In our daily interactions with others, oral communication is advantageously complemented by visual information that allows discriminating between sometimes ambiguous or contradictory signals. In optical imaging too, multimodality has emerged a powerful tool for better diagnosis or functional imaging. In pathology for instance, different dyes are used to stain different structures of the cell or different kinds of cell.

However, just like it is difficult to follow a conversation in a noisy room, there is another challenge for microscopy. While a thin fixed slice is easily imaged through a conventional microscope, imaging a thick tissue is hard: it is difficult to isolate the signal coming from a single depth from the background coming from all other positions in the sample. Some optical techniques provide a solution: the so-called “optical sectioning”, that allow recovering axial resolution and therefore reconstructing 3D images of samples.

In this paper, the authors combine two optical techniques with optical sectioning ability into one single microscope. Both are full field techniques (meaning that information is simply obtained on a camera, and not via scanning) and provide a very different contrast. Optical Coherence tomography (here FFOCM) relies on low-coherence interferometry to achieve sectioning and is sensitive to refractive index contrast, i.e. to the structure without the need for staining. Structured light Fluorescence microscopy, on the other hand, is specific and versatile depending on the label used.

The authors show the interest of this dual-modality 3D microscopy by performing volume images of fixed thick mouse retina. They demonstrate that it provides simultaneously, and with direct co-localization, complementary information (structure and function), hence obtaining an information-rich tridimensional image.

While dual modality techniques are not new, this approach combines very different contrast mechanisms to optical sectioning. It uses a simple full-field configuration, does not require an expensive laser source, and en-face images can be directly compared to histological methods. The relative simplicity of the design, the important added value of a dual-modality microscope over the sum of its parts for diagnosis, makes it an important advance for the field, and an interesting reading for anyone interested in biomedical optics.

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