In the imaging of airway tissue, optical coherence tomography (OCT) provides cross-sectional images of tissue structures, shows cilia movement and mucus secretion, but does not provide sufficient contrast to differentiate individual cells. By using fast sequences of microscopic resolution OCT (mOCT) images, OCT can use small signal fluctuations to overcome lack in contrast and speckle noise. In this way, OCT visualizes airway morphology on a cellular level and allows the tracking of the dynamic behavior of immune cells, as well as mucus transport and secretion. Here, we demonstrate that mOCT, by using temporal tissue fluctuation as contrast (dynamic mOCT), provides the possibility to study physiological and pathological tissue processes in vivo.

An original method to heat cultured cells using a 1.94 µm continuous-wave thulium laser for biological assessment is introduced here. Thulium laser radiation is strongly absorbed by water, and the cells at the bottom of the culture dish are heated through thermal diffusion. A laser fiber with a diameter of 365 µm is set about 12 cm above the culture dish, without any optics, such that the laser beam diameter is almost equivalent to the inner diameter of the culture dish (30 mm). By keeping a consistent amount of culture medium in each experiment, it is possible to irradiate the cells with a highly reproducible temperature increase. To calibrate the temperature increase and its distribution in one cell culture dish for each power setting, the temperature was measured during 10 s of irradiation at different positions and at the cellular level. The temperature distribution was represented using a mathematical graphics software program, and its pattern across the culture dish was in Gaussian form. After laser irradiation, different biological experiments could be performed to assess temperature-dependent cell responses. In this manuscript, viability staining (i.e., distinguishing live, apoptotic, and dead cells) is introduced to help determine the threshold temperatures for cell apoptosis and death after different points in time. The advantages of this method are the preciseness of the temperature and the time of heating, as well as its high efficiency in heating cells in a whole cell culture dish. Furthermore, it allows for study with a wide variety of temperatures and time durations, which can be well-controlled by a computerized operating system.

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