Sven Meissner

Alveolar dynamics in acute lung injury: heterogeneous distension rather than cyclic opening and collapse.

Objectives:To analyze alveolar dynamics in healthy and acid-injured lungs of ventilated mice. Protective ventilation is potentially lifesaving in patients with acute lung injury. However, optimization of ventilation strategies is hampered by an incomplete understanding of the effects of mechanical ventilation at the alveolar level. Design:In anesthetized and ventilated Balb/c mice, subpleural alveoli were visualized by darkfield intravital microscopy and optical coherence tomography. Setting:Animal research laboratory. Subjects:Male Balb/c mice. Interventions:Lung injury was induced by intratracheal instillation of hydrochloric acid. In control animals and mice with lung injury, ventilation pressures were varied between 0 and 24 cm H2O at baseline, 60 mins, and 120 mins, and alveolar distension and cyclic opening and collapse of alveolar clusters were analyzed. Measurements and Main Results:In normal lungs, alveolar clusters distend with increasing ventilation pressure in a sigmoid relationship. Although an increase in ventilation pressure from 0 to 24 cm H2O increases alveolar size by 41.5 ± 2.3% in normal lungs, alveolar distension is reduced to 20.6 ± 2.2% 120 mins after induction of lung injury by acid aspiration. Cyclic opening and collapse of alveolar clusters are neither observed in normal nor acid-injured lungs. Alveolar compliance is highest in small and distensible alveolar clusters, which are also most prone to acid-induced injury. Conclusions:Over the applied pressure range, volume changes in control and acid-injured mouse lungs result predominantly from alveolar distension rather than cyclic opening and collapse of alveolar clusters. Preferential loss of compliance in small alveolar clusters redistributes tidal volume to larger alveoli, which increases spatial heterogeneity in alveolar inflation and may promote alveolar overdistension.

Optical coherence tomography in biomedical research.

Optical coherence tomography (OCT) is a noninvasive, high-resolution, interferometric imaging modality using near-infrared light to acquire cross-sections and three-dimensional images of the subsurface microstructure of biological specimens. Because of rapid improvement of the acquisition speed and axial resolution of OCT over recent years, OCT is becoming increasingly attractive for applications in biomedical research. Therefore, OCT is no longer used solely for structural investigations of biological samples but also for functional examination, making it potentially useful in bioanalytical science. The combination of in vivo structural and functional findings makes it possible to obtain thorough knowledge on basic physiological and pathological processes. Advanced applications, for example, optical biopsy in visceral cavities, have been enabled by combining OCT with established imaging modalities. This report gives an outline of the state of the art and novel trends of innovative OCT approaches in biomedical research in which the main focus is on applications in fundamental research and pre-clinical utilization.

Simultaneous three-dimensional optical coherence tomography and intravital microscopy for imaging subpleural pulmonary alveoli in isolated rabbit lungs

There is a growing interest in analyzing lung mechanics at the level of the alveoli in order to understand stress-related pathogenesis and possibly avoid ventilator associated lung injury. Emerging quantitative models to simulate fluid mechanics and the associated stresses and strains on delicate alveolar walls require realistic quantitative input on alveolar geometry and its dynamics during ventilation. Here, three-dimensional optical coherence tomography (OCT) and conventional intravital microscopy are joined in one setup to investigate the geometric changes of subpleural alveoli during stepwise pressure increase and release in an isolated and perfused rabbit lung model. We describe good qualitative agreement and quantitative correlation between the OCT data and video micrographs. Our main finding is the inflation and deflation of individual alveoli with noticeable hysteresis. Importantly, this three-dimensional geometry data can be extracted and converted into input data for numerical simulations.

Three-dimensional Fourier domain optical coherence tomography in vivo imaging of alveolar tissue in the intact thorax using the parietal pleura as a window

In vivo determination of 3-D and dynamic geometries of alveolar structures with adequate resolution is essential for developing numerical models of the lung. A thorax window is prepared in anesthetized rabbits by removal of muscle tissue between the third and fourth rib without harming the parietal pleura. The transparent parietal pleura allows contact-free imaging by intravital microscopy (IVM) and 3-D optical coherence tomography (3-D OCT). We demonstrate that dislocation of the lung surface is small enough to observe identical regions in the expiratory and inspiratory plateau phase, and that OCT in this animal model is suitable for generating 3-D geometry of in vivo lung parenchyma. To our knowledge, we present a novel thorax window preparation technique for 3-D imaging of alveolar dynamics for the first time. The 3-D datasets of the fine structure of the lung beneath the pleura could provide a basis for the development of 3-D numerical models of the lung.

Improved three-dimensional Fourier domain optical coherence tomography by index matching in alveolar structures

Three-dimensional Fourier domain optical coherence tomography (3-D FDOCT) is used to demonstrate that perfusion fixation with a mixture of glutaraldehyde and paraformaldehyde does not alter the geometry of subpleural lung parenchyma in isolated and perfused rabbit lungs. This is confirmed by simultaneous imaging of lung parenchyma with intravital microscopy. To eliminate the diffraction index interfaces between alveolar pockets and walls, we fill the fixed lungs with ethanol by perfusing with gradually increasing concentrations. This bottom-up filling process leaves no remaining air bubbles in the alveolar structures, thus drastically improving the resolution and penetration depth of 3-D FDOCT imaging. We observe an approximately 18% increase in alveolar area after ethanol filling, likely due in large part to elimination of the air/tissue interfaces. 3-D OCT datasets acquired from ethanol-filled lungs allow segmentation of the ethanol-filled structures, which were formerly air-filled, and 3-D reconstruction of larger areas of subpleural alveolar structures. Our innovative process of filling the lungs with ethanol postperfusion fixation thus enables more accurate quantification of alveolar geometries, a critical component of modeling lung function.

Virtual four-dimensional imaging of lung parenchyma by optical coherence tomography in mice

In this feasibility study, we present a method for virtual 4-D imaging of healthy and injured subpleural lung tissue in the ventilated mouse. We use triggered swept source optical coherence tomography (OCT) with an A-scan frequency of 20 kHz to image murine subpleural alveoli during the inspiratory phase. The data acquisition is gated to the ventilation pressure to take single B-scans in each respiration cycle for different pressure levels. The acquired B-scans are combined off-line into one volume scan for each pressure level. The air fraction in healthy lungs and injured lungs is measured using 2-D OCT en-face images. Upon lung inspiration from 2 to 12 cm H(2)O ventilation pressure, the air fraction increases in healthy lungs by up to 11% and in injured lungs by 8%. This expansion correlates well with results of previous studies, reporting increased alveolar area with increased ventilation pressures. We demonstrate that OCT is a useful tool to investigate alveolar dynamics in spatial dimensions.

Four‐dimensional imaging of murine subpleural alveoli using high‐speed optical coherence tomography

The investigation of lung dynamics on alveolar scale is crucial for the understanding and treatment of lung diseases, such as acute lung injury and ventilator induced lung injury, and to promote the development of protective ventilation strategies. One approach to this is the establishment of numerical simulations of lung tissue mechanics where detailed knowledge about three-dimensional alveolar structure changes during the ventilation cycle is required. We suggest four-dimensional optical coherence tomography (OCT) imaging as a promising modality for visualizing the structural dynamics of single alveoli in subpleural lung tissue with high temporal resolution using a mouse model. A high-speed OCT setup based on Fourier domain mode locked laser technology facilitated the acquisition of alveolar structures without noticeable motion artifacts at a rate of 17 three-dimensional stacks per ventilation cycle. The four-dimensional information, acquired in one single ventilation cycle, allowed calculating the volume-pressure curve and the alveolar compliance for single alveoli.

Intravital microscopy of subpleural alveoli via transthoracic endoscopy

Transfer of too high mechanical energy from the ventilator to the lungs alveolar tissue is the main cause for ventilator-induced lung injury (VILI). To investigate the effects of cyclic energy transfer to the alveoli, we introduce a new method of transthoracic endoscopy that provides morphological as well as functional information about alveolar geometry and mechanics. We evaluate the new endoscopic method to continuously record images of focused subpleural alveoli. The method is evaluated by using finite element modeling techniques and by direct observation of subpleural alveoli both in isolated rat lungs as well as in intact animals (rats). The results confirm the overall low invasiveness of the endoscopic method insofar as the mechanical influences on the recorded alveoli are only marginal. It is, hence, a suited method for intravital microscopy in the rat model as well as in larger animals.

Defense of fake fingerprint attacks using a swept source laser optical coherence tomography setup

The most established technique for the identification at biometric access control systems is the human fingerprint. While every human fingerprint is unique, fingerprints can be faked very easily by using thin layer fakes. Because commercial fingerprint scanners use only a two-dimensional image acquisition of the finger surface, they can only hardly differentiate between real fingerprints and fingerprint fakes applied on thin layer materials. A Swept Source OCT system with an A-line rate of 20 kHz and a lateral and axial resolution of approximately 13 μm, a centre wavelength of 1320 nm and a band width of 120 nm (FWHM) was used to acquire fingerprints and finger tips with overlying fakes. Three-dimensional volume stacks with dimensions of 4.5 mm x 4 mm x 2 mm were acquired. The layering arrangement of the imaged finger tips and faked finger tips was analyzed and subsequently classified into real and faked fingerprints. Additionally, sweat gland ducts were detected and consulted for the classification. The manual classification between real fingerprints and faked fingerprints results in almost 100 % correctness. The outer as well as the internal fingerprint can be recognized in all real human fingers, whereby this was not possible in the image stacks of the faked fingerprints. Furthermore, in all image stacks of real human fingers the sweat gland ducts were detected. The number of sweat gland ducts differs between the test persons. The typical helix shape of the ducts was observed. In contrast, in images of faked fingerprints we observe abnormal layer arrangements and no sweat gland ducts connecting the papillae of the outer fingerprint and the internal fingerprint. We demonstrated that OCT is a very useful tool to enhance the performance of biometric control systems concerning attacks by thin layer fingerprint fakes.

Three-dimensional simultaneous optical coherence tomography and confocal fluorescence microscopy for investigation of lung tissue

Abstract. Although several strategies exist for a minimal-invasive treatment of patients with lung failure, the mortality rate of acute respiratory distress syndrome still reaches 30% at minimum. This striking number indicates the necessity of understanding lung dynamics on an alveolar level. To investigate the dynamical behavior on a microscale, we used three-dimensional geometrical and functional imaging to observe tissue parameters including alveolar size and length of embedded elastic fibers during ventilation. We established a combined optical coherence tomography (OCT) and confocal fluorescence microscopy system that is able to monitor the distension of alveolar tissue and elastin fibers simultaneously within three dimensions. The OCT system can laterally resolve a 4.9 μm line pair feature and has an approximately 11 μm full-width-half-maximum axial resolution in air. confocal fluorescence microscopy visualizes molecular properties of the tissue with a resolution of 0.75 μm (laterally), and 5.9 μm (axially) via fluorescence detection of the dye sulforhodamine B specifically binding to elastin. For system evaluation, we used a mouse model in situ to perform lung distension by application of different constant pressure values within the physiological regime. Our method enables the investigation of alveolar dynamics by helping to reveal basic processes emerging during artificial ventilation and breathing.