Reflectance spectrometry for real-time hemoglobin determination of placental vessels during endoscopic laser surgery for twin-to-twin transfusion syndrome.

OBJECTIVE: The objective of the study was to develop a noninvasive technique to determine hemoglobin (Hb) content through spectral analysis of diffusely reflected broadband visible illumination from individual blood vessels during endoscopic fetal surgery for twin-to-twin transfusion syndrome (TTTS). METHODS: The reflection of an incoming xenon endoscopic light source was captured through a 630-mum-diameter optic fiber coupled to a fixed grating spectrometer (2-nm resolution). A 450- to 700-nm wavelength range was used for analysis. Three data-capturing methods were studied: (1) fixed-image spectrum capture with fiber aimed at (but not touching) center of a vessel, (2) no-touch scanning perpendicular to the vessel and dynamic spectral capture, and (3) dynamic spectral capture and analysis of the reflectance spectra during brief vessel touch. RESULTS: Eight controls (elective laparoscopic and thoracoscopic operations in children aged 1-17 years) were enrolled. Four vessels were analyzed in each case. The brief-touch technique with intensity peak analysis yielded the most reproducible results between multiple vessels in the same patient. Spectrometry was also applied to 2 TTTS patients. The (anemic) donor and (polycythemic) recipient twin fetuses could be differentiated with good correlation between vessels (arteries and vein) of the same fetus. CONCLUSIONS: It is possible to differentiate donor from recipient placental vessels by spectral analysis of the reflected light through the endoscope using a noninvasive and real-time method. This may improve the accuracy of endoscopic laser ablation of placental vessels in TTTS and may allow instant endoscopic Hb determination for laparoscopic procedures as well.

Fourier transform spectroscopy using stressed liquid crystal.

A Stressed Liquid Crystal (SLC) is proposed for application as a single panel retardance element in a Fourier transform (FT) spectrometer. Volume alignment in SLCs increase the maximum retardance and subsequent FT resolution by creating greater path lengths. Here, the relationship between transmission and shear for thick SLC cells is characterized and the spectral resolution using the SLC phase modulators in a single and double pass FT spectrometer system is quantified. For a 100 microm thick SLC, the resolution of a single frequency peak was observed at 60 nm full width half maximum.

Surgical performance with head-mounted displays in laparoscopic surgery.

INTRODUCTION: The difficulties of laparoscopic surgery include two-dimensional image projection and loss of alignment between the surgeon's hands and visual field. Head-mounted displays (HMDs) allow freedom from gazing at a stationary overhead monitor, thus improving ergonomics. Modern HMDs offer greatly improved image quality and reduced bulk and weight. We compared two types of HMDs with conventional overhead image display. MATERIALS AND METHODS: Twelve preclinical medical students (i.e., laparoscopic novices) completed the standardized bead-passing task in a Fundamentals of Laparoscopic Surgery box trainer, using a wall-mounted monitor(WALL), a solid-state high-resolution dual full-visual graphic array (VGA) HMD (HIGH-HMD), or a lightweight commercial 1/4 VGA HMD (LOW-HMD). Participants performed each task by using the three image displays. The order in which they performed each test was randomly assigned to minimize the carryover effect.Students were then asked to grade comfort and image quality on a scale from 1 (worst) to 5 (best). Statistical comparison of the time per bead was performed with the Kruskal-Wallis test, and P < 0.05 was considered significant. RESULTS: Average time per bead (total beads = 12/participant/test) was 14.2 seconds for WALL, 13.2 seconds for LOW-HMD, and 12.5 seconds for HIGH-HMD (P 0.05). The comfort ratings were 3.67 +/- 0.82, 3.50 +/- 1.38,and 3.83 +/- 0.75, respectively, and image quality was rated as 3.00 +/- 0.63, 2.83 +/- 1.47 and 4.67 +/- 0.52, respectively. CONCLUSIONS: The high-resolution HMD offered significantly better image quality and allowed faster task performance than a lower resolution model of HMD, and both performed better than the overhead display. The high-resolution HMD was not significantly more comfortable than the low-resolution model, given its added weight. HMDs alone may only be of incremental benefit in improving performance in laparoscopic surgery.However, their greatest promise is in their combination with other advances in imaging and image manipulation technology, as they open the door to individualized image display.

Electro-optical investigations of holographic-polymer-dispersed ferroelectric liquid crystals.

Uniform alignment of ferroelectric liquid-crystal domains encapsulated by a polymer binder was established through a holographic exposure process. The refractive index modulation in these thin films is modeled as a phase grating that can be electrically addressed to erase the optical diffractive properties. A phenomenological model is developed to take into account a distribution of domain sizes and an effective field that stabilizes the ferroelectric liquid-crystal domains. A diffraction model successfully predicts changes in normalized intensities for first-order diffraction with applied field. These gratings demonstrate microsecond-scale response and relaxation times for various grating pitch sizes between approximately 3 and approximately 12 microm.

Liquid-crystal materials find a new order in biomedical applications.

With the maturation of the information display field, liquid-crystal materials research is undergoing a modern-day renaissance. Devices and configurations based on liquid-crystal materials are being developed for spectroscopy, imaging and microscopy, leading to new techniques for optically probing biological systems. Biosensors fabricated with liquid-crystal materials can allow label-free observations of biological phenomena. Liquid-crystal polymers are starting to be used in biomimicking colour-producing structures, lenses and muscle-like actuators. New areas of application in the realms of biology and medicine are stimulating innovation in basic and applied research into these materials.

Switchable circular-to-point converter based on holographic polymer-dispersed liquid-crystal technology.

We demonstrate the use of a switchable circular-to-point converter (SCPC) device based on holographic polymer-dispersed liquid-crystal technology for application in lidar detection and optical telecommunication. A SCPC device converts the Fabry-Perot ring pattern into a single point or an array of points, while an external electrical field on the SCPC deactivates the conversion. Stacking different SCPC elements gives a random optical switch for applications in lidar detection and optical telecommunication. Two types of SCPC designs are analyzed and one is chosen and built for testing.

Holographic diffraction gratings using polymer-dispersed ferroelectric liquid crystals.

By controlling the morphology of holographic polymer-dispersed ferroelectric liquid crystals (FLCs), highly aligned FLC domains are obtained for diffractive optical applications. Rapid, thresholdless switching is observed for various grating pitch sizes between approximately 3 and approximately 12 microm. A simple phenomenological model is presented encompassing a distribution of domain sizes and an effective field that stabilizes the FLC domains to reflect the observed thresholdless switching and optical behavior.

Two-dimensional vectorial photonic crystals formed in azo-dye-doped liquid crystals.

A 2D, square lattice, vectorial photonic crystal is formed by vectorial holography using an azo-dye-doped liquid-crystal material. Four coherent beams are interfered to generate a highly stable, switchable polarization holography structure. The formation of the vectorial lattice by the liquid-crystal molecular orientation is confirmed by diffraction pattern and polarization microscopy analysis. Simulations of the alignment and diffraction pattern are in good agreement with the experimental results. Polarization sensitive diffraction behavior is also consistent with the Kakichashvili model.

Diffuse reflectance spectra of the palpebral conjunctiva and its utility as a noninvasive indicator of total hemoglobin.

The palpebral conjunctiva is an attractive location to qualitatively examine for the presence of anemia; however, this method of diagnosis has not been shown to be accurate. A spectroscopic examination of the palpebral conjunctiva enables the use of a quantitative parameter as a basis for diagnoses. Visible range diffuse reflectance spectra from the palpebral conjunctiva are examined from 30 patients and hemoglobin levels are extracted from these signatures using both a partial least-squares (PLS) multivariate regression model and a discrete spectral region model. Hemoglobin concentration derived from both these models is compared to an in vitro measurement of hemoglobin. Root mean squared errors of cross validation for the two analytical methods are 0.67 g/dL and 1.07 g/dL, respectively. Conjunctival reflectance spectra coupled with a PLS analysis achieve an enhanced specificity and sensitivity for anemia diagnoses over reported observational studies using the palpebral conjunctiva and achieve improved accuracy to other reported methods of noninvasive hemoglobin measurement.

Surface-induced orientational order in stretched nanoscale-sized polymer dispersed liquid-crystal droplets.

We investigate orientational ordering in stretched polymer-dispersed liquid-crystal (PDLC) droplets using deuterium nuclear magnetic resonance, in the nematic and isotropic phases. In the latter case, we estimate the surface order parameter S(0) and the thickness of the interfacial layer from the temperature-independent surface ordering model for an elliptical cavity with a varying aspect ratio. A simple phenomenological model well describes the quadrupole splitting frequency of NMR spectra in the isotropic phase. The strain dependence of S(0) suggests that stretching-induced changes in the orientation of polymer chains in the PDLC matrix noticeably affect liquid-crystal surface anchoring. Experimental results are supported by simulated NMR spectra obtained as output from Monte Carlo simulations of paranematic ordering in ellipsoidal droplets based on the Lebwohl-Lasher lattice model.

In-plane switching of cholesteric liquid crystals for visible and near-infrared applications.

We have investigated the in-plane switching of cholesteric liquid crystals for reflective wavelength shifters for visible and near-infrared applications. These devices are based on the elongation of the cholesteric pitch by an electric field perpendicular to the helical axis. The transmission notch-reflection peak position can be tuned continuously to a longer wavelength (redshift) by application of an in-plane electric field. The helix is completely unwound when the electric field is higher than the cholesteric-to-nematic transition field, and the sample is transformed to a transparent state. We have investigated the electro-optic performance of in-plane switching of cholesteric samples and developed a simple phenomenological model to describe the underlying electro-optic phenomena.

Template synthesis of nanophase mesocarbon.

Templating techniques are used increasingly to create carbon materials with precisely engineered pore systems. This article presents a new templating technique that achieves simultaneous control of pore structure and molecular (crystal) structure in a single synthesis step. With the use of discotic liquid crystalline precursors, unique carbon structures can be engineered by selecting the size and geometry of the confining spaces and selecting the template material to induce edge-on or face-on orientation of the discotic precursor. Here mesophase pitch is infiltrated by capillary forces into a nanoporous glass followed by slow carbonization and NaOH etching. The resulting porous carbon material exhibits interconnected solid grains about 100 nm in size, a monodisperse pore size of 60 nm, 42% total porosity, and an abundance of edge-plane inner surfaces that reflect the favored edge-on anchoring of the mesophase precursor on glass. This new carbon form is potentially interesting for a number of important applications in which uniform large pores, active-site-rich surfaces, and easy access to interlayer spaces in nanometric grains are advantageous.

Deuteron NMR study of molecular ordering in a holographic-polymer-dispersed liquid crystal.

Using deuteron nuclear magnetic resonance (NMR) and dynamic light scattering, we study the orientational order and dynamics of a BL038-5CB liquid-crystal mixture in a holographic polymer dispersed liquid-crystal material (HPDLC) as used for switchable diffractive optical elements. At high temperatures, where the liquid crystal is predominantly in the isotropic phase, the HPDLC deuteron NMR linewidth and transverse spin-relaxation rate T-12 are two orders of magnitude larger than in the bulk. The analysis shows that the surface-induced order parameter in HPDLC is significantly larger than in similar confining systems and that translational diffusion of molecules in the surface layer is at least two orders of magnitude slower than in the rest of the cavity. The unusual temperature dependence of T-12 upon cooling suggests the possibility of a partial separation of the 5CB component in the liquid-crystal mixture. The onset of the nematic phase in HPDLC occurs at considerably lower temperature than in the bulk and takes place gradually due to different sizes and different content of non-liquid-crystalline ingredients in droplets. Parts of the droplets are found isotropic even at room temperature and the structure of the nematic director field in the droplets is only slightly anisotropic. We point out the capability of NMR to detect the actual state of liquid-crystalline order in HPDLCs and to contribute in this way to the improvement of the switching efficiency of diffraction gratings.