Advanced applications of optical time-stretch in ultrafast tissue and single-cell imaging

Abstract

There has been tremendous effort in advancing ultrafast imaging technologies, ranging from a burst-mode imaging platform to a real-time continuous imaging platform. While burst-mode platform can have a frame-rate up to THz favoring fundamental scientific discovery such as taking snapshots of photons’ motion, real-time continuous ultrahigh-speed imaging platform empowers high-throughput screening applications such as rare-cancer cell screening at >100,000 cells/s. However, the conflicting nature among imaging speed, resolution and sensitivity, which virtually appeared in all optical imaging systems, are often overlooked. The sole enthusiasm in pushing imaging frame rate has in fact hindered the translation of ultra-fast imaging technique to clinical application. Over the years, researchers have been seeking for different ways to overcome this fundamental challenge, which is also the focus of this thesis. We aim to apply continuous ultrafast imaging platforms, i.e. optical time-stretch imaging microscopy and swept source optical coherence tomography (OCT), which also provide high-resolution and high-sensitivity required for biological imaging, to biological problems from tissue level to a single-cell level. First, a wavelength-swept light source that is optimized for ultrafast time-stretch imaging is developed. It is the first ultrafast fiber-laser to date that could offer broadband spectrum at short near-infrared wavelengths (1060 nm) with high temporal stability required for multi-MHz OCT. Next, we explore the applicability of swept-source optical coherence tomography to study the regenerative process of a millimeter-long flat worm, planarian. By label-free imaging the whole worm, we decipher the optical properties of regenerated tissue, providing new insights to the field of regenerative medicine. Regarding single-cell analysis (SCA), we perform label-free cell-cycle study of breast cancer cells by optical time-stretch imaging microscopy, multi-ATOM – an ultrafast quantitative phase imaging modality operating at an imaging line-scan rate of 10’s MHz. With the biophysical phenotypes extracted from each individual cell, we can characterize the cell-cycle phase of each cell label-free in a highly scalable manner, from 1,000 to 1,000,000 cells. Such label-free and scalable study has significance in relieving the laborious burden for biologist in handling biochemical markers, that is often slow and costly. More importantly, label-free phenotypes provide new biophysical insight into genetic dependent cancer therapy. Finally, we employ multi-ATOM for label-free detection of cancer cells and cancer-cell-subtype classification. This is made possible by ultrahigh-throughput single-cell imaging capability at sub-cellular resolution, providing high-dimensional and large-scale single-cell analysis. We demonstrate that high-dimension (up to 17) biophysical phenotypes of single cells can be extracted from multi-ATOM and are sensitive to reveal heterogeneity, including cancer sub-types, in a large population of cells (up to 1,000,000). This label-free technique could have the potential to complement start-of-the-art platform for detecting circulating tumor cells (CTCs), that is recognized to be have important prognostic and diagnostic implications, especially in view of the known heterogeneity in cancer which response to specific types of cancer therapies. All in all, this thesis elucidates the utility of ultrafast optical time-stretch imaging technologies, both in-vitro and in-vivo, for next-generation of biological research as well as clinical diagnosis where high-throughput and high-content analysis is needed.