Chronic total occlusions (CTOs) are defined as complete occlusions of an artery older than one month. Minimally invasive catheter-based interventions commonly employed for partial occlusions (e.g., balloon angioplasty followed by stenting) are problematic in CTOs because of the phycisian's inability to pass the device through the occlusion without a significant risk of arterial wall perforations. Furthermore, successfully treated CTOs exhibit a high re-occlusion rate. As a result, these cases are mostly sent to bypass surgery. With the advent of drug-eluting stents that reduce the incidence of re-occlusion, and thus, eliminating the second problem, new devices have begun to emerge that aim to recanalize CTOs without the cost and trauma of bypass surgery. These devices, however, need effective image guidance methods to ensure successful crossing of the CTOs.
Optical coherence tomography (OCT) is being evaluated as an intravascular imaging modality for guiding catheter-based interventions of CTOs. Occluded ex vivo human arterial samples were used to produce longitudinal cross-sections using an OCT system. These OCT images were compared with histology to assess OCT's ability to identify different components of the occluded artery, evaluate the imaging depth, and determine the ability to detect the underlying vessel wall.
Given the inherent difficulties of creating a mechanically scanning OCT probe in the distal tip of a catheter for use in a stenotic artery, we directed our initial efforts towards developing a "motionless" fiber based OCT system using a single mode fiber array. We discuss design considerations for implementing a forward viewing intravascular OCT probe.
The work is devoted to photoluminescent investigation of arterial walls in order to create a new navigation method for minimally invasive treatment of cardiovascular decease in the presence of chronic total occlusions. The method uses the distinct photoluminescent properties of arterial wall and chronic total occlusion plaque to alert the interventionalist when a fiber-optic equipped catheter is in contact with the vessel wall. We conducted a study to compare the photoluminescence properties of healthy and stenosed vessel walls, and a typical chronic total occlusion plaque in the spectral range 300-700 nm. All samples were obtained from human tibial arteries. These groups of arterial samples showed easily differentiable luminescence amplitude and spectral characteristics. The photoluminescent properties of intact and intentionally damaged vessel walls were also investigated to permit detection of artery perforation that could take place during the revascularization. Using optical excitation of different wavelength gives additional opportunities of detecting arterial plaques requiring laser treatment. The results presented are complemented with micro-computed tomography images and histology of the segments analyzed.
Our aim is to use nanocrystals (NC) to study endothelial cell biology, in particular, the cell surface receptor Tie2. Tie2 is highly expressed in endothelial cells and is critical for angiogenesis and vascular maintenance. Conjugating NCs to the Tie2 ligands, the angiopoietins, and tracking their characteristic luminescence lines will allow us to study the regulation of Tie2 in vitro and in vivo. To study NC behavior in a relevant biological system, endothelial cells were grown and cultured in vitro. Two different types of NC were made and tested: two-color core-shell CdSe/ZnS and more complicated nanostructures containing also Au clusters. Measurements were performed in specially prepared media with different pH values, as well as in the cell growth medium. The positions of NC-related luminescence lines were not influenced by the kind of media which makes them suitable for biolabeling the cell surface receptor Tie2. At the same time, the relative magnitude of the NC peaks depends on the pH of the medium and can therefore be used for characterization of the latter. Confocal microscopic images show that NCs with different ligands demonstrate different distributions inside living cells.
Solution-synthesized nanocrystals which manifest strong quantum-confinement effects enable size-tunability of spectral properties and strong excitonic effects. Lead sulfide (PbS) nanocrystals are especially interesting for applications in telecommunication because their resonance is tunable to wavelengths from 1.3-1.55 μm and beyond. In other quantum dot systems, optically-induced bleaching of absorption has been shown to lead to a strong nonlinearity in the vicinity of the exciton peak wavelength [1][2-4][5-8]. We report herein results of picosecond-resolved transient absorption in spin-processible solution-synthesized PbS nanocrystals across the wavelength range 1100 nm to 1600 nm. The sample was synthesized using the solution phase organometallic method (hot injection technique), which provides good control over the size of the nanocrystals [9]. The sample consisted of nanocrystals with diameter around 5.2 nm resulting in an exciton peak at 1330 nm. Since the Bohr radius for the bulk PbS is 18 nm, these nanocrystals lay within the strong quantum-confinement regime [10]. Time-resolved absorption was studied using the single-wavelength collinear pump-probe setup. From the observed double-exponential decay trace of the transmission, fast and slow time constants were extracted. The fast component of few 10is of ps was attributed to Auger recombination. The slow component is on the order of ns. The saturation intensity was also measured in this wavelength range using the Z-Scan technique [11]. The open aperture signals were fit to the intensity-dependent absorption model. The value of the saturation intensity was found to be 0.6 GW/cm2 around the exciton peak.
We analyze the intensity-dependent optical response of passive optical limiters realized using distributed feedback structures which consist of alternating layers of materials possessing opposite Kerr nonlinearities. By elaborating an analytical model and employing numerical simulations, we explore device performance with respect to key requirements for passive optical limiter deployment. We prove that the proposed limiting mechanism results in complete clamping of transmitted intensity to a sensor-safe limiting value, independent of incident intensity. We provide a direct analytical expression for this limiting intensity in terms of structural and material parameters.
We propose and quantify a future role for photonic crystals in optical signal processing. We analyze the optical signal processing functionality of nonlinear periodic structures. By elaborating an analytical model and employing numerical simulations, we explore the performance of proposed devices. We prove that the proposed limiters provide true limiting by clamping the transmitted intensity at a level which is independent of the incident intensity. We explore the response of optical switches for signal and pump beams having the same and different frequencies. We describe and quantify the performance of the proposed structures in the realization of optical hard-limiters. We explore the performance of an all-optical logic gate whose forward- directed output implements a binary AND and whose backward- directed output implements an OR function. In addition we prove that the for fabrication errors as large as 10%, qualitative device functionality remains, with performance only modestly degraded.
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