SignificanceTissue oxygenation is a parameter that allows for determining the health status of human beings. In diabetic patients, it is particularly important to evaluate this parameter as an indicator of microcirculatory problems in the extremities.AimWe aim to obtain tissue oxygen saturation from diffuse reflectance measurements.ApproachA computational algorithm to automate the methodology was implemented with the aim of establishing a medical diagnosis technique that is non-invasive and easy to apply and requires a short evaluation time. Tissue oxygen saturation measurements were performed on a group of volunteers to whom a vascular occlusion was applied. It was observed that, by increasing the applied pressure to the arm of each volunteer, the tissue oxygen saturation progressively decreased.ResultsThe results indicate that the developed technique is an effective method for monitoring changes in blood hemodynamics in patients with some type of pathology in which tissue oxygenation is compromised. In addition, the expected behavior of tissue oxygen saturation during a vascular occlusion was obtained.ConclusionsA methodology to obtain tissue oxygen saturation from diffuse reflectance measurements was successfully developed. It meets the necessary characteristics to be considered a technique for obtaining StO2 because it can be applied in vivo and non-invasively and does not require a high computational cost; thus it is fast and capable of providing an objective and quantifiable evaluation.
Photodynamic therapy (PDT) is a very effective technique for treatment of certain types of cancer, among the most common, skin cancer. PDT requires the presence of three elements: the photosensitizer, light and oxygen. Penetration depth of light into the tumor depends on both the characteristics of the tissue to be treated and the wavelength. As the light dose to be delivered in each lesion depends on the optical properties of the tissue, all the effects that change these properties should be considered in order to choose suitable doses. There are some studies that have determined the maximum dose of radiation tolerated for certain types of skin, but the influence of the temperature on the optical properties, especially for darker skin types, remains unknown. In this study, we analyzed the optical properties of skin in vivo of different Latin volunteers in order to study the influence of the temperature on the optical properties and thereby to define more precisely the dose of light to be received by each patient in a personalized way. The optical properties of skin in vivo were investigated using an optical system that included an integrating sphere, a tungsten lamp and a spectrophotometer. Such experimental set up-allowed to obtain spectra reflectance of various volunteers and from this measurement, the absorption coefficient was recovered by Inverse Adding Doubling (IAD) program.
A focused nanosecond laser pulse produces optical damage to subsurface targets when its intensity is high enough to overcome the required threshold irradiance. However, when the material is highly scattering, the laser pulse irradiance decreases as it propagates through the sample because the temporal pulse profile is stretched due to multiple scattering events. The objective of this work is to determine the transfer function associated to an integrating sphere measurement involving turbid media samples. Integrating spheres are used to measure the total diffuse reflectance and transmittance of homogeneous turbid media samples to retrieve its absorption and scattering coefficients. Reflectance and transmittance measurements, being static properties, are not affected by multiple reflections of light inside the integrating spheres. However, for a time-dependent measurement, such as the temporal profile of a short laser pulse propagating through a turbid medium, the light reflection and multiple scattering events inside the sphere contributes to an additional stretching deformation of the measured temporal pulse profile, which complicates the interpretation of the measurements. In this work we use integrating spheres to analyze the effect of a turbid media on the propagation of a nanosecond laser pulse.
We present a time-resolved study of the interaction of nanosecond laser pulses with tissue phantoms. When a laser pulse interacts with a material, optical energy is absorbed by a combination of linear (heat generation and thermoelastic expansion) and nonlinear absorption (expanding plasma), according to both the laser light irradiance and material properties. The objective is to elucidate the contribution of linear and nonlinear optical absorption to bubble formation. Depending on the local temperatures and pressures reached, both interactions may lead to the formation of bubbles. We discuss three experimental approaches: piezoelectric sensors, time-resolved shadowgraphy, and time-resolved interferometry, to follow the formation of bubbles and measure the pressure originated by 6 ns laser pulses interacting with tissue phantoms. We studied the bubble formation and pressure transients for varying linear optical absorption and for radiant exposures above and below threshold for bubble formation. We report a rapid decay (of 2 orders of magnitude) of the laser-induced mechanical pressure measured (by time-resolved shadowgraphy) very close to the irradiation spot and beyond 1 mm from the irradiation site (by the piezoelectric sensor). Through time-resolved interferometry measurements, we determined that bubble formation can occur at marginal temperature increments as low as 3°C.
Nanosecond long laser pulses are used in medical applications where precise tissue ablation with minimal
thermal and mechanical collateral damage is required. When a laser pulse is incident on a material, optical energy will be
absorbed by a combination of linear and nonlinear absorption according to both: laser light irradiance and material
properties. In the case of water or gels, the first results in heat generation and thermoelastic expansion; while the second
results in an expanding plasma formation that launches a shock wave and a cavitation/boiling bubble. Plasma formation
due to nonlinear absorption of nanosecond laser pulses is originated by a combination of multiphoton ionization and
thermionic emission of free electrons, which is enhanced when the material has high linear absorption coefficient. In this
work, we present three experimental approaches to study pressure transients originated when 6 ns laser pulses are
incident on agar gels and water with varying linear absorption coefficient, using laser radiant exposures above and below
threshold for bubble formation: (a) PVDF sensors, (b) Time-resolved shadowgraphy and (c) Time-resolved
interferometry. The underlying hypothesis is that pressure transients are composed of the superposition of both: shock
wave originated by hot expanding plasma resulting from nonlinear absorption of optical energy and, thermoelastic
expansion originated by heat generation due to linear absorption of optical energy. The objective of this study is to carry
out a comprehensive experimental analysis of the mechanical effects that result when tissue models are irradiated with
nanosecond laser pulses to elucidate the relative contribution of linear and nonlinear absorption to bubble formation.
Furthermore, we investigate cavitation bubble formation with temperature increments as low as 3 °C.
Nanosecond long laser pulses are used in medical applications where precise tissue ablation with minimal thermal and
mechanical collateral damage is required. When a laser pulse is incident on a material, optical energy will be absorbed
by a combination of linear and nonlinear absorption according to both: laser light intensity and material properties. In the
case of water or gels, the first results in heat generation and thermoelastic expansion; while the second results in an
expanding plasma formation that launches a shock wave and a cavitation/boiling bubble. Plasma formation due to
nonlinear absorption of nanosecond laser pulses is originated by a combination of multiphoton ionization and thermionic
emission of free electrons, which is enhanced when the material has high linear absorption coefficient. In this work, we
present measurements of pressure transients originated when 6 ns laser pulses are incident on agar gels with varying
linear absorption coefficient, mechanical properties and irradiation geometry using laser radiant exposures above
threshold for bubble formation. The underlying hypothesis is that pressure transients are composed of the superposition
of both: shock wave originated by hot expanding plasma resulting from nonlinear absorption of optical energy and,
thermoelastic expansion originated by heat generation due to linear absorption of optical energy. The objective of this
work is to evaluate the relative contribution of each absorption mechanism to mechanical effects in agar gel. Real time
pressure transients are recorded with PVDF piezoelectric sensors and time-resilved imaging from 50 μm to 10 mm away
from focal point.
Cell mono-layers were irradiated with nanosecond laser pulses under two distinct scenarios: (a) with culturing medium
positioning the beam waist at different stand-off distances γ and (b) without cell culturing medium, positioning the beam
waist directly on top of the cell mono-layer. Damaged cells were marked with Trypan Blue, a vital cell marker. Three
different zones of damage were identified: (1) a zone of complete cell clearance, surrounded by (2) a ring of dead cells
marked with Trypan Blue and (3) the rest of the cell culture where the cells remain alive and viable. Different
hydrodynamic mechanisms damage cells as it was shown by high speed video for γ=0 and comparison with time
resolved imaging. The cell damage mechanism has its origin on the optical breakdown plasma formation. For the case
with culturing medium, a combination of plasma formation and shear stresses are responsible for cell damage; wheras
for the case without cell culturing medium, the plasma formation is the only mechanism of interaction between laser
pulses and cells. The rapidly expanding plasma generates shock waves whose pressure is most likely responsible for the
cell detachment observed.
Our work presents a study of the physical phenomena produced in biological-tissue irradiated by pulsed lasers. It is of
special interest to us to apply short (Nd:YAG, 5ns) and ultrashort (Ti:Sapphire, 90fs) laser pulses into biological-tissue
samples in order to study and understand the interaction features. These laser pulses posses interaction features with
tissue that could offer, depending on the pulse parameters, good advantages for medical applications; such features
include free heat laser pulse-tissue interaction and excellent mechanical damage confinement. This study was carried out
in porcine corneal tissue and biological-tissue models made by agar gel layers combined with organic dye, which
allowed us to emulate vascular tissue. The main objective of our study is to understand photo-induced thermal effects
and bubble formation in biological-tissue irradiated by short and ultrashort laser pulses. Our results on corneal tissue
showed a good potential for novel ophthalmic applications, especially in refractive surgery. In the case of the agar gel
biological-tissue models, similar results can be applied in dermatology in order to remove port wine stain or other similar
birthmarks.
Bubble formation is a well identified phenomenon within short (ns) and ultrashort (fs) laser pulses-aqueous media interactions. Bubble formation might be produced by three different mechanisms: (1) optical breakdown, (2) rarefraction wave and (3) overheating of the material. Experiments where transparent and scattering tissue models
that mimic biological tissue were irradiated with a Q-switched, 532 nm, 5 nanosecond, Nd:YAG and Ti:sapphire femtosecond laser systems. The type of bubble (transient or permanent) and initial bubble diameter were characterized as a function of time as well as the number of pulses and repetition rate at which they were delivered.
Threshold fluence for bubble formation in scattering tissue model was also studied. Two types of bubbles were identified depending on the number of pulses and the repetition rate at which they were delivered: transient (type 1) and permanent (type 2) bubbles. There is an insignificant difference in the fluence required to form a bubble in transparent tissue models regardless of the depth at which the beam was focused; in contrast, for scattering tissue models, the fluence required to form a bubble in deep positions is significantly higher than that of more superficial beam focus positions.
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