Biomedical subjects

Gracie Vargas

Publications and source records attributed to Gracie Vargas.

6 recordsLinked to original sources

PubMed2005

Depth-resolved phase retardation measurements for laser-assisted non-ablative cartilage reshaping.

Since polarization-sensitive optical coherence tomography (PS-OCT) is emerging as a new technique for determining phase retardation in biological materials, we measured phase retardation changes in cartilage during local laser heating for application to laser-assisted cartilage reshaping. Thermally-induced changes in phase retardation of nasal septal cartilage following Nd:YAG laser irradiation were investigated using a PS-OCT system. A PS-OCT system and infrared imaging radiometer were used to record, respectively, depth-resolved images of the Stokes parameters of light backscattered from ex vivo porcine nasal septal cartilage and radiometric temperature changes following laser irradiation. PS-OCT images of cartilage were recorded before (control), during and after laser irradiation. From the measured Stokes parameters (I, Q, U and V), an estimate of the relative phase retardation between two orthogonal polarizations was computed to determine birefringence in cartilage. Phase retardation images of light backscattered from cartilage show significant changes in retardation following laser irradiation. To investigate the origin of retardation changes in response to local heat generation, we differentiated two possible mechanisms: dehydration and thermal denaturation. PS-OCT images of cartilage were recorded after dehydration in glycerol and thermal denaturation in heated physiological saline. In our experiments, observed retardation changes in cartilage are primarily due to dehydration. Since dehydration is a principal source for retardation changes in cartilage over the range of heating profiles investigated, our studies suggest that the use of PS-OCT as a feedback control methodology for non-ablative cartilage reshaping requires further investigation.

Animals↗

PubMed2004

Inverted colloidal crystals as three-dimensional cell scaffolds.

A new type of three-dimensional scaffold with inverted colloidal crystal geometry for the investigation of topological effects in cell cultures is introduced in this publication. The scaffolds are made by infiltration of the hexagonal crystal lattice of polystyrene spheres with sol-gel formulation and subsequent annealing. It possesses a relatively high degree of order among existing cell scaffolds and affords tight control over the scaffold porosity and tissue organization. The prepared scaffolds can be a convenient system for the investigation of cell-cell and cell-matrix interactions. Their biocompatibility is demonstrated for human hepatocellular carcinoma HEP G2 and human bone marrow HS-5 cell cultures. A preliminary effect of the scaffold topology on cell proliferation is observed. HEP G2 hepatocytes form a large number of 10-15 cell colonies on scaffolds made from 75-microm spheres, while their number diminishes for scaffolds from 10- and 160-microm spheres. Under similar conditions, HS-5 forms smaller colonies consisting of three to four cells in 90-microm cavities.

Bone Marrow Cells↗

PubMed2003

Morphological changes in blood vessels produced by hyperosmotic agents and measured by optical coherence tomography.

Optical tissue clearing by hyperosmotic chemical agents significantly increases light depth penetration in skin and may improve light-based therapeutics such as laser treatment of cutaneous vascular lesions. A feasibility study was conducted to evaluate the potential role of optical clearing by glycerol in laser treatment of cutaneous vessels. Optical imaging was performed to investigate the morphological effects of glycerol on blood vessels of skin. Blood vessels were imaged using Doppler optical coherence tomography in in vivo hamster skin treated with glycerol. Images were obtained from the subdermal side to assess morphological changes in the blood vessels caused by glycerol and from the epidermal side to assess enhanced Doppler imaging of blood vessels. Application of glycerol to the subdermis resulted in venule stasis and for prolonged treatment times, arteriole stasis. In cases where flow remained in arterioles, an improved Doppler signal was detected from blood vessels when imaging transepidermally compared with the native condition. Intensity images indicated changes in blood optical properties and improved contrast of skin cross sections after glycerol application. The observed optical and morphological effects were reversed upon hydration of the skin with phosphate-buffered saline. The combination of increased depth of light penetration and the temporary slowing or cessation of flow in blood vessels could mean improved laser treatment of vessels.

Animals↗

PubMed2002

Coherent thermal wave imaging of subsurface chromophores in biological materials.

Thermal wave imaging of discrete subsurface chromophores in biological materials is reported using a phase sensitive coherent detection technique applied to recorded infrared (IR) images. We demonstrate that utilization of a periodically modulated laser source for thermal wave excitation and coherent detection applied to each pixel may be used to compute images of thermal wave amplitude and phase at the laser modulation frequency. In comparison to recorded IR images, the narrow-band detection technique significantly improves the quality of thermal wave amplitude images of subsurface chromophores in biological materials. Additionally, the technique provides phase information, which may be used to estimate chromophore depth in tissue. Application of the technique is demonstrated using tissue phantoms and in vivo biological models. We present a theoretical analysis and computer simulations that demonstrate the effect of tissue optical and thermal properties on thermal wave amplitude and phase. In comparison to the pulsed photothermal technique, coherent thermal wave imaging of subsurface chromophores in tissue for diagnostic applications allows reduction of peak incident laser fluence by as much as four orders of magnitude and is safer and more amenable to in vivo imaging.

Animals↗

PubMed2002

Erbium: YAG laser lithotripsy mechanism.

PURPOSE: We tested the hypothesis that the mechanism of long pulse erbium:YAG laser lithotripsy is photothermal. MATERIALS AND METHODS: Human urinary calculi were placed in deionized water and irradiated with erbium:YAG laser energy delivered through a sapphire optical fiber. Erbium:YAG bubble dynamics were visualized with Schlieren flash photography and correlated to acoustic emissions measured by a polyvinylidene fluoride needle hydrophone. The sapphire fiber was placed either parallel or perpendicular to the calculus surface to assess the contribution of acoustic transients to fragmentation. Stones were irradiated using desiccated stone irradiated in air, hydrated stone irradiated in air and hydrated stone irradiated in water. Ablation crater sizes were compared. Uric acid stones were irradiated in water and the water was assayed for cyanide. RESULTS: During the early phase of vapor bubble expansion, acoustic transients had minimal effects on calculus fragmentation. Fragmentation occurred due to direct absorption of laser energy transmitted to the calculus through the vapor channel between the sapphire fiber tip and calculus. The forward axial expansion of the bubble occurred more rapidly than the radial expansion. A parallel oriented fiber on the calculus surface produced no fragmentation but generated larger amplitude acoustic transients compared to perpendicular orientation. In perpendicular orientation the erbium:YAG laser did not generate any collapse acoustic waves but fragmentation occurred. Crater width was greatest for desiccated stones irradiated in air (p <0.03). Cyanide production increased as erbium:YAG irradiation of uric acid calculi increased, (r2 = 0.98). CONCLUSIONS: The erbium:YAG laser fragments stones through a photothermal mechanism.

Humans↗

PubMedDate not supplied

Use of optical coherence tomography to monitor biological tissue freezing during cryosurgery.

The use of optical coherence tomography (OCT) for imaging skin during cryosurgery is evaluated. OCT provides high spatial resolution (5-10 microm) images of optical backscattering due to local variations in refractive index, such as the boundary between liquid and frozen water in tissue. Time resolved OCT images were acquired during freezing of water, Intralipid trade mark, and in vivo hamster skin. Subsurface morphological changes were evident only during freezing of Intralipid and skin. A simple thermal model was applied which predicted freezing times on the same order of magnitude as those observed in OCT images. OCT can be used as a feedback tool during cryosurgical procedures to monitor progression of the freezing front.

Animals↗