Ultradeep optical coherence tomography

Deep optical imaging of intact brain tissue is limited by scattering and absorption, restricting our ability to study subcortical structures. Here, we introduce ultradeep optical coherence tomography (OCT), which combines hardware and computational approaches to enhance the signal from deep tissue regions. Ultradeep OCT utilizes a longer illumination center wavelength of 1575 nm, spectral-domain balanced detection and imaging in the conjugate configuration with the zero optical path difference positioned deep in the tissue. Autocorrelation artifacts are measured and subtracted, and multiple frames are coherently averaged to improve the signal-to-noise ratio further. These techniques, along with computational refocusing and bandwidth control, allowed us to extend structural imaging depth in biological tissues. Ultradeep OCT demonstrated physical imaging depths of 2.3 to 2.8 mm (mean 2.5 mm) in the ex vivo mouse brain and 2.5 to 3.0 mm (mean 2.8 mm) in chicken tissue. The resulting volumes in the ex vivo mouse brain samples depict the dorsal hippocampus and its two major subregions with sufficient contrast to allow segmentation and three-dimensional reconstructions. Features in the thalamus could also be discerned. These findings demonstrate the potential of ultradeep OCT as a label-free approach for visualizing deep-brain structures at a spatial scale complementary to magnetic resonance imaging and microscopy. This approach could enable optical imaging studies of hippocampal structural changes in mouse models of neurodegenerative disease.