  Optical coherence tomography (OCT) is capable of non-destructively obtaining
cross-sectional information of samples with micrometer spatial resolution,
which plays an important role in ophthalmology and endovascular medicine.
Measuring OCT amplitude can obtain three-dimensional structural information of
the sample, such as the layered structure of the retina, but is of limited use
for functional information such as tissue specificity, blood flow, and
mechanical properties. OCT functional imaging techniques based on other optical
field properties including phase, polarization state, and wavelength have
emerged, such as Doppler OCT, optical coherence elastography,
polarization-sensitive OCT, and visible-light OCT. Among them, functional
imaging techniques based on dynamic changes of amplitude have significant
robustness and complexity advantages, and achieved significant clinical success
in label-free blood flow imaging. In addition, dynamic light scattering OCT for
3D blood flow velocity measurement, dynamic OCT with the ability to display
label-free tissue/cell specificity, and OCT thermometry for monitoring the
temperature field of thermophysical treatments are the frontiers in OCT
functional imaging. In this paper, the principles and applications of the above
technologies are summarized, the remaining technical challenges are analyzed,
and the future development is envisioned.
