## 4 FF-OCT and Biomechanics  
Since the early beginning of medicine, palpation has been used to qualitatively assess abnormal tissue stiffness associated with several diseases. Palpation-based diagnosis of heart, abdomen, and wounds diseases or infections has been reported in many ancient cultures, such as Greek, Egyptian, or Chinese medicine [72; 73]. The mechanical properties of tissues and cells are mostly related to their structure and function, and changes in a tissue's biomechanical properties can reveal its pathological state [74]. Notably, biomechanical changes can include the stiffness increase due to an extracellular matrix accumulation, including collagen accumulation, associated with liver fibrosis, and ultimately cirrhosis [75; 76]. Moreover, tumor development significantly and progressively impacts the tissue biomechanics [77]. More importantly, the tumor stiffness actively increases its progression, and thus its malignancy, partly through a mechanically-induced molecular circuit, leading to the reduction in the number of tumor suppressors [78]. Therefore, not only tissue stiffness, but also stiffness anisotropy can be used as relevant diagnosis parameters.  
In the 1980s, the thorough and quantitative measurement of tissue elasticity was developed, giving access to stiffness in the approximation of purely elastic deformation of tissues. Elastography has been  
Figure 2: Comparison between FF-OCT and Histology. (**A**,**B**) Large field image of a breast sample affected by ductal carcinoma in situ (DCIS) acquired with FF-OCT (**A**) and histology (H & E staining; (**B**–**E**) Zoom in from boxes represented in panel A, respectively showing a normal fibrous tissue, enlarged ducts, and the collagen organization around the carcinoma cells in a tumorous region [14]. These images have been acquired by LLTech, and are displayed here with their authorization. Scale bars in A and B represent 1 mm, and 200 \(\upmu\)m in C to E.  
introduced in biomedical imaging with ultrasound and MRI elastography [79; 80], and has been proven to be a precious diagnosis biomarker for physicians since then [81; 82]. In this paper, we will only discuss tissue biomechanics in comparison to palpation-based diagnosis, and therefore at macroscopic and mesoscopic scales. However, we can add that cellular and molecular biomechanics can also be significant biomarkers of diseases, but they stay difficult to measure in tissues. To our knowledge, the most promising technique to study microscopic biomechanics inside a tissue is fluorescent FRET sensors [83].  
In OCT, mechanical contrast has been introduced by [PERSON] in 1998 [84]. Since this pioneering work, many methods have been developed with success such as 2D, 3D, static or dynamic elastography [84; 85; 86; 87; 88]. In FF-OCT, our group adapted two similar approaches to measure mechanical contrast. The first approach is a static method based on a finite element 3D correlation algorithm that gives access to the 3D local strain tensor inside the samples [20], giving access to the local shear modulus, which is itself proportional to the tissue elasticity, or stiffness for soft tissues. The second approach is a quantitative dynamic method based on shear wave speed imaging, inspired by similar experiments performed in ultrasounds developed in our lab. Using a fast FF-OCT system working at up to 30,000 frames per second, we could measure the local shear wave speed which is directly related to the local shear modulus [38].  
In this section, we present a brief description of both methods and some results obtained on biological samples.