A method for imaging, including counting quanta of energy emitted into a range of angles from particles of an energy emitter distributed over a volume in a body, thereby generating a set of counts. The method further includes defining a probability distribution expression that specifies a local concentration of the particles of the energy emitter over the volume as a function of the set of counts, the function being defined in terms of respective coefficients of a plurality of different scales of the local concentration, including at least a first and a second scale. A dependence of the coefficients of the first scale on the coefficients of the second scale is specified, and the local concentration over the volume is computed by applying the probability distribution expression to the set of counts subject to the specified dependence.