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Intralipid: Towards a diffusive reference standard for optical
Physics in Medicine & Biology
Abstract
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.
... The use of ink and Intralipid in tissue-simulating phantoms has been studied and described in the literature, e.g. Di Ninni et al. [81,82] and Spinelli et al. [83,84]. In this section, we summarize how we determined the nominal values of µ a and µ ′ s , which we used as a guide for the experimental protocol and later for a comparison with our TD-NIRS results. ...
... We used SMOFlipid 20% (Fresenius Kabi, Poland) as a substitute for Intralipid, which we confirmed has similar µ ′ s and µ a . We calculated the nominal µ ′ s of a phantom based on the volume fraction of SMOFlipid [81,84]: ...
... where ε ′ s,SL is the specific reduced scattering coefficient of SMOFlipid. We used the value 21.5 mm −1 for 750 nm, which was reported for Intralipid [81,83,84]. V Dilted ink , V SL , and V water are the volumes of diluted ink, SMOFlipid, and water, respectively. ...
Near-infrared spectroscopy (NIRS) is an established technique for measuring tissue oxygen saturation (StO2), which is of high clinical value. For tissues that have layered structures, it is challenging but clinically relevant to obtain StO2 of the different layers, e.g. brain and scalp. For this aim, we present a new method of data analysis for time-domain NIRS (TD-NIRS) and a new two-layered blood-lipid phantom. The new analysis method enables accurate determination of even large changes of the absorption coefficient (Δµa) in multiple layers. By adding Δµa to the baseline µa, this method provides absolute µa and hence StO2 in multiple layers. The method utilizes (i) changes in statistical moments of the distributions of times of flight of photons (DTOFs), (ii) an analytical solution of the diffusion equation for an N-layered medium, (iii) and the Levenberg–Marquardt algorithm (LMA) to determine Δµa in multiple layers from the changes in moments. The method is suitable for NIRS tissue oximetry (relying on µa) as well as functional NIRS (fNIRS) applications (relying on Δµa). Experiments were conducted on a new phantom, which enabled us to simulate dynamic StO2 changes in two layers for the first time. Two separate compartments, which mimic superficial and deep layers, hold blood-lipid mixtures that can be deoxygenated (using yeast) and oxygenated (by bubbling oxygen) independently. Simultaneous NIRS measurements can be performed on the two-layered medium (variable superficial layer thickness, L), the deep (homogeneous), and/or the superficial (homogeneous). In two experiments involving ink, we increased the nominal µa in one of two compartments from 0.05 to 0.25 cm⁻¹, L set to 14.5 mm. In three experiments involving blood (L set to 12, 15, or 17 mm), we used a protocol consisting of six deoxygenation cycles. A state-of-the-art multi-wavelength TD-NIRS system measured simultaneously on the two-layered medium, as well as on the deep compartment for a reference. The new method accurately determined µa (and hence StO2) in both compartments. The method is a significant progress in overcoming the contamination from the superficial layer, which is beneficial for NIRS and fNIRS applications, and may improve the determination of StO2 in the brain from measurements on the head. The advanced phantom may assist in the ongoing effort towards more realistic standardized performance tests in NIRS tissue oximetry. Data and MATLAB codes used in this study were made publicly available.
... The most dominant choice in the field of biomedical optics for a turbid phantom has been the various forms of commercially available lipid emulsions, used for intravenous feeding of patients. The leading version of this is called Intralipid, 79,80 shown in Figure 5, however other trade names from other companies are also used, such Not biological/organic Rigorous creation process F I G U R E 5 A schematic of Intralipid composed largely of soybean oil droplets in water is illustrated (a) with a histogram of particle sizes measured by electron microscopy (b). 80 A vial of Intralipid is shown as supplied by one manufacturer (c) with example Intralipid-blood phantoms 153 (d) with an aqueous mix of 1% intralipid and 1% blood, at full oxygenation (top) and deoxygenated (bottom), and extinction spectra of Intralipid phantoms with added constituents for fluorescence measurement (e). ...
... Nonetheless, this is likely the most widely utilized tissue phantom matrix material in scientific laboratories, and there is widespread literature on its optical characteristics and use. [79][80][81][82][83] The use of blood as an absorber is widely utilized since it perfectly mimics the blood and water absorption, which dominates soft tissue 84 and is widely commercially available from non-human sources. Also, most fluorescent agents used in humans can then be directly dissolved in the phantom, allowing for a good match to the in vivo situation with nearly identical spectral characteristics and calibration approaches. ...
The last decade has seen a large growth in fluorescence‐guided surgery (FGS) imaging and interventions. With the increasing number of clinical specialties implementing FGS, the range of systems with radically different physical designs, image processing approaches, and performance requirements is expanding. This variety of systems makes it nearly impossible to specify uniform performance goals, yet at the same time, utilization of different devices in new clinical procedures and trials indicates some need for common knowledge bases and a quality assessment paradigm to ensure that effective translation and use occurs. It is feasible to identify key fundamental image quality characteristics and corresponding objective test methods that should be determined such that there are consistent conventions across a variety of FGS devices. This report outlines test methods, tissue simulating phantoms and suggested guidelines, as well as personnel needs and professional knowledge bases that can be established. This report frames the issues with guidance and feedback from related societies and agencies having vested interest in the outcome, coming from an independent scientific group formed from academics and international federal agencies for the establishment of these professional guidelines.
... In contrast, intralipid solutions may change their optical properties due to factors like temperature changes or particle aggregation, leading to variability in imaging outcomes. 38,39 Moreover, tumor phantoms can replicate the structural characteristics of tumors, including their dimensions, and morphology. By regulating the concentration of fluorophores within these phantoms, concentrations closer to those obtained in vivo mouse models can be achieved. ...
Afterglow imaging, leveraging persistent luminescence following light cessation, has emerged as a promising modality for surgical interventions. However, the scarcity of efficient near-infrared (NIR) responsive afterglow materials, along with their inherently low brightness and lack of cyclic modulation in afterglow emission, has impeded their widespread adoption. Addressing these challenges requires a strategic repurposing of afterglow materials that improve on such limitations. Here, we have developed an afterglow probe, composed of bovine serum albumin (BSA) coated with an afterglow material, a semiconducting polymer dye (PFODBT/SP1), called BSA@SP1 demonstrating a substantial amplification of the afterglow luminescence (∼3-fold) compared to polymer-lipid coated PFODBT (DSPE-PEG@SP1). This enhancement is believed to be attributed to the electron-rich matrix provided by BSA that immobilizes SP1 and enhances the generation of ¹ O2 radicals, which improves the afterglow luminescence brightness. Through molecule docking, physicochemical characterization, and optical assessments, we highlight BSA@SP1’s superior afterglow properties, cyclic afterglow behavior, long-term colloidal stability, and biocompatibility. Furthermore, we demonstrate superior tissue permeation profiling of afterglow signals of BSA@SP1’s compared to fluorescence signals using ex vivo tumor-mimicking phantoms and various porcine tissue types (skin, muscle, and fat). Expanding on this, to showcase BSA@SP1’s potential in image-guided surgeries, we implanted tumor-mimicking phantoms within porcine lungs and conducted direct comparisons between fluorescence and afterglow-guided interventions to illustrate the latter’s superiority. Overall, our study introduces a promising strategy for enhancing current afterglow materials through protein complexation, resulting in both ultrahigh signal-to-background ratios and cyclic afterglow signals.
Abstract Figure
... Finally, 100 mL homogeneous mixed solution was poured in the acrylic container and put in the freezer at −4 • C to cool for 20 min, and it was removed after solidification, as shown in Figure 2a. In this experiment, the µ a reference value of the phantoms was obtained by using a spectrometer (QE65pro, Ocean Insight, Orlando, FL, USA) according to Beer Lambert's law, and then, the µ s reference value in the phantom of the same volumetric concentration was obtained from the reduced scattering coefficient in a pure solution of 20% lipid emulsion [27]. ...
As a new imaging inspection method with characteristics of a wide view field and non-contact, spatial frequency domain imaging (SFDI) is very suitable to evaluate the optical properties of agricultural products to ensure the sustainable development of agriculture. However, due to the unique forward scattering characteristics of fruit skin, only a few photons can return to the skin surface after interacting with the flesh, thus affecting the detection accuracy of the flesh layer. This study aims to propose a more accurate and wider applicable method to extract the optical properties of two-layer tissue from SFDI measurements. Firstly, a two-layer model was proposed by optimizing the reflectivity of the flesh layer through the optical properties and thickness of the skin layer. Secondly, the influence of the optical properties and thickness of different skin layers on the reflectivity optimization of the flesh layer was investigated by a Monte Carlo simulation, and then, the accuracy and effectiveness of the proposed model was evaluated for practical inspection by phantom experiments. Finally, this model was used to obtain the optical properties, layer by layer, of four thin-skinned fruits (pear, apple, peach and muskmelon) to verify its universality. The results showed that, for the skin layer, the average errors of the absorption coefficient (μa1) and the reduced scattering coefficient (μ′s1) were 10.87% and 7.91%, respectively, and for the flesh layer, the average errors of the absorption coefficient (μa2) and the reduced scattering coefficient (μ′s2) were 16.76% and 8.64%, respectively. This study provides the basis for the SFDI detection of optical properties of two-layer tissue such as thin-skinned fruits, which can be further used for nondestructive fruit quality evaluations.
Optical phantoms are important tools in biomedical imaging for calibration and validation of photonic instrumentation medicine, especially with readily available and relatively inexpensive materials to characterize the scattering property. In this work, using laser collimated transmission and diffuse reflectance, the optical properties of pasteurized milk-based phantoms are characterized and compared with the optical properties of intralipid-based phantoms to construct phantoms comparable to intralipid-based phantoms at 405 nm. To that end, three different types of pasteurized milk corresponding to three different manufacturers and intralipid 20% samples with varying concentrations were prepared and their optical properties were determined. The results show that the optical properties of intralipid-based phantoms and pasteurized milk-based phantoms are roughly comparable. The stability of pasteurized milk samples was also studied, and it showed that emulsions exhibit varying levels of stability up to three hours. Despite the short time stability of the constructed milk-based phantoms, the results demonstrate the potential of using readily available simple laser-based transmission and reflection measurements to prepare milk-based optical phantoms for rapid calibration and testing of optical measuring devices particularly for biphotonic applications.
Intralipid is a material widely employed for the preparation of phantoms for optical imaging and biophotonics applications in medical field. The development of new optical diagnostic equipment in these fields requires the use of well-designed phantoms with optical properties (including scattering and absorption) mimicking those of biological tissues in all the pre-clinical stages of investigations. For this reason, great research effort has been devoted to optically characterize Intralipid and at preparing optimal phantoms. In this short review, we summarize the principal physico-chemical characteristics of Intralipid and the main contributions in the assessment of its scattering and absorption properties. In addition, the most largely used Intralipid-based homogeneous and non-homogeneous phantoms are discussed. Even though other materials are available for the preparation of phantoms, the use of Intralipid still offers an inexpensive and easy-to-use method for preparing phantoms with finely tuned optical properties.
The absorption and transport scattering coefficients of biological tissues determine the radial dependence of the diffuse reflectance that is due to a point source. A system is described for making remote measurements of spatially resolved absolute diffuse reflectance and hence noninvasive, noncontact estimates of the tissue optical properties. The system incorporated a laser source and a CCD camera. Deflection of the incident beam into the camera allowed characterization of the source for absolute reflectance measurements. It is shown that an often used solution of the diffusion equation cannot be applied for these measurements. Instead, a neural network, trained on the results of Monte Carlo simulations, was used to estimate the absorption and scattering coefficients from the reflectance data. Tests on tissue-simulating phantoms with transport scattering coefficients between 0.5 and 2.0 mm(-1) and absorption coefficients between 0.002 and 0.1 mm(-1) showed the rms errors of this technique to be 2.6% for the transport scattering coefficient and 14% for the absorption coefficients. The optical properties of bovine muscle, adipose, and liver tissue, as well as chicken muscle (breast), were also measured ex vivo at 633 and 751 nm. For muscle tissue it was found that the Monte Carlo simulation did not agree with experimental measurements of reflectance at distances less than 2 mm from the incident beam.
The absorption, scattering, and anisotropy coefficients of the fat emulsion Intralipid-10% have been measured at 457.9, 514.5, 632.8, and 1064 nm. The size and shape distributions of the scattering particles in Intralipid-10% were determined by transmission electron microscopy. Mie theory calculations performed by using the particle size distribution yielded values for the scattering and anisotropy coefficients from 400 to 1100 nm. The agreement with experimental values is better than 6%.
Measurements of optical properties (scattering coefficient mu(s), absorption coefficient mu(a), reduced scattering coefficient mu(s)', and asymmetry factor g) have been carried out up to a volume particle concentration of rho = 0.227. The results for mu(s) and mu(s)' show significant deviations from the linear dependence on rho as expected when the independent scattering assumption is fulfilled. The asymmetry factor also changed significantly. In contrast, the dependence of mu(a) remained linear even at the largest concentration investigated. The simple linear dependence of absorption on the chromophore concentration expected from the independent scattering assumption is thus applicable also to spectroscopic measurements of dense media. A comparison with an approximate theoretical model based on the Foldy-Twersky equation is also reported. The model provides a good description of the dependence of mu(s) on particle concentration.
We propose a comprehensive protocol for the performance assessment of photon migration instruments. The protocol has been developed within the European Thematic Network MEDPHOT (optical methods for medical diagnosis and monitoring of diseases) and is based on five criteria: accuracy, linearity, noise, stability, and reproducibility. This protocol was applied to a total of 8 instruments with a set of 32 phantoms, covering a wide range of optical properties.
Optical spectroscopy, imaging, and therapy tissue phantoms must have the scattering and absorption properties that are characteristic of human tissues, and over the past few decades, many useful models have been created. In this work, an overview of their composition and properties is outlined, by separating matrix, scattering, and absorbing materials, and discussing the benefits and weaknesses in each category. Matrix materials typically are water, gelatin, agar, polyester or epoxy and polyurethane resin, room-temperature vulcanizing (RTV) silicone, or polyvinyl alcohol gels. The water and hydrogel materials provide a soft medium that is biologically and biochemically compatible with addition of organic molecules, and are optimal for scientific laboratory studies. Polyester, polyurethane, and silicone phantoms are essentially permanent matrix compositions that are suitable for routine calibration and testing of established systems. The most common three choices for scatters have been: (1.) lipid based emulsions, (2.) titanium or aluminum oxide powders, and (3.) polymer microspheres. The choice of absorbers varies widely from hemoglobin and cells for biological simulation, to molecular dyes and ink as less biological but more stable absorbers. This review is an attempt to indicate which sets of phantoms are optimal for specific applications, and provide links to studies that characterize main phantom material properties and recipes.
In a dense distribution of particles, the propagation characteristics of the coherent field are strongly affected by the pair-correlated distributions of scatterers. This paper presents an optical experimental study to show that, when the particle density is greater than about 0.1%, the attenuation constant departs markedly from the formula based on an uncorrelated scatter assumption. It decreases sharply when ka <1, whereas it shows a slight increase when ka ≫ 1. Experimental data are shown for the volume densities ranging from 10-3 to 40% and ha ranging from 0.529 to 82.793. Comparisons are given with some theoretical calculations.
Attenuation measurements of an He-Ne laser beam in suspensions of polystyrene latex spheres are presented. Measurements were performed with a transmissometer having a receiver with a variable field of view (FOV) in a lenspinhole geometry. Attenuation measurements taken with different fields of view for any fixed optical depth, enabled us to separate the scattered received power Ps and the beam-attenuated power P0 from the total received power P. In this way the true value τ of the optical depth was obtained from P0 and compared with the apparent optical depth τA obtained from P. Thus the error ε = (τ − τA)/τ due to the contribution of scattered received power in attenuation measurements was investigated. Many measurements were taken in order to see how τA and ε vary with the optical depth, with the size of the diffusing spheres and with geometric arrangement (radius and angular FOV of the receiver, distance between scattering cell and receiver).
In spite of many progresses achieved both with theories and with experiments in studying light propagation through diffusive media, a reliable method for accurate measurements of the optical properties of diffusive media at NIR wavelengths is, in our opinion, still missing. It is therefore difficult to create a diffusive medium with well known optical properties to be used as a reference. In this paper we describe a method to calibrate the reduced scattering coefficient, mu'(s) , of a liquid diffusive medium and the absorption coefficient, mu(a), of an absorbing medium with a standard error smaller than 2% both on mu'(s) and on mu(a). The method is based on multidistance measurements of fluence into an infinite medium illuminated by a CW source. The optical properties are retrieved with simple inversion procedures (linear fits) exploiting the knowledge of the absorption coefficient of the liquid into which the diffuser and the absorber are dispersed. In this study Intralipid diluted in water has been used as diffusive medium and Indian ink as absorber. For a full characterization of these media measurements of collimated transmittance have also been carried out, from which the asymmetry factor of the scattering function of Intralipid and the single scattering albedo of Indian ink have been determined.
We estimated the absorption and reduced scattering coefficients of tissue-simulating phantoms from interstitial measurements of the phase difference and relative amplitude signals at two distances from a sinusoidally modulated isotropic source. It was found that absorption and reduced scattering coefficients can be recovered within 10% and slightly over 10% respectively, using either the data collected by two detectors 3mm apart or by two detectors 5mm apart with light collected by one detector attenuated by a neutral density filter. This accuracy was achieved over a wide range of optical properties, mu(a)=0.008 to 0.17mm(-1) and mu(s)'=0.3 to 1.8mm(-1). Additional factors affecting accuracy including source anisotropy, uncertainty in fiber placement, phase amplitude crosstalk, and the forward light propagation model (the combined isotropic similarity model and standard diffusion approximation versus the modified spherical harmonics method) were studied by Monte Carlo simulations (first two factors) and experiments (last two factors).
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.