In vitro measurements of optical properties of porcine brain using a novel compact device

Knowledge of the optical properties of tissues can be applied in numerous medical and scientific fields, including cancer diagnostics and therapy. There are many different ways of determining the optical properties of turbid media. The paper describes measurements of the optical properties of porcine brain tissue using novel instrumentation for simultaneous absorption and scattering characterisation of small turbid samples. Integrating sphere measurements are widely used as a reference method for determination of the optical properties of relatively thin turbid samples. However, this technique is associated with bulky equipment, complicated measuring techniques, interference compensation techniques and inconvenient sample handling. It is believed that the sphere for some applications can be replaced by a new, compact device, called the combined angular and spatially resolved head sensor, to measure the optical properties of thin turbid samples. The results compare very well with data obtained with an integrating sphere for well-defined samples. The instrument was shown to be accurate to within 12% for μ_a and 1% for μ _s ^′ in measurements of intralipid-ink samples. The corresponding variations of data were 17% and 2%, respectively. The reduced scattering coefficient for porcine white matter was measured to be 100 cm⁻¹ at 633 nm, and the value for coagulated brain tissue was 65 cm⁻¹. The corresponding absorption coefficients were 2 and 3 cm⁻¹, respectively.     

Multiple scattering effects in Doppler optical coherence tomography of flowing blood

We investigate the effect of multiple scattering on the optical coherence tomography (OCT) signal and the Doppler OCT signal of flowing blood. Doppler OCT measurements at 1300 nm are performed on flowing diluted porcine blood with hematocrit ranging between 0% and 15%. Measured blood hematocrit and mean red blood cell volume are used to calculate, using the discrete dipole approximation model, the (single) scattering coefficient and scattering anisotropy of blood. Monte Carlo simulations, based on the calculated scattering coefficients and scattering anisotropies, are compared to Doppler OCT measurements for hematocrit smaller than 10%. Good quantitative agreement between Doppler OCT measurements and Monte Carlo simulations is observed. Our measurements, calculations and simulations explain the relatively low attenuation coefficients and well preserved flow profiles measured with Doppler OCT for flowing blood. Monte Carlo simulations demonstrate the effect of the scattering anisotropy of the medium on the strength of multiple scattering effects in Doppler OCT signals. With increasing scattering anisotropy the OCT attenuation decreases; the distortion of the flow profile is strongest at intermediate scattering anisotropies (≈0.6).     

Changes in tissue optical properties due to radio-frequency ablation of myocardium

The optical properties of pig heart tissue were measured after in vivo ablation therapy had been performed during open-heart surgery. In vitro samples of normal and ablated tissue were subjected to measurements with an optically integrating sphere set-up in the region 470–900 nm. Three independent measurements were made: total transmittance, total reflectance and collimated transmittance, which made it possible to extract the absorption and scattering coefficients and the scattering anisotropy factor g, using an inverse Monte Carlo model. Between 470 and 700 nm, only the reduced scattering coefficient and absorption could be evaluated. The absorption spectra were fitted to known tissue chromophore spectra, so that the concentrations of haemoglobin and myoglobin could be estimated. The reduced scattering coefficient was compared with Mie computations to provide Mie equivalent average radii. Most of the absorption was from myoglobin, whereas haemoglobin absorption was negligible. Metmyoglobin was formed in the ablated tissue, which could yield a spectral signature to distinguish the ablated tissue with a simple optical probe to monitor the ablation therapy. The reduced scattering coefficient increased by, on average, 50% in the ablated tissue, which corresponded to a slight decrease in the Mie equivalent radius.     

Optical properties of normal and diseased human breast tissues in the visible and near infrared

The optical absorption and scattering coefficients have been determined for specimens of normal and diseased human breast tissues over the range of wavelengths from 500 to 1100 nm. Total attenuation coefficients were measured for thin slices of tissue cut on a microtome. The diffuse reflectance and transmittance were measured for 1.0 mm thick samples of these tissues, using standard integrating sphere techniques. Monte Carlo simulations were performed to derive the scattering and absorption coefficients, as well as the mean cosine of the scattering angle. The results indicate that scatter exceeds absorption by at least two orders of magnitude. Absorption is most significant at wavelengths below 600 nm. The scattering coefficients lie in the range 30-90 mm-1 at 500 nm, and fall smoothly with increasing wavelength to between 10 and 50 mm-1 at 1100 nm. The scattering coefficient for adipose tissue differs, in that it is invariant with wavelength over this spectral range. For all tissues examined, the scattered light is highly forward peaked, with the mean cosine of the scattering angle in the range 0.945-0.985. Systematic differences between the optical properties of some tissue types are demonstrated.     

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 properties of human skin in the near infrared wavelength range of 1000 to 2200 nm

In this paper we present the absorption coefficient mu(a) and the isotropic scattering coefficient mu(s)(') for 22 human skin samples measured using a double integrating sphere apparatus in the wavelength range of 1000-2200 nm. These in vitro results show that values for mua) follow 70% of the absorption coefficient of water and values for mu(s)(') range from 3 to 16 cm(-1). From the measured optical properties, it was found that a 2% Intralipid solution provides a suitable skin tissue phantom.     

Detecting glucose-induced changes in in vitro and in vivo experiments with optical coherence tomography

Optical clearing is a well-known phenomenon. It is based on the matching of refractive indices of a bulk material and scattering particles. The same principle is also used in scattering-based optical measurements of different constituents, such as glucose. By registering changes in scattering, it is possible to evaluate changes in the concentration of a solvent. This work describes the use of optical coherence tomography (OCT) to monitor glucose-induced changes in the optical properties of samples. Intralipid, mouse skin tissue, and mice (C57BL) are used as samples. Differences between in vitro and in vivo measurement conditions, the effect of glucose on the samples' optical properties, as well as possible problems in OCT experiments are discussed. A comparison of OCT signals from mouse skin samples and mice in vivo shows that the intensity of backscattered radiation is stronger in a living animal than in cultured tissue. Moreover, the effect of glucose on the scattering properties is larger in an in vivo case than in an in vitro case. In comparison with tissue, the effect of glucose is the smallest in Intralipid. The results increase the value of using cultured tissue in developing optical sensing techniques.     

Differences in optical properties between healthy and pathological human colon tissues using a Ti:sapphire laser: an in vitro study using the Monte Carlo inversion technique

The purpose of the study is to analyze and compare differences in the optical properties between normal and adenomatous human colon tissues in vitro at 630-, 680-, 720-, 780-, 850-, and 890-nm wavelengths using a Ti:sapphire laser. The optical parameters of tissue samples are determined using a double integrating sphere setup at seven different laser wavelengths. The inverse Monte Carlo simulation is used to determine the optical properties from the measurements. The results of measurement show that the optical properties and their differences vary with a change of laser wavelength for normal and adenomatous colon mucosa/submucosa and normal and adenomatous colon muscle layer/chorion. The maximum absorption coefficients for normal and adenomatous human colon mucosa/submucosa are 680 nm, and the minimum absorption coefficients for both are 890 nm. The maximum difference of the absorption coefficients between both is 56.8% at 780 nm. The maximum scattering coefficients for normal and adenomatous colon mucosa/submucosa are 890 nm, and the minimum scattering coefficients for both are 780 nm. The maximum difference of the scattering coefficients between both is 10.6% at 780 nm. The maximum absorption coefficients for normal and adenomatous colon muscle layer/chorion are 680 nm, and the minimum absorption coefficients for both are 890 nm. The maximum difference of the absorption coefficients between both is 47.9% at 780 nm. The maximum scattering coefficients for normal and adenomatous colon muscle layer/chorion are 890 nm, and the minimum scattering coefficients for both are 680 nm. The maximum difference of the scattering coefficients between both is 9.61% at 850 nm. The differences in absorption coefficients between normal and adenomatous tissues are more significant than those in scattering coefficients.     

Effect of light losses of sample between two integrating spheres on optical properties estimation

A Monte Carlo algorithm is applied to simulate the measurements of a sample with glass slides sandwiched between the double integrating sphere (DIS) setup. The effects caused by various parameters, such as the sample port of integrating sphere, thicknesses, and optical properties of the sample, on light losses and optical properties estimated by the inverse adding-doubling method (IAD) have been investigated. The results show that the light loss greatly increases the estimated error of absorption coefficient and slightly affects the estimated scattering coefficient. When the increase of apparent absorption of the sample induced by the light loss is 59%, the relative error of the scattering coefficient is less than 2% and that of the absorption coefficient reaches 28%. Enhancing the sample port diameter or decreasing the thickness of the sample can reduce the error effectively, and the effect of the former is much greater than that of the latter. In addition, the IAD method is proved to be valid for estimating the optical properties of a highly scattering or highly absorbing sample. This study can not only evaluate the error of optical properties estimation, but also provide optimal ways for the design of DIS and a scheme for acquiring accurate optical properties of tissue.     

Application of optical coherence tomography, pulsed photoacoustic technique, and time-of-flight technique to detect changes in the scattering properties of a tissue-simulating phantom

Intralipid is a well-known emulsion used as a tissue-simulating phantom in developing optical imaging and diagnostic techniques for medical applications. The optical coherence tomography (OCT), pulsed photoacoustic (PA), and time-of-flight (TOF) techniques were used to detect glucose-induced changes in the optical properties of Intralipid. A comparison of the applicability of these techniques to register changes in the scattering properties of Intralipid samples showed that OCT is the most effective method, whereas the sensitivity of the PA technique was less pronounced. Photon migration studies with the TOF technique showed changes in pulse amplitude, pulse width, and arrival time of the pulse maximum as a function of changes in Intralipid concentration. Also the measured signal parameters showed changes when measuring high glucose concentrations.     

Thermal coagulation-induced changes of the optical properties of normal and adenomatous human colon tissues in vitro in the spectral range 400-1,100 nm

The absorption coefficients, the reduced scattering coefficients and the optical penetration depths for native and coagulated human normal and adenomatous colon tissues in vitro were determined over the range of 400-1,100 nm using a spectrophotometer with an internal integrating sphere system, and the inverse adding-doubling method was applied to calculate the tissue optical properties from diffuse reflectance and total transmittance measurements. The experimental results showed that in the range of 400-1,100 nm there were larger absorption coefficients (P < 0.01) and smaller reduced scattering coefficients (P < 0.01) for adenomatous colon tissues than for normal colon tissues, and there were smaller optical penetration depths for adenomatous colon tissues than for normal colon tissues, especially in the near-infrared wavelength. Thermal coagulation induced significant increase of the absorption coefficients and reduced scattering coefficients for the normal and adenomatous colon tissues, and significantly reduced decrease of the optical penetration depths for the normal and adenomatous colon tissues. The smaller optical penetration depth for coagulated adenomatous colon tissues is a disadvantage for laser-induced thermotherapy (LITT) and photodynamic therapy (PDT). It is necessary to adjust the application parameters of lasers to achieve optimal therapy.     

Analyzing quantitative light scattering spectra of phantoms measured with optical coherence tomography

We demonstrate the ability of multiple forms of optical coherence tomography (OCT) in the frequency domain to quantitatively size scatterers. Combined with a variety of distinct phantoms, we gain insight into the measurement uncertainties associated with using scattering spectra to size scatterers. We size spherical scatterers on a surface using swept-source OCT with an analysis based on a simple slab-mode resonance model. Automating this technique, a two-dimensional (2-D) image is created by raster scanning across a surface phantom designed to have a distinct size transition to demonstrate accuracy and repeatability. We also investigate the potential of a novel sphere-nanotube structure as a quantitative calibration artifact for use in comparing measured intensity and phase scattering spectra directly to Mie theory predictions. In another experiment, we demonstrate tissue-relevant sizing of scatterers as small as 5 microm on a surface by use of a Fourier domain OCT system with 280 nm of bandwidth from a supercontinuum source. We perform an uncertainty analysis for our high-resolution sizing system, estimating a sizing error of 9% for measurements of spheres with a diameter of 15 microm. With appropriate modifications, our uncertainty analysis has general applicability to other sizing techniques utilizing scattering spectra.     

Optical scattering coefficient estimated by optical coherence tomography correlates with collagen content in ovarian tissue

Optical scattering coefficient from ex vivo unfixed normal and malignant ovarian tissue was quantitatively extracted by fitting optical coherence tomography (OCT) A-line signals to a single scattering model. 1097 average A-line measurements at a wavelength of 1310 nm were performed at 108 sites obtained from 18 ovaries. The average scattering coefficient obtained from the normal tissue group consisted of 833 measurements from 88 sites was 2.41 mm(-1) (± 0.59), while the average coefficient obtained from the malignant tissue group consisted of 264 measurements from 20 sites was 1.55 mm(-1) (± 0.46). The malignant ovarian tissue showed significant lower scattering than the normal group (p < 0.001). The amount of collagen within OCT imaging depth was analyzed from the tissue histological section stained with Sirius Red. The average collagen area fraction (CAF) obtained from the normal tissue group was 48.4% (± 12.3%), while the average CAF obtained from the malignant tissue group was 11.4% (± 4.7%). A statistical significance of the collagen content was found between the two groups (p < 0.001). These results demonstrated that quantitative measurements of optical scattering coefficient from OCT images could be a potential powerful method for ovarian cancer detection.     

Accuracy limits in the determination of absolute optical properties using time-resolved NIR spectroscopy.

We assess typical systematic experimental errors involved in a time-resolved measurement as applied to NIR diffuse optical spectroscopy and investigate their effect on the quantification accuracy of the absorption and the reduced scattering coefficient. We demonstrate that common systematic experimental uncertainties may lead to quantification errors of 10% or more, even when excellent signal to noise ratio conditions exist and accurate photon propagation models are employed. We further demonstrate that the accuracy of the calculation depends nonlinearly on the optical properties of the medium measured. High scattering and low absorbing media can be quantified more accurately than media with low scattering or high absorption using measurements of the same signal to noise ratio. We further discuss curve-shape fitting schemes that aid in improving the quantification accuracy in the presence of experimental errors. Finally, we identify uncertainties that set quantification accuracy limits and we find temporal resolution as the ultimate limiting factor in the quantification accuracy achieved. Our findings suggest that temporal resolution of the order of 10 ps is necessary for quantifying the absorption and reduced scattering coefficient of diffuse media with accuracy better than 5% using curve fitting methods. In that sense this analysis can be used in time-resolved system design and in predicting the expected errors given the technology selected for time-resolved measurements.     

A method for determination of the absorption and scattering properties interstitially in turbid media

We have developed a method to quickly determine tissue optical properties (absorption coefficient mu(a) and transport scattering coefficient mu'(s)) by measuring the ratio of light fluence rate to source power along a linear channel at a fixed distance (5 mm) from an isotropic point source. Diffuse light is collected by an isotropic detector whose position is determined by a computer-controlled step motor, with a positioning accuracy of better than 0.1 mm. The system automatically records and plots the light fluence rate per unit source power as a function of position. The result is fitted with a diffusion equation to determine mu(a) and mu'(s). We use an integrating sphere to calibrate each source-detector pair, thus reducing uncertainty of individual calibrations. To test the ability of this algorithm to accurately recover the optical properties of the tissue, we made measurements in tissue simulating phantoms consisting of Liposyn at concentrations of 0.23, 0.53 and 1.14% (mu'(s) = 1.7-9.1 cm(-1)) in the presence of Higgins black India ink at concentrations of 0.002, 0.012 and 0.023% (mu(a) = 0.1-1 cm(-1)). For comparison, the optical properties of each phantom are determined independently using broad-beam illumination. We find that mu(a) and mu'(s) can be determined by this method with a standard (maximum) deviation of 8% (15%) and 18% (32%) for mu(a) and mu'(s), respectively. The current method is effective for samples whose optical properties satisfy the requirement of the diffusion approximation. The error caused by the air cavity introduced by the catheter is small, except when mu(a) is large (mu(a) > 1 cm(-1)). We presented in vivo data measured in human prostate using this method.     

Optical properties of normal and cancerous human skin in the visible and near-infrared spectral range

Differences in absorption and/or scattering of cancerous and normal skin have the potential to provide a basis for noninvasive cancer detection. In this study, we have determined and compared the in vitro optical properties of human epidermis, dermis, and subcutaneous fat with those of nonmelanoma skin cancers in the spectral range from 370 to 1600 nm. Fresh specimens of normal and cancerous human skin were obtained from surgeries. The samples were rinsed in saline solution and sectioned. Diffuse reflectance and total transmittance were measured using an integrating sphere spectrophotometer. Absorption and reduced scattering coefficients were calculated from the measured quantities using an inverse Monte Carlo technique. The differences between optical properties of each normal tissue-cancer pair were statistically analyzed. The results indicate that there are significant differences in the scattering of cancerous and healthy tissues in the spectral range from 1050 to 1400 nm. In this spectral region, the scattering of cancerous lesions is consistently lower than that of normal tissues, whereas absorption does not differ significantly, with the exception of nodular basal cell carcinomas (BCC). Nodular BCCs exhibit significantly lower absorption as compared to normal skin. Therefore, the spectral range between 1050 and 1400 nm appears to be optimal for nonmelanoma skin cancer detection.     

Parametric electrical impedance tomography for measuring bone mineral density in the pelvis using a computational model

Osteoporosis is defined as bone microstructure deterioration resulting a decrease of bone's strength. Measured bone mineral density (BMD) constitutes the main tool for Osteoporosis diagnosis, management, and defines patient's fracture risk.In the present study, parametric electrical impedance tomography (pEIT) method was examined for monitoring BMD, using a computerized simulation model and preliminary real measurements. A numerical solver was developed to simulate surface potentials measured over a 3D computerized pelvis model. Varying cortical and cancellous BMD were simulated by changing bone conductivity and permittivity.Up to 35% and 16% change was found in the real and imaginary modules of the calculated potential, respectively, while BMD changes from 100% (normal) to 60% (Osteoporosis). Negligible BMD relative error was obtained with SNR > 60 [dB]. Position changes errors indicate that for long term monitoring, measurement should be taken at the same geometrical configuration with great accuracy. The numerical simulations were compared to actual measurements that were acquired from a healthy male subject using a five electrodes belt bioimpedance device.The results suggest that pEIT may provide an inexpensive easy to use tool for frequent monitoring BMD in small clinics during pharmacological treatment, as a complementary method to DEXA test.     

Determination of a standard site for the measurement of bone mineral density of the human calcaneus

Ultrasound of the calcaneus may be used as a cheap, ionising radiation-free and easy to use indicator of skeletal status, and hence of osteoporotic fracture risk. At present ultrasound is not widely used as it suffers from high precision errors. As ultrasound parameters are determined in part by bone mineral density (BMD), an increase in the accuracy and precision of BMD measurements should reduce the precision error associated with ultrasound measurements. The aim of this study was to define an anatomical site on the calcaneus at which accurate and precise measurements of BMD can be made. Ten dry calcanei and 10 cadaveric feet were scanned using a DXA scanner; 9 anatomically defined regions (1 cm²) were selected in the posterior part of the calcaneus for analysis. The centre of region 1 was positioned halfway along the line joining the anterior border of the calcaneal tubercle and the peak of the posterior superior tubercle, and the remaining 8 regions were placed around this central area. The BMD in these 9 regions was compared with the whole bone BMD and the variability of BMD within each of the 9 regions was measured. The reproducibility of the technique was assessed by taking 10 repeated measurements of 2 bone and 2 cadaveric specimens, each specimen being removed and repositioned between measurements. Region 1 was found to be the most representative of total BMD in cadaveric feet. This region also showed the least variability of BMD and consistently gave the lowest coefficients of variation in the reproducibility study both in the bone and the cadaveric specimens. This region is hence the most suitable site on the calcaneus for measuring absolute values of and changes in BMD. The surface position of region 1 was found to be consistently 5/9 along the line at 45° to the vertical, from the lateral malleolus to the heel. The identification of the surface location of region 1 relative to anatomical landmarks of the foot has enabled the same anatomical site to be measured in all subjects. This allows meaningful intersubject comparisons to be made. Preliminary data suggest that precision errors using ultrasound are also reduced when measurements are taken at this region of the calcaneus. The reduction in the precision error of ultrasound assessment of skeletal status may provide a cheap and safe way to identify individuals at risk from osteoporotic fracture.     

蛋清热变过程中的光学特性变化

目的为解决手术中激光切割能力下降的问题,以鸡蛋蛋清为样本,研究热变过程中生物组织的光学特性变化。方法使用积分球系统测量650nm激光照射下的样本在热风枪加热下,从26％升温到99℃过程中的准直透射光、漫透射光和反射光光强随温度变化的数据,计算吸收系数和散射系数变化曲线。结果样本热变前光学特性受温度影响较小,热变过程中透射率降低,吸收系数和散射系数增大。结论手术中低强度激光照射组织发生热变使吸收能力下降,应保持较高激光强度以达到消融阈值。