Second harmonic imaging of collagen in mammalian tissue

It has recently been demonstrated that collagen is a very effective 'upconverter' of light by second harmonic generation (SHG) but hitherto the potential this offers for biomedical imaging has not been realised. We show that bright SHG images van be obtained over a wide excitation range at illumination levels comparable to or lower than those required for two-photon excitation of fluorescent labels, with no damage to the collagen structure. Both paraffin and cryostat sections have been used, and medically significant results have been obtained in several fields. We show that the signal is easily distinguished from single and two-photon excited fluorescence by its forward propagation and narrow spectral width; in principle it could also be distinguished by lifetime. Key microscope requisites are: immersion objectives and condensers, high-efficiency PMT detectors for transmitted light, suitable filters, and effective blocking of stray light, especially from the mercury lamp.