaries. The principal advantage of RPT over other non-intrusive methods is that it enables the visualization of rather large systems. However, the underlying reconstruction algorithm is limited to cylindrically shaped systems such as fluidized beds and columns. It excludes a wide variety of systems involving multiphase flows such as, for instance, spherical reactors, cyclones and powder silos, hoppers and blenders, all of which are thus currently out of reach of current RPT capabilities. This work addresses these limitations and proposes an approach that solves the inverse map problem to reconstruct the tracer position with time by using a mesh of unstructured cells to discretize the system geometry and kinematics. The anisotropy induced by the gas-solid interface is discussed and taken into account in the proposed model. To show the possibilities and assess the performance of the developed technique, the flow of particles in a 16-qt V-blender is mapped and the mean velocity field is computed. (c) 2006 Elsevier B.V. All rights reserved.