Optical coherence tomography
Optical coherence tomography (OCT) is a 3-D imaging technique based on low- coherence interferometry (Huang et al., 1991; Schmitt, 1999). OCT has a typical spatial resolution of 1-15 pm in the axial direction, which is decoupled from lateral resolution and solely determined by the bandwidth of the imaging light source. Generally, the laser source of OCT is in the infrared spectral range, providing a depth of penetration of up to several millimeters in scattering tissues. To ensure the imaging depth and avoid the axial scanning, a scan lens with low numerical aperture is usually utilized in OCT, which leads to a typical microscale lateral resolution. Fourier domain OCT (FDOCT) is currently the most widely used OCT technique, as it features greater sensitivity and higher imaging speed as compared to time domain OCT (TDOCT) (Leitgeb et al., 2003). In FDOCT, the depth-resolved scan (A-scan) is obtained through a simple Fourier transform of the interference fringes with respect to the wavenumber, where the amplitude information is utilized to form the structural OCT image, and the phase of light is extracted to achieve high sensitivity of tissue and cell displacement for functional imaging, such as Doppler OCT (Fercher et al.,
2003) . Three-dimensional OCT reconstruction is achieved by lateral scanning of the imaging beam with a field of view generally at the millimeter level. By employing a swept laser source (Choma et al., 2003), current FDOCT imaging provides a typical A-scan rate of up to hundreds of kHz (Drexler et al., 2014), and with an ultrafast swept laser source (such as Fourier domain mode-locked laser), OCT A-scan rate at MHz level has been demonstrated (Klein et al., 2013). With the unique imaging scale and only relying on the endogenous contrast, both structural and functional OCT imaging has been applied as useful tools for tissue engineering (Liang et al., 2009; Boppart et al., 2008).