Back-illumination interference tomography for imaging weak scattering in thick tissues

Biological tissues are composed of discrete compartments of biochemical media that often exhibit subtle differences in refractive index. Light propagating through these compartments partially diffracts in a forward direction with a $\pi /2$ phase shift. We introduce a microscopy technique to image this scattering signal in thick tissues, called back-illumination interference tomography (BIT). An incoherent source is demagnified and imaged past the focal plane of a high-numerical aperture objective lens, producing a small, semicoherent source of backscattered light. This backscattered light undergoes a phase inversion over the narrow depth of field of the microscope, providing interference contrast to weakly scattering objects at the focal plane. BIT offers a different source of contrast to conventional illumination and oblique back-illumination microscopy. Compared to these techniques, we show that BIT improves contrast to blood cells in vitro in microfluidic chambers and in vivo in a human capillary. Finally, we apply BIT to unstained, unlabeled bulk human tissue ex vivo and compare side-by-side to adjacent frozen sections stained with hematoxylin and eosin. These results demonstrate the potential of BIT to provide high-resolution, high-speed, 3D imaging of unprocessed biological tissues.