Microscopic optical coherence tomography for imaging of mucus transport

 Summary (Translation from the German original)

Several lung diseases such as cystic fibrosis (CF), asthma or COPD are associated with abnormalities of the mucus. In many cases, these changes lead to an obstruction of the airway purification. However, the exact mechanisms are often unknown. Therefore, the role of the mucus in lung diseases has been an active field of research for years. In many places, the effectiveness of clinical research in this field could be increased if it were possible to study the mucus and its response to therapeutic interventions directly and not via exogenous factors such as survival time in animal experiments or the improvement of lung function.

In this thesis, a technology was developed which enables the investigation of the transport of mucus in small animals such as mice as well as in the upper respiratory tract of humans. Essential criteria are a high spatial resolution to visualize dynamic changes on a cellular level as well as a high acquisition speed to be insensitive to any movement.

In the first part of the thesis a new microscopy system for intravital imaging of mucus transport in mice was developed. For this purpose, optical coherence tomography was further developed into microscopic optical coherence tomography (mOCT). By using a fast line scan camera a A-scan rate of 127 kHz (130 MHz pixel rate) could be achieved, which allowed up to 160 B-scans per second. By increasing the spatial resolution tenfold, both axially and laterally, it was possible to display microscopic structures. An axial resolution of 1.3 μm in air and 1 μm in tissue was achieved. The lateral resolution could be varied by selecting the objective lens and thus adapted to the measuring tasks and the object to be examined. With the objectives used in this work resolutions in the range of 1.75 μm up to 0.36 μm were reached. The investigation of enucleated tracheal tissue has shown that relevant cellular structures such as cilia, epithelial cells and mucus can be visualized by mOCT with microscopic resolution. The possibility of visualizing and quantifying different transport mechanisms of mucus transport by mOCT was demonstrated in an experiment on healthy, spontaneously breathing mice. For this purpose, saline solution mixed with particles was applied nasally and both the aspiration phase and the exhalation phase were investigated by means of mOCT. Not only the well investigated ciliary transport, but also a hundredfold faster transport mechanism caused by a cough-like reflex-like air flow, which has hardly been described in mice in the literature so far, could be shown. In an animal study on ß-ENaC overexpressed mice, which serve as a model for cystic fibrosis, the effect of the hydration of the mucus by isotonic and hypertonic saline solutions, which are used as therapeutics in CF therapy, was investigated. For comparison, the reaction of healthy animals to the saline solutions was studied. While no mucus was released in healthy animals, massive mobilization of mucus occurred in ß-ENaC overexpressed mice. Also, evidence could be found that hypertonic saline solution leads to a better transport of mucus. Thus, mOCT has the potential to provide direct feedback in the development of cystic fibrosis drugs.

In the second part of the thesis, microscopic OCT was further developed into an endoscopic version for the investigation of mucus transport in humans. Therefore, an endoscopic version of the mOCT was developed for the imaging of the concha nasalis. The 4.5 cm long endoscope based on GRIN lenses with a diameter of 2.5 mm offers an isotropic spatial resolution of 1.5 μm. Spherical and chromatic aberrations were used to significantly increase the depth of focus. This made it possible to visualize cellular structures over a depth measurement range of more than 100 μm Initial measurements on subjects with a frame rate of 80 Hz showed B-scans of the concha nasalis on which the silial movements, cellular structures of the epithelium, vessels and blood cells were visible. The transport velocity of the mucus was successfully determined to 0.165 mm/s. Endomicroscopic OCT has the potential to be used for the diagnosis of lung diseases already manifesting in the nasal mucosa, such as cystic fibrosis, asthma or primary ciliary dyskinesia.


The original language of the thesis was German entitled: “Mikroskopisch optische Kohärenztomographie für die Darstellung von Mukustransport.” 
Further parts of my thesis will be published on my site in the near future.