ST-ODT: Spatiotemporal Optical Diffraction Tomography

The University Of Central Florida Board Of Trustees, Orlando FL

Investigators

Abstract

The development of tools for high-resolution three-dimensional optical imaging of biological cells and tissues is highly desirable for fundamental and applied research in biology and medicine. It enables quantitative biology: understanding complex interactions and functional properties of biological systems through quantitative measurements, for example monitoring of spatiotemporal activity patterns in neuronal networks. Since cells and tissues are transparent to light, existing imaging methods have often relied on foreign contrast agents and fluorescent markers. Label-free three-dimensional imaging based on light refraction, i.e., measurement of the spatial distribution of the refractive index, obviates the need of chemical clearing, thereby preserving the functional information. However, label-free refractive-index imaging of cells and tissues is difficult because they are highly scattering. This challenge will be addressed in this project by use of optical diffraction tomography with ultrashort laser pulses, together with techniques from digital holography and processing tools developed for communication systems. Our proposed approach, called spatiotemporal optical diffraction tomography combines ballistic imaging and optical diffraction tomography. Optical diffraction tomography cannot be directly applied to refractive-index imaging of highly-scattering objects because it is based on the first-Born/Rytov approximation. Optical diffraction tomography cannot use pulsed illumination because the inversion algorithm is based on continuous-wave diffraction. We show that with pulsed illumination, the time-integrated impulse response field is exactly the same as the diffracted field with continuous-wave illumination. Therefore, by adjusting the time-integration window for coherently-detected diffraction of pulsed plane-wave illumination, we not only obtain the equivalent continuous-wave diffraction field but also reject multiply-scattered and diffuse light, making continuous mapping of the refractive index of highly-scattering three-dimensional phase objects possible. Our proposed research includes the following components: 1) Determining classes of three-dimensional objects suitable for spatiotemporal optical diffraction tomography through simulation; 2) Optimizing performance of spatiotemporal optical diffraction tomography and, investigating the performance limits; and 3) Experimental demonstration of spatiotemporal optical diffraction tomography. Spatiotemporal optical diffraction tomography is based on the equivalence of the time-integrated impulse response field and the continuous-wave diffracted field, a property that has not been heretofore applied to imaging. The interplay between temporal and spatial propagation effects represents a paradigm shift in tomography that is uniquely suitable for three-dimensional imaging in the presence of multiple scattering. The problem is similar to multiple-input-multiple-output communication systems and can benefit from that body of knowledge. The idea of characterizing an inhomogeneous medium with sufficient accuracy to solve an inverse propagation problem is also most interesting and challenging, and has applications to other disciplines. Since spatiotemporal optical diffraction tomography can provide continuous three-dimensional maps of the refractive index, it also enables focusing light onto specific locations within a highly-scattering three-dimensional phase object. The implications are far reaching.