Optimum selection of input polarization states in determining the sample Mueller matrix: a dual photoelastic polarimeter approach.

Dual photoelastic modulator polarimeter systems are widely used for the measurement of light beam polarization, most often described by Stokes vectors, that carry information about an interrogated sample. The sample polarization properties can be described more thoroughly through its Mueller matrix, which can be derived from judiciously chosen input polarization Stokes vectors and correspondingly measured output Stokes vectors. However, several sources of error complicate the construction of a Mueller matrix from the measured Stokes vectors. Here we present a general formalism to examine these sources of error and their effects on the derived Mueller matrix, and identify the optimal input polarization states to minimize their effects in a dual photoelastic modulator polarimeter configuration. The input Stokes vector states leading to the most robust Mueller matrix determination are shown to form Platonic solids in the Poincaré sphere space; we also identify the optimal 3D orientation of these solids for error minimization.

Effects of formalin fixation on tissue optical polarization properties.

Formalin fixation is a preparation method widely used in handling tissue specimens, such as biopsies, specifically in optical studies such as microscopy. In this note, we examine how formalin fixation affects the polarization properties of porcine myocardium and liver as assessed by optical polarimetry. Spatial maps of linear retardance and depolarization were derived from four myocardial and four liver samples before and after formalin fixation. Overall, linear retardance and depolarization increased after fixation for both myocardium (15% and 23% increase, respectively) and liver (38% and 51%, respectively). The relative increase in retardance was greater in liver compared to myocardium, although the absolute increase in retardance was comparable for both. The effect of fixation on bulk optical properties was also investigated for myocardium where the scattering coefficient increased from 92 to 132 cm(-1) and the absorption coefficient remained constant at 1.1 cm(-1).

Doppler optical cardiogram gated 2D color flow imaging at 1000 fps and 4D in vivo visualization of embryonic heart at 45 fps on a swept source OCT system.

We report a Doppler optical cardiogram gating technique for increasing the effective frame rate of Doppler optical coherence tomography (DOCT) when imaging periodic motion as found in the cardiovascular system of embryos. This was accomplished with a Thorlabs swept-source DOCT system that simultaneously acquired and displayed structural and Doppler images at 12 frames per second (fps). The gating technique allowed for ultra-high speed visualization of the blood flow pattern in the developing hearts of African clawed frog embryos (Xenopus laevis) at up to 1000 fps. In addition, four-dimensional (three spatial dimensions + temporal) Doppler imaging at 45 fps was demonstrated using this gating technique, producing detailed visualization of the complex cardiac motion and hemodynamics in a beating heart.

Balanced detection for low-noise precision polarimetric measurements of optically active, multiply scattering tissue phantoms.

The use and advantages of balanced detection for making low-noise polarimetric measurements in turbid materials are demonstrated. The technique reduces the intensity noise originating from the laser and, in addition, makes possible a direct measurement of a component of the Stokes vector. When phase-locked detection is used with either amplitude or polarization modulation for polarimetric measurements in turbid media, one can obtain elements of the scattering matrix of very small magnitude. This methodology is used to measure optical activity and surviving linear polarization fractions in clear and turbid media containing glucose at physiologically relevant concentrations. The results are in agreement with Monte Carlo simulations of polarized light propagation in turbid media.

Changes in relative light fluence measured during laser heating: implications for optical monitoring and modelling of interstitial laser photocoagulation.

Dynamic changes in internal light fluence were measured during interstitial laser heating of tissue phantoms and ex vivo bovine liver. In albumen phantoms, the results demonstrate an unexpected rise in optical power transmitted approximately I cm away from the source during laser exposure at low power (0.5-1 W), and a decrease at higher powers (1.5-2.5 W) due to coagulation and possibly charring. Similar trends were observed in liver tissue, with a rise in interstitial fluence observed during 0.5 W exposure and a drop in interstitial fluence seen at higher powers (1-1.5 W) due to tissue coagulation. At 1.5 W irradiation an additional, later decrease was also seen which was most likely due to tissue charring. Independent spectrophotometric studies in Naphthol Green dye indicate the rise in fluence observed in the heated albumen phantoms may have been primarily due to light exposure causing photobleaching of the absorbing chromophore. and not due to heat effects. Experiments in liver tissue demonstrated that the observed rise in fluence is dependent on the starting temperature of the tissue. Correlating changes in light fluence with key clinical endpoints/events such as the onset of tissue coagulation or charring may be useful for on-line monitoring and control of laser thermal therapy via interstitial fluence sensors.

Theoretical, experimental, and computational aspects of optical property determination of turbid media by using frequency-domain laser infrared photothermal radiometry.

In this work, the optical and thermal properties of tissuelike materials are measured by using frequency-domain infrared photothermal radiometry. This technique is better suited for quantitative multiparameter optical measurements than the widely used pulsed-laser photothermal radiometry (PPTR) because of the availability of two independent signal channels, amplitude and phase, and the superior signal-to-noise ratio provided by synchronous lock-in detection. A rigorous three-dimensional (3-D) thermal-wave formulation with a 3-D diffuse and coherent photon-density-wave source is applied to data from model phantoms. The combined theoretical, experimental, and computational methodology shows good promise with regard to its analytical ability to measure optical properties of turbid media uniquely, as compared with PPTR, which exhibits uniqueness problems. From data sets obtained by using calibrated test phantoms, the reduced optical scattering and absorption coefficients were found to be within 20% and 10%, respectively, of the values independently derived by using Mie theory and spectrophotometric measurements.

Laser thermal therapy: utility of interstitial fluence monitoring for locating optical sensors.

Multipoint optical fluence measurements can potentially be used to detect coagulation-induced changes in optical propagation during interstitial laser thermal therapy. Estimating the dimensions of coagulation using on-line optical monitoring, which is applicable to treatments where the tip of the source fibre is not precharred, may be limited by the accuracy of the placement of optical sensors with respect to source fibres. A strategy has been developed to determine accurately the position of a four-sensor linear array, prior to treatment, using optical fluence data obtained from the sensors for low-power (< or = 0.5 W) irradiation. A minimum of four sensors in an array was required in order to develop a mathematical formulation for position determination that did not require tissue optical properties or laser power as input. Optical propagation was based on diffusion theory for homogeneous tissues in spherical geometry. Low input laser power is needed to ensure that there are no thermally induced changes in tissue optical properties not accounted for in the mathematical description. Experimental evaluation was performed in a tissue-equivalent liquid phantom using 0.5 W of 805 nm optical energy and a translatable isotropic optical sensor. For sensor locations with 2 mm spacing, placement accuracy of 0.67 mm was achieved. The accuracy improved to 0.13 mm as the sensor spacing increased to 5 mm.

Methodology for examining polarized light interactions with tissues and tissuelike media in the exact backscattering direction.

The properties of polarized light emerging from turbid media in the exact backscattering direction are studied by modulating the incident light polarization state and isolating the synchronous signal with lock-in amplifier detection. The results are reported for polystyrene microsphere suspensions in distilled water, with and without glucose, and for both ex vivo and in vivo biological tissues. A new theoretical formulation based on Mueller calculus is developed to describe the observed behavior of the backscattered light in terms of two sample parameters: optical rotation and depolarization. This technique proved successful in modeling both phantom and tissue samples. Results showed the presence of a significant surviving polarization fraction in the backscattering direction even in extremely dense optical phantom media, an important finding that has not been observed at other detection angles. Substantial polarized light preservation in biological tissue samples is also demonstrated for this detection geometry. This illustrates the potential of using polarized light to investigate turbid biological materials in vivo in retroreflection geometry.

The effects of dynamic optical properties during interstitial laser photocoagulation.

A nonlinear mathematical model was developed and experimentally validated to investigate the effects of changes in optical properties during interstitial laser photocoagulation (ILP). The effects of dynamic optical properties were calculated using the Arrhenius damage model, resulting in a nonlinear optothermal response. This response was experimentally validated by measuring the temperature rise in albumen and polyacrylamide phantoms. A theoretical study of ILP in liver was conducted constraining the peak temperatures below the vaporization threshold. The temperature predictions varied considerably between the static and dynamic scenarios, and were confirmed experimentally in phantoms. This suggests that the Arrhenius model can be used to predict dynamic changes in optical and thermal fields. An increase in temperature rise due to a decrease in light penetration within the coagulated region during ILP of the liver was also demonstrated. The kinetics of ILP are complex and nonlinear due to coagulation, which changes the tissue properties during treatment. These complex effects can be adequately modelled using an Arrhenius damage formulation.

Changes in optical properties of ex vivo rat prostate due to heating.

This study examines the effectiveness of a single, first-order Arrhenius process in accurately modelling the thermally induced changes in the optical properties, particularly the reduced scattering coefficient, mu(s)', and the absorption coefficient, mu(a), of ex vivo rat prostate. Recent work has shown that mu(s)' can increase as much as five-fold due to thermal coagulation, and the observed change in mu(s)' has been modelled well according to a first-order rate process in albumen. Conversely, optical property measurements conducted using pig liver suggest that this change in mu(s)' cannot suitably be described using a single rate parameter. In canine prostate, measurements have indicated that while the absorption coefficient varies with temperature, it does not do so according to first-order kinetics. A double integrating sphere system was used to measure the reflectance and transmittance of light at 810 nm through a thin sample of prostate. Using prostate samples collected from Sprague Dawley rats, optical properties were measured at a constant elevated temperature. Tissue samples were measured over the range 54-83 degrees C. The optical properties of the sample were determined through comparison with reflectance and transmittance values predicted by a Monte Carlo simulation of light propagation in turbid media. A first order Arrhenius model was applied to the observed change in mu(s)' and mu(a) to determine the rate process parameters for thermal coagulation. The measured rate coefficients were Ea = (7.18 +/- 1.74) x 10(4) J mol(-1) and Afreq = 3.14 x 10(8) s(-1) for mu(s)'. It was determined that the change in mu(s)' is well described by a single first-order rate process. Similar analysis performed on the changes in mu(a) due to increased temperatures yielded Ea = (1.01 +/- 0.35) x 10(5) J mol(-1) and Afreq = 8.92 x 10(12) s(-1). The results for mu(a) suggest that the Arrhenius model may be applicable to the changes in absorption.

Optical phantom materials for near infrared laser photocoagulation studies.

BACKGROUND AND OBJECTIVE: Phantoms were developed that simulate tissue with dynamic and static optical properties with which to study the effects of laser irradiation. STUDY DESIGN/MATERIALS AND METHODS: Albumen, agar, and an absorbing dye (Naphthol Green) were combined to form a phantom with heat sensitive optical properties to mimic tissue response. The optical properties of this phantom were measured by using the added absorber technique. A polyacrylamide phantom with static optical properties was designed with the equivalent values of micro(a) and micro'(s) by combining appropriate concentrations of Naphthol Green and Intralipid-10%. RESULTS: The absorption and reduced scattering coefficient of the phantoms were 0. 50 +/- 0.04 cm(-1) and 2.67 +/- 0.07 cm(-1) respectively, in the native state at 805 nm. In the coagulated state, the absorption and scattering coefficient were 0.7 +/- 0.1 cm(-1) and 13.1 +/- 0.5 cm(-1) respectively. CONCLUSION: Two phantoms with dynamic or static optical properties were developed with properties similar to tissue. They may be used in future studies of opto-thermal effects in tissues.

Magnetic resonance imaging of temperature changes during interstitial microwave heating: a phantom study.

Changes in magnetic resonance (MR) signals during interstitial microwave heating are reported, and correlated with simultaneously acquired temperature readings from three fiber-optic probes implanted in a polyacrylamide gel phantom. The heating by a MR-compatible microwave antenna did not interfere with simultaneous MR image data acquisition. MR phase-difference images were obtained using a fast two-dimensional-gradient echo sequence. From these images the temperature-sensitive resonant frequency of the 1H nuclei was found to decrease approximately by 0.008 ppm/ degree C. The method and results presented here demonstrate that noninvasive MR-temperature imaging can be performed simultaneously with interstitial microwave thermal treatment.

Why do veins appear blue? A new look at an old question.

We investigate why vessels that contain blood, which has a red or a dark red color, may look bluish in human tissue. A CCD camera was used to make images of diffusely reflected light at different wavelengths. Measurements of reflectance that are due to model blood vessels in scattering media and of human skin containing a prominent vein are presented. Monte Carlo simulations were used to calculate the spatially resolved diffuse reflectance for both situations. We show that the color of blood vessels is determined by the following factors: (i) the scattering and absorption characteristics of skin at different wavelengths, (ii) the oxygenation state of blood, which affects its absorption properties, (iii) the diameter and the depth of the vessels, and (iv) the visual perception process.

Analysis of layered scattering materials by pulsed photothermal radiometry: application to photon propagation in tissue.

A model of pulsed photothermal radiometry (PPTR) based on optical diffusion theory is presented for a turbid, two-layer, semi-infinite medium containing a surface layer whose optical absorption and scattering properties differ from that of the underlying layer. Assuming one-dimensional geometry, we develop expressions for the depth-dependent fluence distributions and radiant-energy-density profiles and for the time dependence of the PPTR signal. Experimental tests of the PPTR model in a series of layered phantoms of varying optical properties are described. The results of these tests are consistent with the model predictions.

Pulsed photothermal radiometry in optically transparent media containing discrete optical absorbers.

A description of heat transport by conduction and radiation in inhomogeneous materials following absorption of a brief optical pulse is presented, and investigated experimentally using pulsed photothermal radiometry (PPTR). The model indicates that the role of radiation as an intramedium heat transfer modality increases with increasing temperatures and decreasing infrared (IR) absorption of the medium. However, for the range of conditions analysed in this study, conductive transfer dominates. Thus, the inclusion of radiation does not significantly perturb the internal temperature profiles, although it does influence the radiometric emission from the sample, and hence the PPTR signal. The thermal confinement effects described in this study may be relevant in photomedicine, for example in pulsed laser irradiation of tissues containing small absorbing targets.

Optical and thermal characterization of natural (Sepia officinalis) melanin.

The optical properties and the thermal diffusivity of natural cuttlefish (Sepia officinalis) melanin have been measured. The optical absorption and scattering properties of melanin particles were determined at 580 nm and 633 nm, using photometric and photothermal techniques. For the photometric studies, the absorption and the transport scattering coefficients were determined from the measurements of diffuse reflectance and transmittance. The scattering anisotropy was obtained from an additional measurement of the total attenuation coefficient and independently obtained by goniometry. For photothermal studies, pulsed photothermal radiometry was used to deduce the absorption and transport scattering coefficients via a model based on optical diffusion theory. Pulsed photothermal radiometry was also used to provide the thermal diffusivity of solid melanin pressed pellets.

Determination of optical properties of turbid media using pulsed photothermal radiometry.

Pulsed photothermal radiometry (PPTR) measures blackbody radiation emitted by a sample after absorption of an optical pulse. Three techniques for obtaining the absorption coefficient of absorbing-only, semi-infinite samples are examined and shown to give comparable results. An analytic theory for the time dependence of the PPTR signal in semi-infinite scattering and absorbing media has been derived and tested in a series of controlled gel phantoms. This theory, based on the diffusion approximation of the radiative transport equation, is shown to model the time course of the detected signal accurately. Furthermore, when the incident fluence is known, the theory can be used in a non-linear, two-parameter fitting algorithm to determine the absorption and reduced scattering coefficients of a turbid sample with an accuracy of 10-15% for transport albedos ranging from 0.42-0.88.