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

A novel investigational preclinical model to assess fluence rate for dental oral craniofacial tissues

OBJECTIVE

Photodynamic Therapy (PDT) and Photobiomodulation (PBM) are recognized for their potential in treating head and neck conditions. The heterogeneity of human tissue optical properties presents a challenge for effective dosimetry. The porcine mandible cadaver serves as an excellent model and has several similarities to human tissues of the dental oral craniofacial complex. This study aims to validate a novel modeling system that will help refine PDT and PBM dosimetry for the head and neck region.

METHODS AND MATERIALS

Light transmission was analyzed through several tissue combinations at distances of 2 mm to 10 mm. Maximum light fluence rates (mW/cm2) were compared across tissue types to reveal the effects of tissue heterogeneity.

RESULTS

The study revealed that light fluence is affected by tissue composition, with dentin/enamel showing reduced transmission and soft tissue regions exhibiting elevated values. The porcine model has proven to be efficient in mimicking human tissue responses to light, enabling the potential to optimize future protocols.

CONCLUSION

The porcine mandible cadaver is a novel model to understand the complex interactions between light and tissue. This study provides a foundation for future investigations into dosimetry optimization for PDT and PBM.

Photobiomodulation in dental extraction therapy: Postsurgical pain reduction and wound healing

BACKGROUND

This scoping review and analysis were designed to assess the amount of time spent delivering photobiomodulation (PBM) light therapy after dental extraction to improve postoperative pain and wound healing.

TYPES OF STUDIES REVIEWED

The scoping review was performed according to the Cochrane Collaboration and Preferred Reporting Items for Systematic Reviews and Meta-Analyses criteria. Publications were specific for human randomized controlled clinical trials, PBM after dental extraction therapy, and related clinical outcomes. Online databases searched included PubMed, Embase, Scopus, and Web of Science. Analyses were conducted to analyze the prescribed intervals of time (seconds) per application of PBM.

RESULTS

Of the 632 studies initially identified, 22 studies fulfilled the inclusion criteria. Postoperative pain and PBM were reported in 20 articles for 24 treatment groups, with treatment times ranging from 17 through 900 seconds and wavelengths from 550 through 1,064 nm. Clinical wound healing outcomes were reported in 6 articles for 7 groups with treatment times ranging from 30 through 120 seconds and wavelengths from 660 through 808 nm. PBM therapy was not associated with adverse events.

CONCLUSIONS AND PRACTICAL IMPLICATIONS

There is future potential to integrate PBM after dental extraction therapy to improve postoperative pain and clinical wound healing. The amount of time spent delivering PBM will vary by wavelength and the type of device. Further investigation is needed to translate PBM therapy into human clinical care.

Improving Titanium Implant Stability with Photobiomodulation: A Review and Meta-Analysis of Irradiation Parameters

Objective: This analysis was designed to present a summary of available evidence that will inform practice and guide future research for photobiomodulation (PBM) after titanium implant placement procedures. Materials and methods: A systematic review was performed according to the Cochrane Collaboration and in line with Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) criteria. Two investigators screened the titles and abstracts, and reviewed articles for risk of bias. Online databases searched included PubMed, Embase, Scopus, and Web of Science. Terms were specific to the effects of PBM on dental implant stability. Results: Eight hundred fifty-six studies were identified, and 15 studies fulfilled the inclusion criteria. Light sources included both laser and light emitting diode (LED) devices. Wavelengths ranged from 618 to 1064 nm. The meta-analysis concluded that all 15 published studies were able to safely apply PBM near dental implants without adverse events. Laser and LED wavelengths that reported significant results included 618, 626, 830, 940 (2 × ), and 1064 nm. Conclusions: The use of adjunctive PBM can be safely prescribed after surgical placement of titanium implants. Six groups reported statistical significance for improving implant stability (four laser diode, two LED) in wavelengths ranging from 618 to 1064 nm. The amount of time spent delivering PBM was not a variable that differentiated whether a study reported significant results.

Validating Homogeneity for a Novel 3-Dimensional Tissue Phantom Modeling System of the Human Maxilla

Silicon phantom models have been utilized to calculate light fluence in patients being treated with Photodynamic Therapy (PDT). This application can be utilized for other non-ionizing wavelength therapies such as Photobiomodulation (PBM). We have developed a novel protocol to validate homogeneity for 3-dimensional silicon phantom models of the human maxilla. Accurately quantifying the light profiles of human tissue can accommodate for varying optical properties that occur between subjects. More importantly, this can help optimize light fluence dosimetry calculations to achieve intended results. Silicon models of identical composition were fabricated into two different shapes: 1 flat-planar cylindrical shaped model, 2) non-flat planar (3-dimensional) mold of the human maxilla. Fabricating homogenous silicon phantom models continues to be a challenge as micro-bubbles can contaminate the compound during the curing process. Integrating both proprietary CBCT and handheld surface acquisition imaging devices confirmed our results to be within 0.5mm of accuracy. This protocol was specifically used to cross-reference and validate homogeneity at various depths of penetration. These results present the first known successful validation of identical silicon tissue phantoms with a flat-planar surface vs. a non-flat 3D planar surface. This proof-of-concept phantom validation protocol is sensitive to the specific variations of 3-dimensional surfaces and can be applied to a workflow used to capture accurate light fluence calculations in the clinical setting.

Light Technology for Efficient and Effective Photodynamic Therapy: A Critical Review

Photodynamic therapy (PDT) is a cancer treatment with strong potential over well-established standard therapies in certain cases. Non-ionising radiation, localisation, possible repeated treatments, and stimulation of immunological response are some of the main beneficial features of PDT. Despite the great potential, its application remains challenging. Limited light penetration depth, non-ideal photosensitisers, complex dosimetry, and complicated implementations in the clinic are some limiting factors hindering the extended use of PDT. To surpass actual technological paradigms, radically new sources, light-based devices, advanced photosensitisers, measurement devices, and innovative application strategies are under extensive investigation. The main aim of this review is to highlight the advantages/pitfalls, technical challenges and opportunities of PDT, with a focus on technologies for light activation of photosensitisers, such as light sources, delivery devices, and systems. In this vein, a broad overview of the current status of superficial, interstitial, and deep PDT modalities-and a critical review of light sources and their effects on the PDT process-are presented. Insight into the technical advancements and remaining challenges of optical sources and light devices is provided from a physical and bioengineering perspective.

A Comparison of Two Probes to Determine Rectum Optical Properties

Tissue optical properties are crucial for determining the light dose delivered to the tumor. Two probes are compared: the two-catheter probe is based on transmittance measurement between one point source and one isotropic detector inside parallel catheters spaced at 0.5 cm along a 1-inch diameter transparent cylinder; and a 1-inch trans-rectal diffuse optical tomography (DOT) probe designed for prostate measurements, using a multiple fiber-array with source-detector separations of 1.4-10 mm. The two-catheter probe uses an empirical model for primary and scatter light fluence rates in the cylindrical cavity condition for anal PDT to determine optical properties along the source catheter using dual motors to move the source and detector along the catheters. The DOT probe uses finite element method (FEM) to obtain distribution of optical properties in 3D. Validations for the two probes were performed in liquid and solid phantoms. For each method, validation was performed in tissue-mimicking liquid phantoms for a range of known optical properties (μa between 0.05 and 0.9 cm-1 and μs' between 5.5 and 16.5 cm-1). To cross-check the two methods, solid phantoms were created of known optical properties at the University of Pennsylvania and sent for measurement to Princess Margaret Cancer Centre (PMH) to mimic realistic patient simulating conditions. Measurements were taken and optical properties were then recovered without knowing the expected values to cross-validate each probe. The results show modest agreement between the measured μa and μs'values, but high degree of agreement between the measured μeff performed independently using the two methods.

Determination of the distribution of drug concentration and tissue optical properties for ALA-mediated anal photodynamic therapy

PDT efficacy depends on the availability and dynamic interactions of photosensitizer, light, and oxygen. Tissue optical properties influence the delivered light dose and impact PDT outcome. In-vivo measurements of tissue optical properties and photosensitizer concentration enable determination of explicit and implicit dose factors affecting PDT and helps to understand the underlying biophysical mechanism of PDT. In this study, we measure tissue optical properties (absorption μa (λ) and scattering μs' (λ) coefficients) and PpIX concentration in tissue simulating liquid phantoms with a geometry that resembles anal canal. In-vivo light fluence rate and photosensitizer fluorescence of 405nm excitation light source were acquired using a dual-motor continuous wave transmittance spectroscopy system. We characterized the tissue optical properties correction factor of fluorescence signal using a series of tissue simulating phantoms with known PpIX concentrations and with absorption coefficient between 0.1 - 0.9 cm-1 and reduced scattering coefficient between 5 - 40 cm-1. The results demonstrated that our spectroscopy system could determine the distribution of tissue optical properties and PPIX concentration during anal PDT.

An Optical Surface Applicator for Intraoperative Photodynamic Therapy

BACKGROUND AND OBJECTIVES

Intraoperative photodynamic therapy (IO-PDT) is typically administered by a handheld light source. This can result in uncontrolled distribution of light irradiance that impacts tissue and tumor response to photodynamic therapy. The objective of this work was to characterize a novel optical surface applicator (OSA) designed to administer controlled light irradiance in IO-PDT.

STUDY DESIGN/MATERIALS AND METHODS

An OSA was constructed from a flexible silicone mesh applicator with multiple cylindrically diffusing optical fibers (CDF) placed into channels of the silicone. Light irradiance distribution, at 665 nm, was evaluated on the OSA surface and after passage through solid tissue-mimicking optical phantoms by measurements from a multi-channel dosimetry system. As a proof of concept, the light administration of the OSA was tested in a pilot study by conducting a feasibility and performance test with 665-nm laser light to activate 2-(1'-hexyloxyethyl) pyropheophorbide-a (HPPH) in the thoracic cavity of adult swine.

RESULTS

At the OSA surface, the irradiance distribution was non-uniform, ranging from 128 to 346 mW/cm2 . However, in the tissue-mimicking phantoms, beam uniformity improved markedly, with irradiance ranges of 39-153, 33-87, and 12-28 mW/cm2 measured at phantom thicknesses of 3, 5, and 10 mm, respectively. The OSA safely delivered the prescribed light dose to the thoracic cavities of four swine.

CONCLUSIONS

The OSA can provide predictable light irradiances for administering a well-defined and potentially effective therapeutic light in IO-PDT. Lasers Surg. Med. © 2019 Wiley Periodicals, Inc.

Optimized Cylindrical Diffuser Powers for Interstitial PDT Breast Cancer Treatment Planning: A Simulation Study

Purpose

It is well known that interstitial photodynamic therapy (iPDT) of large tumors requires effective planning to ensure efficient delivery of therapeutic dose to the target tumors. This should be achieved in parallel with minimal damage to the nearby intact tissues. To that end, clinical iPDT can be attained using cylindrical diffusing optical fibers (CDFs) as light sources. In this work, we optimize output CDF powers in order to deliver a prescribed light dose to a spherical volume such as a tumor node.

Methods

Four CDFs are placed vertically inside the tumor node. The fluence rate is calculated using the diffusion equation. Therapeutic target dose is (20-50) J·cm⁻². The optical properties (μ_a = 0.085 cm⁻¹, μ_s′ = 16 cm⁻¹) of a breast tumor and the treatment time of 150 sec are used to calculate the fluence rate.

Results

For four CDFs, the therapeutic target dose (20-50) J·cm⁻² is delivered to more than 90%. This is the ratio of the total points that receive the target dose in proportion to the total points in the volume of the node of 3 cm in diameter, whereas, in larger nodes, the ratio is decreased to approximately 67%. Five CDFs are required to improve this ratio by more than 10%.

Conclusion

Optimizing delivered powers enables the distribution of the therapeutic dose uniformly in the medium. In addition, this simulation study represents an essential part of a development dosimetry system for measuring and controlling the optical dose in the breast tumors.

Validation of tissue optical properties measurement using diffuse reflectance spectroscopy (DRS)

The effectiveness of photodynamic treatment depends on several factors including an accurate knowledge of optical properties of the tissue to be treated. Transmittance and diffuse reflectance spectroscopic techniques are commonly used to determine tissue optical properties. Although transmittance spectroscopy technique is accurate in determining tissue optical properties, it is only valid in an infinite medium and can only be used for interstitial measurements. Diffuse reflectance spectroscopy, on the other hand, is easily adapted to most tissue geometries including skin measurements that involve semi-infinte medium. However, the accuracy of the measured optical properties can be affected by uncertainty in the measurements themselves and/or due to the uncertainty in the fitting algorithm. In this study, we evaluate the accuracy of optical properties determination using diffuse reflectance spectroscopy implemented using a contact probe setup. We characterized the error of the optical properties fitted using two fitting algorithms, a wavelength wise fitting algorithm and a full reflectance spectral fitting algorithm. By conducting systematic investigation of the measurements and fitting algorithm of DRS, we gained an understanding of the uncertainties in the measured optical properties and outlined improvement measures to minimize these errors.

Method of interpreting Mueller matrix of anisotropic medium

The differential Mueller matrix is an important concept for analyzing the polarization properties of an optically homogeneous anisotropic sample, both nondepolarizing and depolarizing. In this work, we present a new method of interpreting Mueller matrix of anisotropic medium based on the relationships that exist between the components of a differential Mueller matrix and the polar components of the corresponding macroscopic Mueller matrix, and the necessary conditions are determined that guarantee the physical realizability of the generating matrices. Finally, a group of the experimental data of a sample from the literature with some known polarization properties was used to demonstrate the analysis. The work is helpful for obtaining new insights or new interpretations of the measured Mueller matrix of the medium.

Optimizing the light delivery of linear-array-based photoacoustic systems by double acoustic reflectors

Although linear transducer arrays have been intensely used in photoacoustic imaging, their geometrical shape constrains light illumination. Today, most linear array based photoacoustic systems utilize side-illumination geometry, which consists of two line fiber bundles attached to the side of the probe. The angled light illumination increases the light travel distance in deep tissue, consequently limiting the imaging depth. This issue was partially addressed by adding a right angle prism in front of the transducer. While this design makes the light illumination and acoustic detection co-axial, the transducer and the fiber bundles are orthogonal to each other, making the system inconvenient for handheld use. To overcome this limitation, here we propose a double-reflector design, in which the second reflector redirects the acoustic signals by another 90°, so that the transducer and the fiber bundle are now parallel to each other. In this design, both the transducer and fiber bundle output are fitted into a compact housing for convenient handheld imaging. To evaluate the efficiency of our design, we performed various phantom and human in vivo experiments. Our results demonstrate that the double-reflector design indeed provides deeper imaging depth and it also allows for easy imaging of objects with uneven surfaces.

Reactive Oxygen Species Explicit Dosimetry (ROSED) of a Type 1 Photosensitizer

Type I photodynamic therapy (PDT) is based on the use of photochemical reactions mediated through an interaction between a tumor-selective photosensitizer, photoexcitation with a specific wavelength of light, and production of reactive oxygen species (ROS). The goal of this study is to develop a model to calculate reactive oxygen species concentration ([ROS]_rx) after Tookad®-mediated vascular PDT. Mice with radiation-induced fibrosarcoma (RIF) tumors were treated with different light fluence and fluence rate conditions. Explicit measurements of photosensitizer drug concentration were made via diffuse reflective absorption spectrum using a contact probe before and after PDT. Blood flow and tissue oxygen concentration over time were measured during PDT as a mean to validate the photochemical parameters for the ROSED calculation. Cure index was computed from the rate of tumor regrowth after treatment and was compared against three calculated dose metrics: total light fluence, PDT dose, reacted [ROS]_rx. The tumor growth study demonstrates that [ROS]_rx serves as a better dosimetric quantity for predicting treatment outcome, as a clinically relevant tumor growth endpoint.

31 P NMR Evidence for Peroxide Intermediates in Lipid Emulsion Photooxidations: Phosphine Substituent Effects in Trapping

Intralipid is a lipid emulsion used in photodynamic therapy (PDT) for its light scattering and tissue-simulating properties. The purpose of this study is to determine whether or not Intralipid undergoes photooxidation, and we have carried out an Intralipid peroxide trapping study using a series of phosphines [2'-dicyclohexylphosphino-2,6-dimethoxy-1,1'-biphenyl-3-sulfonate, 3-(diphenylphosphino)benzenesulfonate, triphenylphosphine-3,3',3''-trisulfonate and triphenylphosphine]. Our new findings are as follows: (1) An oxygen atom is transferred from Intralipid peroxide to the phosphine traps in the dark, after the photooxidation of Intralipid. 3-(Diphenylphosphino)benzenesulfonate is the most suitable trap in the series owing to a balance of nucleophilicity and water solubility. (2) Phosphine trapping and monitoring by 31 P NMR are effective in quantifying the peroxides in H2 O. An advantage of the technique is that peroxides are detected in H2 O; deuterated NMR solvents are not required. (3) The percent yield of the peroxides increased linearly with the increase in fluence from 45 to 180 J cm-2 based on our trapping experiments. (4) The photooxidation yields quantitated by the phosphines and 31 P NMR are supported by the direct 1 H NMR detection using deuterated NMR solvents. These data provide the first steps in the development of Intralipid peroxide quantitation after PDT using phosphine trapping and 31 P NMR spectroscopy.

Determination of optical properties of turbid medium from relative interstitial CW radiance measurements using the incomplete P3 approximation

Interstitial determination of the tissue optical properties is important in biomedicine, especially for interstitial laser therapies. Continuous wave (CW) radiance techniques which examine light from multiple directions have been proposed as minimally invasive methods for determining the optical properties under an interstitial probe arrangement. However, both the fitting algorithm based on the P3 approximation and the analytical method based on the diffusion approximation (DA), which are currently used recovery algorithms, cannot extract the optical properties of tissue with low transport albedos accurately from radiance measurements. In this paper, we proposed an incomplete P3 approximation for the radiance, the P3_in for short, which is the asymptotic part of the solution for the P3 approximation. The relative differences between the P3_in and the P3 were within 0.48% over a wide range of clinically relevant optical properties for measurements at source detector separations (SDS) from 5 mm to 10 mm and angles from 0° to 160°. Based on the P3_in, we developed an analytical method for extracting the optical properties directly using simple expressions constructed from the radiance measurements at only two SDSs and four angles. The developed recovery algorithm was verified by simulated and experimental radiance data. The results show that both the absorption and reduced scattering coefficients were recovered accurately with relative errors within 5.28% and 3.86%, respectively, from the simulated data and with relative errors within 10.82% and 10.67%, respectively, from the experimental data over a wide range of albedos from 0.5 to 0.99. Since the developed P3_in-based radiance technique can obtain the optical properties rapidly from the measurements at only two SDSs and four angles, it is expected to be used for in vivo and in situ determination of the optical properties in online treatment planning during laser therapies.

A Method for Medical Diagnosis Based on Optical Fluence Rate Distribution at Tissue Surface

Optical differentiation is a promising tool in biomedical diagnosis mainly because of its safety. The optical parameters’ values of biological tissues differ according to the histopathology of the tissue and hence could be used for differentiation. The optical fluence rate distribution on tissue boundaries depends on the optical parameters. So, providing image displays of such distributions can provide a visual means of biomedical diagnosis. In this work, an experimental setup was implemented to measure the spatially-resolved steady state diffuse reflectance and transmittance of native and coagulated chicken liver and native and boiled breast chicken skin at 635 and 808 nm wavelengths laser irradiation. With the measured values, the optical parameters of the samples were calculated in vitro using a combination of modified Kubelka-Munk model and Bouguer-Beer-Lambert law. The estimated optical parameters values were substituted in the diffusion equation to simulate the fluence rate at the tissue surface using the finite element method. Results were verified with Monte-Carlo simulation. The results obtained showed that the diffuse reflectance curves and fluence rate distribution images can provide discrimination tools between different tissue types and hence can be used for biomedical diagnosis.

A Comparison of Dose Metrics to Predict Local Tumor Control for Photofrin-mediated Photodynamic Therapy

This preclinical study examines light fluence, photodynamic therapy (PDT) dose and "apparent reacted singlet oxygen," [¹ O₂ ]_rx , to predict local control rate (LCR) for Photofrin-mediated PDT of radiation-induced fibrosarcoma (RIF) tumors. Mice bearing RIF tumors were treated with in-air fluences (50-250 J cm^-2 ) and in-air fluence rates (50-150 mW cm^-2 ) at Photofrin dosages of 5 and 15 mg kg^-1 and a drug-light interval of 24 h using a 630-nm, 1-cm-diameter collimated laser. A macroscopic model was used to calculate [¹ O₂ ]_rx and PDT dose based on in vivo explicit dosimetry of the drug concentration, light fluence and tissue optical properties. PDT dose and [¹ O₂ ]_rx were defined as a temporal integral of drug concentration and fluence rate, and singlet oxygen concentration consumed divided by the singlet oxygen lifetime, respectively. LCR was stratified for different dose metrics for 74 mice (66 + 8 control). Complete tumor control at 14 days was observed for [¹ O₂ ]_rx ≥ 1.1 mm or PDT dose ≥1200 μm J cm^-2 but cannot be predicted with fluence alone. LCR increases with increasing [¹ O₂ ]_rx and PDT dose but is not well correlated with fluence. Comparing dosimetric quantities, [¹ O₂ ]_rx outperformed both PDT dose and fluence in predicting tumor response and correlating with LCR.

Evaluation of the 2-(1-Hexyloxyethyl)-2-devinyl pyropheophorbide (HPPH) mediated photodynamic therapy by macroscopic singlet oxygen modeling

Photodynamic therapy (PDT) is known as a non-invasive treatment modality that is based on photochemical reactions between oxygen, photosensitizer, and a special wavelength of light. However, a dosimetric predictor for PDT outcome is still elusive because current dosimetric quantities do not account for the differences in the PDT oxygen consumption rate for different fluence rates. In this study, we evaluate several dose metrics, total fluence, photobleaching ratio, PDT dose, and mean reacted singlet oxygen (mean [1 O2 ]rx ) for predicting the PDT outcome and a clinically relevant tumor re-growth endpoint. For this reason, radiation-induced fibrosarcoma (RIF) mice tumors are treated with 2-(1-Hexyloxyethyl)-2-devinyl pyropheophorbide (HPPH) and different in-air fluences (30 J/cm2 , 50 J/cm2 , 135 J/cm2 , 250 J/cm2 , and 350 J/cm2 ) and in-air fluence rates (20, 50, 75, 150 mW/cm2 ). Explicit measurements of HPPH and oxygen concentration as well as tissue optical properties are performed pre- and post-treatment. Then, this information is incorporated into a macroscopic model to calculate the photobleaching, PDT dose, and mean [1 O2 ]rx . Changes in tumor volume are tracked following the treatment and compared with the dose metrics. The correlation demonstrates that mean [1 O2 ]rx  serves as a better dosimetric quantity for predicting treatment outcome and a clinically relevant tumor re-growth endpoint.

Analytic function for predicting light fluence rate of circular fields on a semi-infinite turbid medium

Accurate determination of in-vivo light fluence rate is critical for preclinical and clinical studies involving photodynamic therapy (PDT). The light fluence distribution in tissue depends on both the tissue optical properties and the incident field size. This study compares the longitudinal light fluence distribution inside biological tissue in the central axis of circular uniform light field with different radii for a range of in-vivo tissue optical properties (absorption coefficients (µa) between 0.01 and 1 cm-1 and reduced scattering coefficients (µs') between 2 and 40 cm-1). This was done using Monte-Carlo simulations for a semi-infinite turbid medium in an air-tissue interface. The end goal is to develop simple analytical expressions that would fit the results from the Monte Carlo simulation for circular beams with different radii. A 6-parameter model (ϕ/ϕair=(1-b⋅e-λ1d)(C2e-λ2d+C3e-λ3d)) can be used to fit MC simulation. Each of these parameters (b, C2, C3, λ1, λ2, and λ3) is expressed as a function of tissue optical properties and beam radius. These results can then be compared against the existing expressions in the literature for broad beam for analysis in both accuracy and applicable range. The analytical function can be used as rapid guide in PDT to calculate in vivo light fluence distribution for known tissue optical properties.

Explicit macroscopic singlet oxygen modeling for benzoporphyrin derivative monoacid ring A (BPD)-mediated photodynamic therapy

Photodynamic therapy (PDT) is an effective non-ionizing treatment modality that is currently being used for various malignant and non-malignant diseases. In type II PDT with photosensitizers such as benzoporphyrin monoacid ring A (BPD), cell death is based on the creation of singlet oxygen (¹O₂). With a previously proposed empirical five-parameter macroscopic model, the threshold dose of singlet oxygen ([¹O₂]_rx,sh]) to cause tissue necrosis in tumors treated with PDT was determined along with a range of the magnitude of the relevant photochemical parameters: the photochemical oxygen consumption rate per light fluence rate and photosensitizer concentration (ξ), the probability ratio of ¹O₂ to react with ground state photosensitizer compared to a cellular target (σ), the ratio of the monomolecular decay rate of the triplet state photosensitizer (β), the low photosensitizer concentration correction factor (δ), and the macroscopic maximum oxygen supply rate (g). Mice bearing radiation-induced fibrosarcoma (RIF) tumors were treated interstitially with a linear light source at 690nm with total energy released per unit length of 22.5-135J/cm and source power per unit length of 12-150mW/cm to induce different radii of necrosis. A fitting algorithm was developed to determine the photochemical parameters by minimizing the error function involving the range between the calculated reacted singlet oxygen ([¹O₂]_rx) at necrosis radius and the [¹O₂]_rx,sh. [¹O₂]_rx was calculated based on explicit dosimetry of the light fluence distribution, the tissue optical properties, and the BPD concentration. The initial ground state oxygen concentration ([³O₂]₀) was set to be 40μM in this study. The photochemical parameters were found to be ξ=(55±40)×10^-3cm²mW^-1s^-1, σ=(1.8±3)×10^-5μM^-1, and g=1.7±0.7μMs^-1. We have taken the literature values for δ=33μM, and β=11.9μM. [¹O₂]_rx has shown promise to be a more effective dosimetry quantity for predicting necrosis than either light dose or PDT dose, where the latter is simplistically a temporal integral of the products of the photosensitizer concentration and light fluence rate.

Macroscopic singlet oxygen modeling for dosimetry of Photofrin-mediated photodynamic therapy: an in-vivo study

Although photodynamic therapy (PDT) is an established modality for cancer treatment, current dosimetric quantities, such as light fluence and PDT dose, do not account for the differences in PDT oxygen consumption for different fluence rates ( ? ). A macroscopic model was adopted to evaluate using calculated reacted singlet oxygen concentration ( [ O 2 1 ] rx ) to predict Photofrin-PDT outcome in mice bearing radiation-induced fibrosarcoma tumors, as singlet oxygen is the primary cytotoxic species responsible for cell death in type II PDT. Using a combination of fluences (50, 135, 200, and 250 ?? J / cm 2 ) and ? (50, 75, and 150 ?? mW / cm 2 ), tumor regrowth rate, k , was determined for each condition. A tumor cure index, CI = 1 ? k / k control , was calculated based on the k between PDT-treated groups and that of the control, Available on the SPIE Digital Library.

Dosimetry study of PHOTOFRIN-mediated photodynamic therapy in a mouse tumor model

It is well known in photodynamic therapy (PDT) that there is a large variability between PDT light dose and therapeutic outcomes. An explicit dosimetry model using apparent reacted ¹O₂ concentration ([¹O₂]_rx) has been developed as a PDT dosimetric quantity to improve the accuracy of the predicted ability of therapeutic efficacy. In this study, this explicit macroscopic singlet oxygen model was adopted to establish the correlation between calculated reacted [¹O₂]_rx and the tumor growth using Photofrin-mediated PDT in a mouse tumor model. Mice with radiation-induced fibrosarcoma (RIF) tumors were injected with Photofrin at a dose of 5 mg/kg. PDT was performed 24h later with different fluence rates (50, 75 and 150 mW/cm²) and different fluences (50 and 135 J/cm²) using a collimated light applicator coupled to a 630nm laser. The tumor volume was monitored daily after PDT and correlated with the total light fluence and [¹O₂]_rx. Photophysical parameters as well as the singlet oxygen threshold dose for this sensitizer and the RIF tumor model were determined previously. The result showed that tumor growth rate varied greatly with light fluence for different fluence rates while [¹O₂]_rx had a good correlation with the PDT-induced tumor growth rate. This preliminary study indicated that [¹O₂]_rx could serve as a better dosimetric predictor for predicting PDT outcome than PDT light dose.

Investigating the impact of oxygen concentration and blood flow variation on photodynamic therapy

Type II photodynamic therapy (PDT) is used for cancer treatment based on the combined action of a photosensitizer, a special wavelength of light, oxygen (³O₂) and generation of singlet oxygen (¹O₂). Intra-patient and inter-patient variability of oxygen concentration ([³O₂]) before and after the treatment as well as photosensitizer concentration and hemodynamic parameters such as blood flow during PDT has been reported. Simulation of these variations is valuable, as it would be a means for the rapid assessment of treatment effect. A mathematical model has been previously developed to incorporate the diffusion equation for light transport in tissue and the macroscopic kinetic equations for simulation of [³O₂], photosensitizers in ground and triplet states and concentration of the reacted singlet oxygen ([¹O₂]_rx) during PDT. In this study, the finite-element based calculation of the macroscopic kinetic equations is done for 2-(1-Hexyloxyethyl)-2-devinyl pyropheophorbide (HPPH)-mediated PDT by incorporating the information of the photosensitizer photochemical parameters as well as the tissue optical properties, photosensitizer concentration, initial oxygen concentration ([³O₂]₀), blood flow changes and ϕ that have been measured in mice bearing radiation-induced fibrosarcoma (RIF) tumors. Then, [¹O₂]_rx calculated by using the measured [³O₂] during the PDT is compared with [¹O₂]_rx calculated based on the simulated [³O₂]; both calculations showed a reasonably good agreement. Moreover, the impacts of the blood flow changes and [³O₂]₀ on [¹O₂]_rx have been investigated, which showed no pronounced effect of the blood flow changes on the long-term ¹O₂ generation. When [³O₂]₀ becomes limiting, small changes in [³O₂] have large effects on [¹O₂]_rx.

Determination of the low concentration correction in the macroscopic singlet oxygen model for PDT

The macroscopic singlet oxygen model has been used for singlet oxygen explicit dosimetry in photodynamic therapy (PDT). The photophysical parameters for commonly used sensitizers, HPPH and BPD, have been investigated in pre-clinical studies using mouse models. So far, studies have involved optimizing fitting algorithms to obtain the some of the photophysical parameters (ξ, σ, g) and the threshold singlet oxygen dose ([¹O₂]_rx,sh), while other parameters such as the low concentration correction, δ, has been kept as a constant. In this study, using photobleaching measurements of mice in vivo, the value of δ was also optimized and fit to better describe experimental data. Furthermore, the value of the specific photobleaching ratio (σ) was also fine-tuned using the photobleaching results. Based on literature values of δ, σ for photosensitizers can be uniquely determined using the additional photobleaching measurements. This routine will further improve the macroscopic model of singlet oxygen production for use in explicit dosimetry.

Explicit dosimetry for 2-(1-hexyloxyethyl)-2-devinyl pyropheophorbide-a-mediated photodynamic therapy: macroscopic singlet oxygen modeling

Type II photodynamic therapy (PDT) is based on the photochemical reactions mediated through an interaction between a photosensitizer, ground-state oxygen ([(3)O2]), and light excitation at an appropriate wavelength, which results in production of reactive singlet oxygen ([(1)O2]rx). We use an empirical macroscopic model based on four photochemical parameters for the calculation of [(1)O2]rx threshold concentration ([(1)O2]rx,sh) causing tissue necrosis in tumors after PDT. For this reason, 2-(1-hexyloxyethyl)-2-devinyl pyropheophorbide-a (HPPH)-mediated PDT was performed interstitially on mice with radiation-induced fibrosarcoma (RIF) tumors. A linear light source at 665 nm with total energy released per unit length of 12 to 100  J/cm and source power per unit length (LS) of 12 to 150  mW/cm was used to induce different radii of necrosis. Then the amount of [(1)O2]rx calculated by the macroscopic model incorporating explicit PDT dosimetry of light fluence distribution, tissue optical properties, and HPPH concentration was correlated to the necrotic radius to obtain the model parameters and [(1)O2]rx,sh. We provide evidence that [(1)O2]rx is a better dosimetric quantity for predicting the treatment outcome than PDT dose, which is proportional to the time integral of the products of the photosensitizer concentration and light fluence rate.

Tissue-mimicking gel phantoms for thermal therapy studies

Tissue-mimicking phantoms that are currently available for routine biomedical applications may not be suitable for high-temperature experiments or calibration of thermal modalities. Therefore, design and fabrication of customized thermal phantoms with tailored properties are necessary for thermal therapy studies. A multitude of thermal phantoms have been developed in liquid, solid, and gel forms to simulate biological tissues in thermal therapy experiments. This article is an attempt to outline the various materials and techniques used to prepare thermal phantoms in the gel state. The relevant thermal, electrical, acoustic, and optical properties of these phantoms are presented in detail and the benefits and shortcomings of each type are discussed. This review could assist the researchers in the selection of appropriate phantom recipes for their in vitro study of thermal modalities and highlight the limitations of current phantom recipes that remain to be addressed in further studies.

Determination of tissue optical properties in PDT treated Head & Neck patients

Determination of optical properties (absorption (μ_a) and scattering (μ_s') coefficients) in human tissue is important when it comes to accurate calculation of fluence rate in and around tissue area. ALA application to the tissue induces production of protoporphyrin IX when activated by red light. Changes in the tissue optical properties can send information such as treatment outcome and tissue drug concentration. Patients in this study were treated with PDT for head and neck mucosal dysplasia. They were enrolled in a phase I study of escalating light doses and oral ALA with 60mg/kg. Red light at 630nm was administered to the tumor from a laser. The light dose was escalated from 50-200J/cm² with a measured fluence rate at tissue surface of 100mW/cm². We developed a light detection device for the purpose of determining optical properties in vivo using the semi-infinite method. The light detection device consists of two parallel, placed 5mm apart. In one of the catheters a 2 mm long linear diffusing light source is placed while in the second catheter, a calibrated isotropic detector is placed. The detector is scanned along the length of the light source containing catheter. Scans are done with the device placed on the treatment area (tumor) and on the normal tissue. Optical properties were measured in-vivo before and after PDT delivery for both normal tissue and tumor.

Parameter determination for BPD mediated vascularPDT

The cell killing mechanism of benzoporphyrin derivative monoacid ring A (BPD) is known to be predominantly apoptotic or vascular, depending on the drug-light interval (DLI). With a 3 hour DLI, necrosis develops secondary to tumor cell damage, while with a 15 minute DLI, necrosis results from treatment-created vascular damage. The purpose of this study is to examine if the different mechanisms of cell death will affect the photochemical parameters for the macroscopic singlet oxygen model. Using the RIF model of murine fibrosarcoma, we determined the four photochemical parameters (ξ, σ, β, γ) and the threshold singlet oxygen dose for BPD-mediated PDT through evaluation of the extent of tumor necrosis as a function of PDT fluence rate and total fluence. Mice were treated with a linear source at fluence rates from 12-150 mW/cm and total fluences from 24-135 J/cm. BPD was administered at 1mg/kg with a 15 minute DLI, followed by light delivery at 690nm. Tumors were excised at 24 hours after PDT and necrosis was analyzed via H&E staining. The in-vivo BPD drug concentration is determined to be in the range of 0.05-0.30 μM. The determination of these parameters specific for BPD and the 15 minute DLI provides necessary data for predicting treatment outcome in clinical BPD-mediated PDT. Photochemical parameters will be compared between 1mg/kg DLI 3 hours and 1mg/kg DLI 15 minutes.

Tagging photons with gold nanoparticles as localized absorbers in optical measurements in turbid media

We analyze a role of a localized inclusion as a probe for spatial distributions of migrating photons in turbid media. We present new experimental data and two-dimensional analysis of radiance detection of a localized absorptive inclusion formed by gold nanoparticles in Intralipid-1% when the target is translated along the line connecting the light source and detector. Data are analyzed using the novel analytical expression for the relative angular photon distribution function for radiance developed by extending the perturbation approach for fluence. Obtained photon maps allow predicting conditions for detectability of inclusions for which proximity to the detector is essential.

Determination of optical properties by interstitial white light spectroscopy using a custom fiber optic probe

We demonstrate interstitial recovery of absorption and scattering coefficients using a custom optical probe and a Monte Carlo (MC)-based recovery algorithm. The probe consists of six side-firing spectroscopy fibers contained in a 1.1-mm outer diameter cladding, with each fiber having a different axial and angular position on the probe. Broadband white light is delivered by one of the fibers and is detected steady-state by the remaining fibers. These spatially and spectrally resolved data are analyzed using a MC-based fitting algorithm in order to extract the local optical properties. The technique was verified in tissue-simulating phantoms consisting of Intralipid-20% as a scatterer and either manganese meso-tetra (4-sulfanatophenyl) porphine or intact human erythrocytes as an absorber. Absorption coefficients were recovered with a mean error of 9% and scattering coefficients were recovered with a mean error of 19%, whereas the hemoglobin oxygen saturation was recovered with a mean error of 12%. These results demonstrate the feasibility of optical property recovery for situations in which surface-contact spectroscopy is not a possibility, and where only a single probe can be inserted into the tissue.

Extraction of anisotropic parameters of turbid media using hybrid model comprising differential- and decomposition-based Mueller matrices

A hybrid model comprising the differential Mueller matrix formalism and the Mueller matrix decomposition method is proposed for extracting the linear birefringence (LB), linear dichroism (LD), circular birefringence (CB), circular dichroism (CD), and depolarization properties (Dep) of turbid optical samples. In contrast to the differential-based Mueller matrix method, the proposed hybrid model provides full-range measurements of all the anisotropic properties of the optical sample. Furthermore, compared to the decomposition-based Mueller matrix method, the proposed model is insensitive to the multiplication order of the constituent basis matrices. The validity of the proposed method is confirmed by extracting the anisotropic properties of a compound chitosan-glucose-microsphere sample with LB/CB/Dep properties and two ferrofluidic samples with CB/CD/Dep and LB/LD/Dep properties, respectively. It is shown that the proposed hybrid model not only yields full-range measurements of all the anisotropic parameters, but is also more accurate and more stable than the decomposition method. Moreover, compared to the decomposition method, the proposed model more accurately reflects the dependency of the phase retardation angle and linear dichroism angle on the direction of the external magnetic field for ferrofluidic samples. Overall, the results presented in this study confirm that the proposed model has significant potential for extracting the optical parameters of real-world samples characterized by either single or multiple anisotropic properties.

Feasibility of interstitial diffuse optical tomography using cylindrical diffusing fibers for prostate PDT

Interstitial diffuse optical tomography (DOT) has been used to characterize spatial distribution of optical properties for prostate photodynamic therapy (PDT) dosimetry. We have developed an interstitial DOT method using cylindrical diffuse fibers (CDFs) as light sources, so that the same light sources can be used for both DOT measurement and PDT treatment. In this novel interstitial CDF-DOT method, absolute light fluence per source strength (in unit of 1 cm(-2)) is used to separate absorption and scattering coefficients. A mathematical phantom and a solid prostate phantom including anomalies with known optical properties were used, respectively, to test the feasibility of reconstructing optical properties using interstitial CDF-DOT. Three dimension spatial distributions of the optical properties were reconstructed for both scenarios. Our studies show that absorption coefficient can be reliably extrapolated while there are some cross talks between absorption and scattering properties. Even with the suboptimal reduced scattering coefficients, the reconstructed light fluence rate agreed with the measured values to within ±10%, thus the proposed CDF-DOT allows greatly improved light dosimetry calculation for interstitial PDT.

Calibration of a prototype NIRS oximeter against two commercial devices on a blood-lipid phantom

In a blood-lipid liquid phantom the prototype near-infrared spectroscopy oximeter OxyPrem was calibrated against the INVOS® 5100c adult sensor in respect to values of regional tissue oxygen haemoglobin saturation (rStO2) for possible inclusion in the randomised clinical trial - SafeBoosC. In addition different commercial NIRS oximeters were compared on changing haemoglobin oxygen saturation and compared against co-oximetry. The best calibration was achieved with a simple offset and a linear scaling of the OxyPrem rStO2 values. The INVOS adult and pediatric sensor gave systematically different values, while the difference between the NIRO® 300 and the two INVOS sensors were magnitude dependent. The co-oximetry proved unreliable on such low haemoglobin and high Intralipid levels.

Separation of absorption and scattering properties of turbid media using relative spectrally resolved cw radiance measurements

We present a new method for extracting the effective attenuation coefficient and the diffusion coefficient from relative spectrally resolved cw radiance measurements using the diffusion approximation. The method is validated on both simulated and experimental radiance data sets using Intralipid-1% as a test platform. The effective attenuation coefficient is determined from a simple algebraic expression constructed from a ratio of two radiance measurements at two different source-detector separations and the same 90° angle. The diffusion coefficient is determined from another ratio constructed from two radiance measurements at two angles (0° and 180°) and the same source-detector separation. The conditions of the validity of the method as well as possible practical applications are discussed.

Determination of optical properties in heterogeneous turbid media using a cylindrical diffusing fiber

For interstitial photodynamic therapy (PDT), cylindrical diffusing fibers (CDFs) are often used to deliver light. This study examines the feasibility and accuracy of using CDFs to characterize the absorption (μ(a)) and reduced scattering (μ'(s)) coefficients of heterogeneous turbid media. Measurements were performed in tissue-simulating phantoms with μ(a) between 0.1 and 1 cm(-1) and μ'(s) between 3 and 10 cm(-1) with CDFs 2 to 4 cm in length. Optical properties were determined by fitting the measured light fluence rate profiles at a fixed distance from the CDF axis using a heterogeneous kernel model in which the cylindrical diffusing fiber is treated as a series of point sources. The resulting optical properties were compared with independent measurement using a point source method. In a homogenous medium, we are able to determine the absorption coefficient μ(a) using a value of μ'(s) determined a priori (uniform fit) or μ'(s) obtained by fitting (variable fit) with standard (maximum) deviations of 6% (18%) and 18% (44%), respectively. However, the CDF method is found to be insensitive to variations in μ'(s), thus requiring a complementary method such as using a point source for determination of μ'(s). The error for determining μ(a) decreases in very heterogeneous turbid media because of the local absorption extremes. The data acquisition time for obtaining the one-dimensional optical properties distribution is less than 8 s. This method can result in dramatically improved accuracy of light fluence rate calculation for CDFs for prostate PDT in vivo when the same model and geometry is used for forward calculations using the extrapolated tissue optical properties.

Experimental spectro-angular mapping of light distribution in turbid media

We present a new approach to the analysis of radiance in turbid media. The approach combines data from spectral, angular and spatial domains in a form of spectro-angular maps. Mapping provides a unique way to visualize details of light distribution in turbid media and allows tracking changes with distance. Information content of experimental spectro-angular maps is verified by a direct comparison with simulated data when an analytical solution of the radiative transfer equation is used. The findings deepen our understanding of the light distribution in a homogenous turbid medium and provide a first step toward applying the spectro-angular mapping as a diagnostic tool for tissue characterization.

Study of optical parameters of polystyrene spheres in dense aqueous suspensions

We investigated the dependence of the scattering and absorption coefficients of particles in dense suspensions by the low-coherence fiber optic dynamic light scattering (FODLS) technique. The estimated particle size was used to calculate the scattering coefficient of particles suspended in dense suspensions. The path-length resolved intensity distributions of light backscattered from absorbing dense suspensions were investigated experimentally. The absorption coefficient can be obtained by applying the measured path-length resolved intensity distributions to the modified Lambert-Beer law. As a result, the low-coherence FODLS technique can simultaneously measure the scattering and absorption coefficients of particles in absorbing dense suspensions, and the scattering and absorption coefficients are independent of each other in dense suspensions in the low-scattering regime of 2l(d) < 10ℓ*.

Singlet oxygen dosimetry modeling for photodynamic therapy

Photodynamic therapy (PDT) is an important treatment modality for cancer and other localized diseases. In addition to PDT dose, singlet oxygen (¹O₂) concentration is used as an explicit PDT dosimetry quantity, because ¹O₂ is the major cytotoxic agent in photodynamic therapy, and the reaction between ¹O₂ and tumor tissues/cells determines the treatment efficacy. ¹O₂ concentration can be obtained by the PDT model, which includes diffusion equation for the light transport in tissue and macroscopic kinetic equations for the generation of the singlet oxygen. This model was implemented using finite-element method (FEM) by COMSOL. In the kinetic equations, 5 photo-physiological parameters were determined explicitly to predict the generation of ¹O₂. The singlet oxygen concentration profile was calculated iteratively by comparing the model with the measurements based on mice experiments, to obtain the apparent reacted ¹O₂ concentration as an explicit PDT dosimetry quantity. Two photo sensitizers including Photofrin and BPD Verteporfin, were tested using this model to determine their photo-physiological parameters and the reacted ¹O₂ concentrations.

A review of in-vivo optical properties of human tissues and its impact on PDT

A thorough understanding of optical properties of biological tissues is critical to effective treatment planning for therapies such as photodynamic therapy (PDT). In the last two decades, new technologies, such as broadband diffuse spectroscopy, have been developed to obtain in vivo data in humans that was not possible before. We found that the in vivo optical properties generally vary in the ranges μ(a) = 0.03-1.6 cm⁻¹ and μ'(s) = 1.2-40 cm⁻¹, although the actual range is tissue-type dependent. We have also examined the overall trend of the absorption spectra (for μ(a) and μ'(s)) as a function of wavelength within a 95% confidence interval for various tissues in vivo. The impact of optical properties on light fluence rate is also discussed for various light application geometries including superficial, interstitial, and within a cavity.

A review of in-vivo optical properties of human tissues and its impact on PDT

A thorough understanding of optical properties of biological tissues is critical to effective treatment planning for therapies such as photodynamic therapy (PDT). In the last two decades, new technologies, such as broadband diffuse spectroscopy, have been developed to obtain in vivo data in humans that was not possible before. We found that the in vivo optical properties generally vary in the ranges μ(a) = 0.03-1.6 cm⁻¹ and μ'(s) = 1.2-40 cm⁻¹, although the actual range is tissue-type dependent. We have also examined the overall trend of the absorption spectra (for μ(a) and μ'(s)) as a function of wavelength within a 95% confidence interval for various tissues in vivo. The impact of optical properties on light fluence rate is also discussed for various light application geometries including superficial, interstitial, and within a cavity.

Localization of an absorber in a turbid semi-infinite medium by spatially resolved continuous-wave diffuse reflectance measurements

A method to locate an absorber embedded in a semi-infinite turbid medium by spatially-resolved continuous-wave (SRCW) diffuse reflectance measurements is introduced. The depth of the absorber is assessed by single wavelength SRCW diffuse reflectance measurements by two detectors in a radial row. The ratio of perturbations introduced by the defect at two detectors is used to be matched with the ratio-versus-depth curve, which are generated by approximate formulas of continuous wave diffuse reflectance. The error due to approximation and the error in depth assessment are studied for different cases revealing favorable source-detector placements with respect to planar position of the defect. The effect of lateral displacement of the source with respect to defect is studied. A strategy to overcome errors introduced by erroneous prediction of background medium optical properties is suggested. Theoretical results indicate that the depth of the absorber can be obtained with 0.1 mm precision independent of its absorption coefficient and its size for the values chosen in the study. The approach is tested experimentally and it is observed that theoretical results fit with experimental data.

Detection of localized inclusions of gold nanoparticles in Intralipid-1% by point-radiance spectroscopy

Interstitial fiber-optic-based approaches used in both diagnostic and therapeutic applications rely on localized light-tissue interactions. We present an optical technique to identify spectrally and spatially specific exogenous chromophores in highly scattering turbid media. Point radiance spectroscopy is based on directional light collection at a single point with a side-firing fiber that can be rotated up to 360 deg. A side firing fiber accepts light within a well-defined, solid angle, thus potentially providing an improved spatial resolution. Measurements were performed using an 800-μm diameter isotropic spherical diffuser coupled to a halogen light source and a 600 μm, ∼43 deg cleaved fiber (i.e., radiance detector). The background liquid-based scattering phantom was fabricated using 1% Intralipid. Light was collected with 1 deg increments through 360 deg-segment. Gold nanoparticles , placed into a 3.5-mm diameter capillary tube were used as localized scatterers and absorbers introduced into the liquid phantom both on- and off-axis between source and detector. The localized optical inhomogeneity was detectable as an angular-resolved variation in the radiance polar plots. This technique is being investigated as a potential noninvasive optical modality for prostate cancer monitoring.

Light penetration in the human prostate: a whole prostate clinical study at 763 nm

Photodynamic therapy (PDT) is being investigated as a treatment for localized prostate cancer. Photodynamic therapy uses a photosensitizing drug which is activated by a specific wavelength of light, in the presence of oxygen. The activated drug reacts with tissue oxygen to produce reactive oxygen species which are responsible for localized tissue necrosis. One of the determinants of the PDT effect is the penetration of light in the prostate. This study assesses the penetration depth of 763 nm light throughout the prostate. Eight men undergoing multiple hollow needle insertion for high dose rate brachytherapy were recruited. 763 nm light, produced by a diode laser, was delivered to the prostate using cylindrically diffusing optical fibers within the plastic needles. Light was detected at different distances from the source, using an isotropic detector within nearby needles. Penetration depth was calculated using the Boltzmann approximation to the diffusion equation. Delivery detector fiber separation was measured on computed tomography. The mean penetration depth was 0.57 cm, but there was within patient variation of a mean factor of 4.3. Further work is ongoing to assess the effect of such variability in light penetration, on the PDT effect.

The use of India ink in tissue-simulating phantoms

The optical properties of India ink, an absorber often used in preparation of tissue simulating phantoms, have been investigated at visible and near infrared wavelengths. The extinction coefficient has been obtained from measurements of collimated transmittance and from spectrophotometric measurements, the absorption coefficient from multidistance measurements of fluence rate in a diffusive infinite medium with small concentrations of added ink. Measurements have been carried out on samples of India ink from five different brands, and for some brands also from different batches. As also reported in previously published papers the results we have obtained showed large inter-brand and inter-batch variations for both the absorption and the extinction coefficient. On the contrary, our results showed small variations for the ratio between the absorption and the extinction coefficient. The albedo is therefore similar for all samples: The values averaged over all samples investigated were 0.161, 0.115, and 0.115 at λ = 632.8, 751, and 833 nm respectively, with maximum deviations of 0.044, 0.019, and 0.035. These results indicate that, using the values we have obtained for the albedo, it should be possible to obtain with uncertainty smaller than about 4% the absorption coefficient of a sample of unknown ink from simple measurements of extinction coefficient. A similar accuracy is not easily obtained with the complicated procedures necessary for measurements of absorption coefficient.

Intralipid: towards a diffusive reference standard for optical tissue phantoms

Measurements of optical properties carried out at visible and NIR wavelengths on many samples of Intralipid 20% showed a high stability and surprisingly small batch-to-batch variations. Measurements have been carried out in a short time interval using samples from nine different batches with expiry dates spreading over ten years. For the specific reduced scattering coefficient, the values we have obtained, averaged over the nine batches, are 25.9, 21.2, and 18.4 mm(-1) at λ = 632.8, 751, and 833 nm, respectively, and the corresponding maximum deviations from the average were 2.2%, 1.1%, and 1.4%. For the absorption coefficient, we obtained values slightly smaller with respect to the absorption coefficient of pure water at 751 and 833 nm, and slightly larger at 632.8 nm. These results suggest that Intralipid 20% can be the first step towards a diffusive reference standard for tissue-simulating phantoms.