1. A method for imaging, comprising: 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; 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, comprising at least a first and a second scale; specifying a dependence of the coefficients of the first scale on the coefficients of the second scale; and computing the local concentration over the volume by applying the probability distribution expression to the set of counts subject to the specified dependence.