## 8 Conclusions and Perspectives  
Full-field optical coherence tomography is a modified version of OCT that can access submicrometric resolution deep inside a sample. Similarly to OCT, it can provide label-free 3D imaging of ex vivo tissues, and can perform measurements similar to histology in terms of contrast and resolution. The interest of FF-OCT lies in its ability to obtain 3D images similar to histology with almost no tissue preparation, which can reduce slicing, staining, and fixation artifacts. FF-OCT can also significantly shorten the biopsy-to-diagnosis time, down to a few minutes. Unfortunately, FF-OCT diagnosis scores are not yet 100% when compared with histology. In this review, we highlighted three promising recent multimodal systems combining FF-OCT structural contrast with other specific contrasts such as mechanical, motility, and molecular contrasts, providing hope that FF-OCT diagnosis scores will increase in the future, and might even ultimately perform better than histology. We need to add that despite our choice here, there are other promising multimodal systems that could be of interest for diagnosis improvement. For example, polarization FF-OCT [21] can be used to increase the sensitivity to fibers, and should be able to detect fiber disorganizations, similarly to OCT measurements, e.g., that could detect polarization changes caused by age-related macular degeneration (AMD) or glaucoma [168]. Multicolor FF-OCT [22] might also help the detection of subtle wavelength-dependent scattering changes, related to tumor malignancy [69]. Despite all these exciting developments, and the possibility to combine many different modalities in a single optical system, we have to emphasize that a compromise will have to be found between speed, processing time, compactness, penetration depth and the number of contrasts available, especially when FF-OCT is used during surgery. We believe that a better characterization of FF-OCT (and D-FF-OCT) contrasts that would be performed specifically for each tissue would help to increase FF-OCT diagnosis ability, without even changing the optical design of the microscope. Beyond the scope of histology, we hope that the development of such multimodal systems will increase the potential interest in FF-OCT, not only as an ex vivo diagnosis tool, but also as a tool to answer complex biological questions. We showed in this article that FF-OCT reveals a label-free original contrast, and can be used either in ex vivo tissues, or in vivo in small animals. If scanning OCT techniques were now to be extensively used in vivo and in clinics, FF-OCT would suffer from the slow speed of 2D detectors that do not allow a dynamic correction of the sample position, which could cancel the sample motion; in vivo implementation is therefore challenging. However, a few in vivo experiments have been successfully performed in paralyzed or anesthetized small animals [35; 61], and in human skins pressed against an imaging window. We recently successfully performed a simultaneous FF-OCT, D-FF-OCT, and fluorescence measurement in living zebrafish larvae. We illustrate our ability to obtain a motility contrast in vivo, even in the presence of blood flow in Figure 9. The dynamic signal is saturated in the blood vessels, and below, but cells can be detected in between the vessels. Additionally, an important effort is being made to enable FF-OCT measurements in patients, with the recent development of two adaptive optics FF-OCT systems [169; 170] for retinal imaging, a FF-OCT system coupled with an OCT system to dynamically compensate for eye motion for cornea and retina assessment, and a dark field FF-OCT system [171] to record internal fingerprints. Additionally, our group recently developed an _en face_ matrix based approach of OCT, which allows to increase the penetration depth by a factor of two [172], and could be advantageous for in vivo experiments. Figure 9: In vivo dynamic FF-OCT in a 2 dpf zebrafish larva.