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Physical methods make it possible to combine analytical imaging with isotopic labelling in biological studies. With radioactive isotopes, trackradioautography may be used (in parallel with conventional grain-density radioautography) for high lateral resolution, even with energetic β-rays; in macroradioautography, filmless methods (gaseous detectors, scintillation counters, and storage phosphor screen devices) have remarkable performances. Neutron capture radiography is used mainly for the detection and imaging of one stable isotope of a few elements which have no radioisotope of practical use. With nuclear microprobes, nuclear reaction analysis and scattering analysis may serve to discriminate between isotopes (including stable isotopes). Secondary ion mass spectrometry images any isotope of almost any element with very good detection limits and a resolution better (sometimes much better) than 1 µm. Preventing the diffusion of mobile substances during the preparation of the biological specimens is still a difficult problem.

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