Optical coherence tomography.

Optical coherence tomography (OCT) is a non-contact method for imaging the topological and internal microstructure of samples in three dimensions. OCT can be configured as a conventional microscope, as an ophthalmic scanner, or using endoscopes and small diameter catheters for accessing internal biological organs. In this Primer, we describe the principles underpinning the different instrument configurations that are tailored to distinct imaging applications and explain the origin of signal, based on light scattering and propagation. Although OCT has been used for imaging inanimate objects, we focus our discussion on biological and medical imaging. We examine the signal processing methods and algorithms that make OCT exquisitely sensitive to reflections as weak as just a few photons and that reveal functional information in addition to structure. Image processing, display and interpretation, which are all critical for effective biomedical imaging, are discussed in the context of specific applications. Finally, we consider image artifacts and limitations that commonly arise and reflect on future advances and opportunities.

Automated noise estimation in polarization-sensitive optical coherence tomography.

Advanced signal reconstruction in polarization-sensitive optical coherence tomography (OCT) frequently relies on an accurate determination of the signal noise floor. However, current methods for evaluating the noise floor are often impractical and subjective. Here we present a method using the degree of polarization uniformity and known speckle intensity statistics to model and estimate the OCT noise floor automatically. We establish the working principle of our method with a series of phantom experiments and demonstrate the robustness of our noise estimation method across different imaging systems and applications in vivo.

Evaluation and review of body fluids saliva, sweat and tear compared to biochemical hydration assessment markers within blood and urine.

Evaluating and testing hydration status is increasingly requested by rehabilitation, sport, military and performance-related activities. Besides commonly used biochemical hydration assessment markers within blood and urine, which have their advantages and limitations in collection and evaluating hydration status, there are other potential markers present within saliva, sweat or tear. This literature review focuses on body fluids saliva, sweat and tear compared to blood and urine regarding practicality and hydration status influenced by fluid restriction and/or physical activity. The selected articles included healthy subjects, biochemical hydration assessment markers and a well-described (de)hydration procedure. The included studies (n=16) revealed that the setting and the method of collecting respectively accessing body fluids are particularly important aspects to choose the optimal hydration marker. To obtain a sample of saliva is one of the simplest ways to collect body fluids. During exercise and heat exposures, saliva composition might be an effective index but seems to be highly variable. The collection of sweat is a more extensive and time-consuming technique making it more difficult to evaluate dehydration and to make a statement about the hydration status at a particular time. The collection procedure of tear fluid is easy to access and causes very little discomfort to the subject. Tear osmolarity increases with dehydration in parallel to alterations in plasma osmolality and urine-specific gravity. But at the individual level, its sensitivity has to be further determined.

Towards high resolution optical imaging of beta cells in vivo.

Endocrine beta cells produce and release insulin in order to tightly regulate glucose homeostasis and prevent metabolic pathologies such as Diabetes Mellitus. Optical imaging has contributed greatly to our current understanding of beta cell structure and function. In vitro microscopy of beta cell lines has revealed the localization of molecular components in the cell and more recently their dynamic behavior. In cultured islets, interactions of beta cells with other islet cells and the matrix as well as paracrine and autocrine signaling or reaction to nutrients have been studied. Lastly, microscopy has been performed on tissue sections, visualizing the islets in an environment closer to their natural surroundings. In most efforts to date, the samples have been isolated for investigation and hence have by definition been divorced from their natural environments and deprived of vascularization and innervations. In such a setting the beta cells lack the metabolic information that is primordial to their basic function of maintaining glucose homeostasis. We review optical microscopy; its general principles, its impact in decoding beta cell function and its recent developments towards the more physiologically relevant assessment of beta cell function within the environment of the whole organism. This requires both large imaging depth and fast acquisition times. Only few methods can achieve an adequate compromise. We present extended focus Optical Coherence Microscopy (xfOCM) as a valuable alternative to both confocal microscopy and two photon microscopy (2PM), and discuss its potential in interpreting the mechanisms underlying glucose homeostasis and monitoring impaired islet function.

In vivo imaging of murine endocrine islets of Langerhans with extended-focus optical coherence microscopy.

AIMS/HYPOTHESIS: Structural and functional imaging of the islets of Langerhans and the insulin-secreting beta cells represents a significant challenge and a long-lasting objective in diabetes research. In vivo microscopy offers a valuable insight into beta cell function but has severe limitations regarding sample labelling, imaging speed and depth, and was primarily performed on isolated islets lacking native innervations and vascularisation. This article introduces extended-focus optical coherence microscopy (xfOCM) to image murine pancreatic islets in their natural environment in situ, i.e. in vivo and in a label-free condition. METHODS: Ex vivo measurements on excised pancreases were performed and validated by standard immunohistochemistry to investigate the structures that can be observed with xfOCM. The influence of streptozotocin on the signature of the islets was investigated in a second step. Finally, xfOCM was applied to make measurements of the murine pancreas in situ and in vivo. RESULTS: xfOCM circumvents the fundamental physical limit that trades lateral resolution for depth of field, and achieves fast volumetric imaging with high resolution in all three dimensions. It allows label-free visualisation of pancreatic lobules, ducts, blood vessels and individual islets of Langerhans ex vivo and in vivo, and detects streptozotocin-induced islet destruction. CONCLUSIONS/INTERPRETATION: Our results demonstrate the potential value of xfOCM in high-resolution in vivo studies to assess islet structure and function in animal models of diabetes, aiming towards its use in longitudinal studies of diabetes progression and islet transplants.

Extended focus depth for Fourier domain optical coherence microscopy.

We report on a new detection scheme for Fourier domain optical coherence microscopy that exhibits high transverse resolution along an axially extended focal range. Nearly constant transverse resolution of approximately 1.5 microm along a focal range of 200 microm is experimentally verified with a maximum sensitivity of 105 dB. A broad-bandwidth Ti:sapphire laser allowed for an axial resolution of 3 microm in air.