Monte Carlo modelling of light propagation in breast tissue

Variations in tissue optical parameters with the incident power of an infrared laser

Infrared (IR) lasers are extensively utilized as an effective tool in many medical practices. Nevertheless, light penetration into the inspected tissue, which is highly affected by tissue optical properties, is a crucial factor for successful optical procedures. Although the optical properties are highly wavelength-dependent, they can be affected by the power of the incident laser. The present study demonstrates a considerable change in the scattering and absorption coefficients as a result of varying the incident laser power probing into biological samples at a constant laser wavelength (808 nm). The optical parameters were investigated using an integrating sphere and Kubelka-Munk model. Additionally, fluence distribution at the sample's surface was modeled using COMSOL-multiphysics software. The experimental results were validated using Receiver Operating Characteristic (ROC) curves and Monte-Carlo simulation. The results showed that tissue scattering coefficient decreases as the incident laser power increases while the absorption coefficient experienced a slight change. Moreover, the penetration depth increases with the optical parameters. The reduction in the scattering coefficients leads to wider and more diffusive fluence rate distribution at the tissue surface. The simulation results showed a good agreement with the experimental data and revealed that tissue anisotropy may be responsible for this scattering reduction. The present findings could be considered in order for the specialists to accurately specify the laser optical dose in various biomedical applications.

Solution of the direct problem in turbid media with inclusions using Monte Carlo simulations implemented in graphics processing units: new criterion for processing transmittance data

The study of light propagation in diffusive media requires solving the radiative transfer equation, or eventually, the diffusion approximation. Except for some cases involving simple geometries, the problem with immersed inclusions has not been solved. Also, Monte Carlo (MC) calculations have become a gold standard for simulating photon migration in turbid media, although they have the drawback large processing times. The purpose of this work is two-fold: first, we introduce a new processing criterion to retrieve information about the location and shape of absorbing inclusions based on normalization to the background intensity, when no inhomogeneities are present. Second, we demonstrate the feasibility of including inhomogeneities in MC simulations implemented in graphics processing units, achieving large acceleration factors ( approximately 10(3)), thus providing an important tool for iteratively solving the forward problem to retrieve the optical properties of the inclusion. Results using a cw source are compared with MC outcomes showing very good agreement.

Analysis of dense-medium light scattering with applications to corneal tissue: experiments and Monte Carlo simulations

Dense-medium scattering is explored in the context of providing a quantitative measurement of turbidity, with specific application to corneal haze. A multiple-wavelength scattering technique is proposed to make use of two-color scattering response ratios, thereby providing a means for data normalization. A combination of measurements and simulations are reported to assess this technique, including light-scattering experiments for a range of polystyrene suspensions. Monte Carlo (MC) simulations were performed using a multiple-scattering algorithm based on full Mie scattering theory. The simulations were in excellent agreement with the polystyrene suspension experiments, thereby validating the MC model. The MC model was then used to simulate multiwavelength scattering in a corneal tissue model. Overall, the proposed multiwavelength scattering technique appears to be a feasible approach to quantify dense-medium scattering such as the manifestation of corneal haze, although more complex modeling of keratocyte scattering, and animal studies, are necessary.

Infrared imaging of subcutaneous veins

BACKGROUND AND OBJECTIVES

Imaging of subcutaneous veins is important in many applications, such as gaining venous access and vascular surgery. Despite a long history of medical infrared (IR) photography and imaging, this technique is not widely used for this purpose. Here we revisited and explored the capability of near-IR imaging to visualize subcutaneous structures, with a focus on diagnostics of superficial veins.

STUDY DESIGN/MATERIALS AND METHODS

An IR device comprising a head-mounted IR LED array (880 nm), a small conventional CCD camera (Toshiba Ik-mui, Tokyo, Japan), virtual-reality optics, polarizers, filters, and diffusers was used in vivo to obtain images of different subcutaneous structures. The same device was used to estimate the IR image quality as a function of wavelength produced by a tunable xenon lamp-based monochrometer in the range of 500-1,000 nm and continuous-wave Nd:YAG (1.06 microm) and diode (805 nm) lasers.

RESULTS

The various modes of optical illumination were compared in vivo. Contrast of the IR images in the reflectance mode was measured in the near-IR spectral range of 650-1,060 nm. Using the LED array, various IR images were obtained in vivo, including images of vein structure in a pigmented, fatty forearm, varicose leg veins, and vascular lesions of the tongue.

CONCLUSION

Imaging in the near-IR range (880-930 nm) provides relatively good contrast of subcutaneous veins, underscoring its value for diagnosis. This technique has the potential for the diagnosis of varicose veins with a diameter of 0.5-2 mm at a depth of 1-3 mm, guidance of venous access, podiatry, phlebotomy, injection sclerotherapy, and control of laser interstitial therapy.

Simulating the effects of multiple scattering on images of dense sprays and particle fields

Most optical measurements in turbid media (including sprays, fogs, particulate and colloidal suspensions) assume single scattering of the detected photons. Multiple scattering introduces error, which has been quantified in very few systems. To quantify this error, we have written a flexible Monte Carlo photon transport simulation code capable of handling any three-dimensional geometry. Simulations of planar laser spray imaging with large, nonabsorbing particles show that up to 50% of the photons reaching the camera are multiply scattered. Because forward scattering dominates, the image is affected little. For particles with more absorption or with size closer to the wavelength of the light than those we have simulated, the effects are expected to be more serious.

Optical properties of native and coagulated human liver tissue and liver metastases in the near infrared range

BACKGROUND AND OBJECTIVE

Knowledge about optical parameters and the resultant light distribution in laser-treated tissue is important for predicting the effects of laser-induced thermotherapy of liver metastases (LITT).

MATERIALS AND METHODS

The absorption and scattering coefficients as well as the anisotropy factors and the optical penetration depths of human liver tissue and colorectal liver metastases were determined at 850, 980, and 1,064 nm under native and thermocoagulated conditions.

RESULTS

Liver metastases had a lower anisotropy factor, absorption, and scattering coefficient than healthy liver (P < 0.01), resulting in a significantly higher optical penetration depth in metastatic tissue. Coagulation significantly changes the optical parameters by reducing the optical penetration depth in both tissue types (P < 0.01).

CONCLUSIONS

A greater optical penetration depth in metastatic tissue is advantageous for LITT, since larger tumor volumes can be coagulated. At the same time, an adjustment of the application parameters during LITT is necessary to achieve optimal therapeutic success.

Hybrid Monte Carlo for photon transport through optically thick scattering media

A Monte Carlo simulation code developed to model time-domain transillumination measurements with small-area detectors through an optically thick scattering slab is presented. A hybrid approach has been implemented to reduce calculation times. Most of the scattering slab is treated stochastically, albeit with variance reduction techniques and the isotropic diffusion similarity rule. The contribution to the output signal per unit area and time of photon packets propagating in a thin slice near the output face of the slab is calculated analytically after each propagation step. This approach drastically reduces the calculation time but produces spikes in the temporal signals.

Effective point-spread function for fast image modeling and processing in microscopic imaging through turbid media

An effective point-spread function (EPSF) for microscopic imaging through turbid media is proposed and calculated. The EPSF incorporates the property of a microscope system as well as the scattering property of a turbid medium. We prove that the image of a thin object embedded in a turbid medium can be expressed by the convolution of the EPSF with an object function. With the help of the convolution relation, image modeling for 5, 000, 000 incident photons can be approximately 15 times faster than the direct Monte Carlo simulation method for a one-dimensional object and can be at least 2 orders of magnitude faster for a two-dimensional object.

Monte carlo procedure for investigating light propagation and imaging of highly scattering media

A Monte Carlo procedure has been developed to study photon migration through highly scattering nonhomogeneous media. When two scaling relationships are used, the temporal response when scattering or absorbing inhomogeneities are introduced can be evaluated in a short time from the results of only one simulation carried out for the homogeneous medium. Examples of applications to the imaging of defects embedded into a diffusing slab, a model usually used for optical mammography, are given. Comparisons with experimental results show the correctness of the results obtained.

Time-resolved imaging on a realistic tissue phantom: μ(s)' and μ(a) images versus time-integrated images

A method is proposed by which we construct images through turbid media, plotting directly either the transport-scattering coefficient μ(s) ' or the absorption coefficient μ(a). These optical parameters are obtained from the best fit of the time-resolved transmittance curves with a diffusion model. Measurements were performed with a time-correlated single-photon counting system on realistic tissue phantoms simulating a tumor mass within a breast. Images were obtained with an incident power of <1 mW and an acquisition time of 1 s/point. Comparison of μ(s) ' and μ(a) images with time-integrated images constructed from the same experimental data shows that the fitting method discriminates between scattering and absorption inhomogeneities and improves image quality for scattering but not for absorption inhomogeneities.

Efficient Monte Carlo simulation of confocal microscopy in biological tissue

A variance-reduction technique is described that greatly improves the efficacy of Monte Carlo simulations of reflection-mode confocal microscopy in anisotropically scattering media. The efficiency gain is large enough that the performance of confocal microscopes probing as deep as 5 scattering lengths can be simulated with a desktop computer. We use the technique to simulate the response of a true confocal microscope probing biological tissue, a problem that has been impractical to undertake by using conventional Monte Carlo methods. Our most important finding is that operation of a confocal microscope in the true confocal mode enables much more effective rejection of undesired scattered light than operation in the partially coherent mode, but the maximum probing depths of microscopes operated in either mode are similar (2-3) scattering lengths) in practice because of sensitivity limitations.

An investigation of light transport through scattering bodies with non-scattering regions

Near-infra-red (NIR) spectroscopy is increasingly being used for monitoring cerebral oxygenation and haemodynamics. One current concern is the effect of the clear cerebrospinal fluid upon the distribution of light in the head. There are difficulties in modelling clear layers in scattering systems. The Monte Carlo model should handle clear regions accurately, but is too slow to be used for realistic geometries. The diffusion equation can be solved quickly for realistic geometries, but is only valid in scattering regions. In this paper we describe experiments carried out on a solid slab phantom to investigate the effect of clear regions. The experimental results were compared with the different models of light propagation. We found that the presence of a clear layer had a significant effect upon the light distribution, which was modelled correctly by Monte Carlo techniques, but not by diffusion theory. A novel approach to calculating the light transport was developed, using diffusion theory to analyze the scattering regions combined with a radiosity approach to analyze the propagation through the clear region. Results from this approach were found to agree with both the Monte Carlo and experimental data.

Recent developments in breast imaging

A review of breast imaging has already appeared in 1982 in this journal. Consequently, the present article concentrates on a discussion of only those developments of a more recent nature. Although the emphasis is placed on the physical aspects of the different imaging methods concerned, the essential factors relating to the clinical background and the associated radiation risk are also outlined. The completeness of detail depends on the present clinical importance of the method under discussion. X-ray mammography, which is still the most important breast imaging technique and has proved to be an effective method for breast cancer screening, is therefore treated in greater detail. Since the early 1980s, ultrasound B-mode scanning has evolved to an indispensable adjunct to x-ray mammography. For Doppler sonography, diaphanography, contrast-enhanced MRI, CT and DSA, the visualization of a tumour depends essentially on the enhanced vascularity of the lesion. Whether this will prove to be a reliable indicator for malignancy remains to be shown in controlled clinical studies. Common to all imaging systems is the increasing use of digital methods for signal processing, which also offers the possibility of computer-aided diagnosis by texture analysis and pattern recognition.

Preliminary clinical evaluation of a combined optical Doppler ultrasound instrument for the detection of breast cancer

Mammography is widely used for imaging the breast, but is known to be less effective in evaluating the younger dense breast. In this study the ability of telediaphanography in conjunction with Doppler ultrasound (TDDU) to detect breast carcinomas was assessed. Light absorption, determined by the number of blood cells per unit volume of breast, results in the detection of an opaque lesion. Subsequent Doppler ultrasound detects the neovascularization at the periphery of tumours. In total, 178 patients were investigated without prior knowledge of the mammographic findings. This consisted of 69 patients presenting to the symptomatic breast clinic with normal mammograms and 109 patients with mammographic detected abnormalities (mean age 54 years). There were 95 neoplastic lesions. The sensitivity and specificity were: telediaphanography alone 73% and 82%; TDDU 61% and 92%, respectively. TDDU was less sensitive for small and impalpable tumours, and did not detect ductal carcinoma in situ (26 false negatives, mean diameter of 1.1 cm (SD of 0.3 cm)). Subsequent Doppler ultrasound did not further increase the sensitivity of the examination, but did increase the specificity. Patients with locally advanced breast cancers showed dramatic changes on repeated optical/Doppler examinations, in concordance with response to chemotherapy. The combined optical/Doppler instrument, with its low sensitivity, is not suitable for screening, even in the young dense breast, but may have a role in assessing the response of large tumours to chemotherapy.

An improved design for a stable and reproducible phantom material for use in near-infrared spectroscopy and imaging

In this note, we describe an improved phantom material for use in near-infrared spectroscopy and imaging. The material consists of a clear epoxy resin with absorbing dyes and amorphous silica spheres as scattering particles. It is possible to calculate the scattering coefficient and angular scattering distribution of the material from Mie theory, using the known size and refractive index of the silica spheres together with the measured refractive index of the resin (approximately 1.56). We show a good agreement between prediction and experimental measurements. The scattering properties of the material closely match those of tissue in the near-infrared wavelength region, having an anisotropy factor, g, of approximately 0.93. The absorption coefficient of the epoxy is low (approximately 0.001 mm-1), and addition of the dyes produces an absorption coefficient that covers the same range as that of tissue.

A Monte Carlo estimation of tissue optical properties for use in laser dosimetry

In certain clinical situations, such as photodynamic therapy, light dosimetry should be considered. The propagation of light in tissues is influenced by fundamental or microscopic optical properties, namely absorption mu a and scattering mu s coefficients, refractive index n and anistropy factor g. These optical parameters can be determined experimentally by direct and/or indirect methods when tissue macroscopic properties, such as reflectance, transmittance or collimated transmittance from a tissue slab, are measured. The method described in this work provides graphical, and in simple cases analytical, 'inverse' solutions to determine tissue microscopic properties from measured macroscopic parameters. The graphs necessary for this inversion have been calculated and are provided. The method can be applied in either direct or indirect techniques and it does not depend on limitations introduced by assumptions and approximations when using theoretical models. It can also be applied for any tissue type, detector geometry and experimental apparatus. The accuracy of the method is very good over a wide range, unlimited in practice, of values of optical properties. Finally, the results of this work are in good agreement with theoretical and experimental results of other investigators.

Investigation of a new light imaging technique to detect incipient caries in teeth

At the present time dental X-rays are the best method to locate carious lesions, however, small lesions can be detected only with difficulty. Even though investigations in the past have shown that light imaging systems are more sensitive to small lesions than X-rays it is difficult to determine the characteristics of these lesions with any of these systems. We developed a new light imaging technique that makes it much easier to determine the size and depth of these lesions on most areas of the teeth even though modifications on the present setup will still be necessary to detect them as easily on occlusal surfaces. This technique is based on raster scans of the teeth with narrow collimated light beams. The results of this investigation show that the areas ( greater than 0.1 mm 2) of small incipient lesions can be measured and their depths estimated.

Monte Carlo calculations of the modulation transfer function of an optical system operating in a turbid medium

Using a Monte Carlo method, we investigate the effect of a turbid medium on image transmission by means of the modulation transfer function approach. We present results that refer to a medium that consists of a random distribution of water spherical particles in air. We analyze the effect of geometric conditions (medium width and position) and source characteristics (Lambertian, beam emission). We present results for small spheres (Rayleigh scattering) and spheres (1.0-microm diameter) that are not small in comparison with the wavelength lambda = 0.6328 microm. Numerical data show a large modulation transfer function dependence on the source emission aperture and a substantial independence of the medium width for a fixed value of the optical depth. In accordance with reciprocity principles, we test an inverse scheme of Monte Carlo calculation, the advantage of this scheme being a substantial reduction in calculation time.

Measurement of the optical properties of the skull in the wavelength range 650-950 nm

The optical properties of samples of bone from pig skull have been measured over the wavelength range 650-950 nm. The scattering phase function was measured on thin samples of the bone using a goniometer, and a value for the mean cosine g, of the scattering angle, was calculated. The scattering and absorption coefficients, mu s and mu a were then determined from measurements of diffuse reflectance and transmittance made with a pair of integrating spheres, by a step-wise search through a table of diffuse reflectance and transmittance versus mu a and mu s generated by a Monte Carlo model incorporating the measured scattering phase function. Values for g measured on six samples varied from 0.925 +/- 0.014 at 650 nm to 0.945 +/- 0.013 at 950 nm. Corresponding values for mu a and mu s measured on 18 samples were mu a = 0.04 +/- 0.002 mm-1, mu s = 35 +/- 0.7 mm-1 at 650 nm to mu a = 0.05 +/- 0.002 mm-1, mu s = 24 +/- 0.6 mm-1 at 950 nm.

Time-resolved optical tomography

We have obtained transaxial slice images of a cylindrical object containing a highly scattering, low-absorbing solution by using picosecond pulses of visible light. Embedded in the solution was an assortment of six glass tubes containing different concentrations of absorbing dye. We reconstructed images tomographically from projections generated by using light transmitted through the object with theshortest flight times.

Increased spatial resolution in transillumination using collimated light

In traditional transillumination of the breast (diaphanography), the abundance of diffuse light resulting from the use of extended noncollimated sources reduces the visibility of deep seated lesions. A prototype scanning imaging system has been developed to investigate the effectiveness of thin collimated light beams (1.5 mm cross section) synchronized with a similarly collimated detector to increase contrast in lesions normally lost due to the detection of diffuse light. The study demonstrates that detection of opaque 1.5 mm details is possible in phantoms simulating breast tissues 6 mm thick regardless of depth. This is about 10 times better than images obtained on the same samples using present transillumination methods. Furthermore, this study indicates that internal structures (lesions, cysts) in up to 12 mm thick excised breast sections can be visualized by exploiting their frequency-dependent attenuation. This is accomplished by inserting 50 nm interference filters in the input light path, which can be varied in a stepwise manner in the range of 400 nm to 1000 nm. These results demonstrate for the first time that images of lesion-bearing 1 cm or larger tissues can be obtained, thus opening promising possibilities for whole-breast imaging.

Analysis of tissue optical coefficients using an approximate equation valid for comparable absorption and scattering

New photosensitizers activated by longer wavelengths than 630 nm light used with Photofrin II are under evaluation by various groups for the treatment of malignancies. Any increase in tumour volume destroyed by these agents as compared to Photofrin II will be partly determined by tissue penetrance at the longer wavelengths. Attenuation coefficients were measured for various tissues at 630 nm and the more penetrative near infrared wavelength of 789 nm. A new model of light propagation in tissue is shown to be accurate for arbitrary ratios of absorption and scattering, by comparison with a rigorous solution to the transport equation. Absorption and transport scattering coefficients of tissues at 630 and 789 nm were obtained by fitting this model to optical attenuation measurements. In vitro tissues included bovine heart, kidney and tongue, pig liver and fat, and chicken muscle; in vivo tissues included Dunning R3327-AT and R3327-H tumours. The penetration depth was found to be 1.35-2.25 times greater at 789 than 630 nm, depending on tissue type. The greatest differences in penetration between the two wavelengths were in the highly pigmented tissues. These substantial increases in penetration in the infrared may be important in future applications of photodynamic therapy.

Optical attenuation characteristics of breast tissues at visible and near-infrared wavelengths

Optical experiments are described for measuring the attenuation characteristics of breast tissues at visible and near-infrared wavelengths. Total attenuation coefficients post mortem were measured directly in thin tissue sections. They are usually within the range from 10 to 30 mm-1, are rather higher in fat than in fibroglandular specimens and decrease with increasing wavelength. The scattering phase function is strongly forward-peaked with the mean cosine of scattering in the range from 0.85 to 0.97 and appearing more forward-peaked in fat than in fibroglandular tissue. The reduced scattering coefficient is of the order of 1 mm-1 in all tissues. Absorption coefficients were measured indirectly in optically thick sections. They are typically between 0.1 and 0.5 mm-1 at wavelengths around 580 nm and an order of magnitude lower at 850 nm. At 580 nm and shorter wavelengths the absorption in carcinoma is significantly higher than in adjacent uninvolved tissue. Significant differences were observed in the first-order derivatives of the transmission spectra of carcinoma and surrounding tissues at certain infrared wavelengths. Transmission spectra measured in vivo across the wavelength range from 500 to 860 nm have a similar form to the spectra of excised samples. Linear absorption coefficients are generally of the same order of magnitude as those found in vitro although they are lower at green wavelengths.