We devised a method to allow for retrospective registration of tomographic images with very different information content, the main emphasis being on sets of positron emission tomography images obtained with different tracers. A multivariate cost-function based on information theory was used as an index of "goodness-of-alignment". The cost-function makes no assumptions regarding the form of the relationship between the two image sets, and is hence very general. Image volumes, with known relative spatial orientation, were simulated for tracers with different uptake patterns and the method was validated by assessing its ability to recover these known orientations. The method was able to recover the known transformations with an accuracy of about 1 mm and 1 degrees along and around all axes, even for tracer combinations with radically different uptake patterns and with large initial misalignment. With the suggested method it is feasible to retrospectively align examinations obtained with different tracers and/or modalities.