  Phase-sensitive coherent imaging exploits changes in the phases of
backscattered light to observe tiny alterations of scattering structures or
variations of the refractive index. But moving scatterers or a fluctuating
refractive index decorrelate the phases and speckle patterns in the images. It
is generally believed that once the speckle pattern has changed, the phases are
scrambled and any meaningful phase difference to the original pattern is
removed. As a consequence, diffusion and tissue motion below the resolution
handicap phase-sensitive imaging of biological specimen. Here, we show that,
surprisingly, a phase comparison between decorrelated speckle patterns is still
possible by utilizing a series of images acquired during decorrelation. The
resulting evaluation scheme is mathematically equivalent to methods for
astronomic imaging through the turbulent sky by speckle interferometry. We thus
adopt the idea of speckle interferometry to phase-sensitive imaging in
biological tissues and demonstrate its efficacy for simulated data and for
imaging of photoreceptor activity with phase-sensitive optical coherence
tomography. The described methods can be applied to any imaging modality that
uses phase values for interferometry.
