Many methods have been developed in the past few decades to determine the optical properties of turbid media, like tissues, that depend on light diffusion. They include diffuse reflectance, diffuse transmission, and reflectance as a function of source-detector distance.1, 2, 3 These techniques can measure the absorption coefficient and the reduced scattering coefficient . But separating into the scattering coefficient and the anisotropy of scattering (g) is difficult unless a tissue sample is removed to allow other measurements, like collimated transmission or goniometry. Knowing both and g can characterize the ultrastructure of tissues that yields the scattering properties, i.e., the apparent particle size distribution like mitochondria, nuclei, lipid membranes, collagen, etc. More information from the and g of tissues may prove useful in monitoring the changes in ultrastructure due to different pathologies.4
In this work, reflectance-mode confocal scanning laser microscopy (rCSLM) was used to determine the optical properties of excised murine skin samples. Contrast in rCSLM is due to the scattering by the tissue ultrastructure, which enables noninvasive imaging of tissues without introducing external contrast agents. The collection of reflectance from the focus of a high-numerical-aperture (NA) lens is a function of the scattering properties of the tissue. The rCSLM provides a signature of the tissue-scattering properties that characterizes the ultrastructure. A simple analysis has being developed in our laboratory to extract optical properties ( and g) from the confocal signal, , obtained as a function of the depth of focus, .5, 6
Other groups have also studied the use of confocal reflectance, or optical coherence tomography (OCT), which is similar, as a function of the depth of the focus to determine the scattering coefficient and anisotropy g.7, 8 However, these previous reports have not considered the contribution of photons that partially scatter but still reach the focus. This paper outlines the analysis that considers such photons when interpreting measurements to yield optical properties.
In this paper, optical measurements are used to sense a single gene defect by its affect on dermal scattering properties in a murine model. The mutation is a dominant negative gene defect that causes osteogenesis imperfecta, also called brittle bone disease, which is caused by a defect in type I collagen structure and characterized by brittle bones that are prone to frequent fractures.9 The gene defect also manifests as a change in dermal scattering due to the failure of collagen fibrils to properly form and assemble into collagen fiber bundles. The structural consequences of this mutation have been studied using second harmonic generation imaging.10
The optical study was conducted on murine skin samples from the University of Connecticut, where the mice carrying the osteogenesis imperfecta mutation were maintained in the B6C3Fe-a/a (C57BL/6JLe X C3HeB/FeJLe) hybrid background under approved animal care protocol. The animals were in age. Skin tissues were harvested from the thigh area immediately after euthanasia, then placed in saline and packed in ice for shipping. Samples were received at the Oregon Health & Science University within , and optical experiments were conducted immediately.
The three skin tissue samples were blindly labeled as #1, #2, and #3, and later revealed to be:10
The rCSLM was performed with a confocal microscope operating in reflectance mode, in which a laser delivered light to a focus within a sample. The light reflected from this focal volume was returned to a detector through a confocal pinhole (Fig. 1 ). The rCSLM system in these experiments was developed for in vivo imaging of mouse skin, in an inverted microscope configuration (Fig. 2 ). The system was comprised of an argon ion laser ( wavelength, ), a beamsplitter to enable delivery yet redirect reflected light to a pinhole/detector, scanning optics that consisted of - and -axis galvo scanning mirrors for lateral scanning at each depth of focus (Nutfield Technology, Inc. RS-15) and a pair of relay lenses for directing the scanned beams into the objective lens, an objective lens (60X water-dipping objective lens, 0.90 NA, Olympus LUMPlanFl), and a motorized -axis scanning stage (Applied Scientific Instrumentation, LS50A) to move the animal for selection of the focal depth. The reflected light was redirected by the beamsplitter into a lens that focused the collimated beam through a pinhole to reach the detector. The -diameter size of the pinhole corresponded to the central lobe of the Airy radius of the beam after magnification by the pair of relay lenses. The detector was a photomultiplier tube (Hamamatsu Photonics, 5773-01) whose voltage output was recorded by a data acquisition board (National Instruments, 6062E). The system was controlled by a laptop computer (Dell, Windows XP operating system) running a program written using Labview™ software (National Instruments). A band-rejection filter (Z488/633) was used to isolate the fluorescence signal from the reflectance signal.
The skin samples were placed epidermal surface down on a metal plate that was the size and shape of a standard microscope slide with a -diameter hole at its center. Tissues were coupled to the objective lens through this central hole by an aqueous saline solution. The microscope viewed the skin sample from below, scanning from the epidermis up into the dermis. The -axis stage stepped through 75 steps, and the - and -axis scanning mirrors captured a image at each depth of focus at a pixel acquisition rate (a rather slow acquisition in this prototype system, requiring about to complete a 3-D image). The pixel size was . Experiments were conducted at a room temperature of .
A 3-D image data set, or image cube, was acquired for each of three sites on each of the two mutant samples, and on each of two sites for the wt/wt skin sample, for a total of eight image cubes. Data was recorded as [Volts] in the range of 0 to 10 . The pixel values for were converted to reflectance units, (dimensionless), by the expressionwas the measurement of an oil-glass coverslip interface, and was the value , where (oil), and (glass).
For each image cube, a set of randomly chosen locations was analyzed. About 60% of the locations were rejected for analysis if the skin surface was not centered within the -axis field of view, which would jeopardize the analysis, or if there was a hair follicle or other surface abnormality. Typically, about 4000 sites were found acceptable for analysis from each of the eight image cubes.
For each acceptable location, the average of 100 profiles over a pixel neighborhood centered around the position of the chosen site was determined. The average at each position was analyzed within the combined epidermal and papillary dermis region and the reticular dermal region of the skin to specify the exponential decays in each region. The basic equation for decay was a simple exponential decay [Fig. 3a ] that was characterized by two parameters, and , where was the amplitude (called reflectivity), and was the exponential decay constant (called attenuation). Figure 3b show a typical average profile. The analysis found the peak reflectance associated with the stratum corneum (sc) to identify the front surface, then moved deeper to find the middle of the epidermis away from the strong influence of the front surface. The papillary dermis began deeper than the front surface, and the reticular dermis began deeper than the front surface. The epidermis beyond and the papillary dermis behaved similarly, and were quite variable. The typical value of attenuation was . The thickness of the combined epidermis and papillary dermis was . Hence, the superficial attenuation by the combined epidermis and papillary dermis was . This paper reports the results of the reticular dermis, which provided less variable data and better analysis than the papillary dermis. The reticular and were fit to the data at depths greater than from the front surface, following the expressionwas the depth from the front surface. The value of was the reflectivity at , based on fitting the data at . The average value of for all sites was used in the analysis, since attempting to use the local value of for each site introduced significant variability into the analysis. In summary, the and for the reticular dermis were determined after correcting for the average attenuation of the overlying epidermis and papillary dermis.
Specifying Optical Properties
An analysis has been developed that maps the two experimental parameters, and , into the two unknown optical properties, the scattering coefficient and the anisotropy of scattering g.5, 11 The value is expressedis the axial length of the focal volume from which light is backscattered to reach the detector through the pinhole. The product gives the fraction of light delivered to the focus that is scattered by the tissue within the confocal volume. The factor is the fraction of the scattered light that is backscattered within the collection solid angle of the objective lens and detected at the pinhole. The value of is dependent on the scattering phase function , where is the angle of deflection of photons by the tissue, and is the numerical aperture of the objective lens. In this work, was specified by the Henyey-Greenstein phase function , which is a unique function of g that closely approximates the scattering function of skin.12 The value is calculated by is given by is the classical axial extent of the focus.12 In these experiments, routine calibration was accomplished by imaging an oil-glass interface for use in Eq. 1. An alternative calibration used -diameter polystyrene microspheres in a 2% agarose gel ( thick, using glass coverslips as spacers). A collimated transmission measurement through the gel specified that equaled , which corresponded to a 0.034 volume fraction of microspheres if the microspheres behaved as predicted by Mie theory. Testing (not shown here) of sphere scattering versus sphere concentration confirmed that the spheres obeyed Mie theory at this concentration. The factor in Eq. 5 equaled 1.3, which allowed the and g predicted by Mie theory to match the experimental data. This need for a correction factor is attributed to the difficulties in using an oil-glass interface for calibration and/or to the possibility that the signal arises from a confocal volume whose axial extent exceeds the classical . This issue is a topic of continued work, and only slightly affects the accuracy of the values deduced in these experiments.
The attenuation is also a function of and g:is a factor that diminishes the effectiveness of scattering to prevent photons from reaching the focus, . The depends on the forward-directed nature of the scattering events, which is a function of g. As g approaches 1, approaches 0, which reduces the effectiveness of scattering in the tissue such that photons can reach the focus despite a few scattering events. The function was determined by Monte Carlo simulations of photon transport to a focus within scattering media for various values of and g. The function was found to follow the expression and .
The factor 2 accounts for the double path traversed by photons into and out of the tissue. The factor is a geometry factor that accounts for the extra pathlength the photons travel relative to the depth of focus when they are launched as a focused cone from an objective lens of numerical aperture . The value of is calculated byis the depth of the focal volume, is the angle of incidence relative to the axis, is the angle of collection, and is the portion of the arc of scattered light that falls within the numerical aperture of the lens. The term , where , is the Henyey-Greenstein scattering function. The term was only weakly affected by the anisotropy of the scattering function. The value of for was 1.37, which was used in the analysis. The value of for murine skin samples at a wavelength was negligible; for example, a value of in the analysis yielded an attenuation of . Experiments on the system (not shown here) showed that the signal due to autofluorescence from the skin collected in the separate fluorescence channel in the spectral range of was 3 orders of magnitude less than the reflectance signal. Thus, the contribution of autofluorescence was not considered in the above analysis.
By using the above equations, the values of and for different values of and g were calculated and plotted as a grid on the versus graph (shown in Fig. 5). The experimentally determined data pair, and , was also plotted and could be compared with the grid to specify the corresponding and g for each data pair. The magnitude of based on the confocal measurement of the spheres in gel matched the magnitude of specified by the separate collimated transmission measurement. In this manner, the method and analysis were calibrated.
Figure 4a shows a typical image of a murine skin, portrayed as a cross-sectional view of from the skin sample, a homozygous mutant. The various layers of the skin are labeled. The reflectance is plotted using a colorbar based on the . The water/stratum corneum interface shows brightly, the epidermis has a lower reflectance, and the dermis has a stronger reflectance than the epidermis. Between and , the edge of a hair follicle is seen, which disrupts the normal planar pattern of the skin. Figure 4b shows an en face image [R(x,y@z)], illustrating the change in collagen fiber bundles in the mutant.
Nine plots of versus are shown in Fig. 5 . Each column of three plots represents a different mouse skin sample except the upper plot in the first column, which shows the calibration and labels for the iso- and iso-g contours used in the figures. The first column shows the homozygous wildtype (wt/wt), the second column shows the heterozygous mutant (wt/oim), and the third column shows the homozygous mutant (oim/oim). Data are plotted as a grayscale encoded histogram on the grid. A dark color signifies a higher frequency of occurrence of a particular data pair. In columns 2 and 3, the mean value of the wildtype data is depicted as a circle for reference. The data lay approximately in the ranges and . The variation in the data spreads in the plot as a coordinate increase in and a decrease in g, which is considered in Sec. 4. The value of g decreases with the presence of the oim mutation. The distribution of data points also seems to be narrower for the wildtype and broader for the mutant tissues. A subpopulation of values in one of the wildtype sites and several of the mutant sites is not yet understood.
The calibration plot (top left graph in Fig. 5) also plots the experimentally observed and for polystyrene microspheres at a 0.034 volume fraction (circle at , ). The grid is designed so that the prediction of Mie theory is aligned to exactly match this experimental measurement. As a test of the calibration, the specified by the collimated transmission measurement (square) is also shown. The circle within the square indicates that the collimated transmission measurement and the confocal measurement were in close agreement.
An interesting feature of the experimental data in Fig. 5 is the characteristic spread of the data, in which the and g appear to vary in a coordinate manner. To clarify the nature of this variation, a simple exercise was conducted in which the analysis was applied to simulated data with added experimental variation. This exercise is shown in Fig. 6 . Figure 6a shows a sampling of the wildtype and mutant data in the most dense portions of the histograms in Fig. 5, replotted as , g, and versus the reflectivity . The values of and g for the wildtype were and 0.81, respectively. For the mutants, the values were and 0.46. As increased, increased. As g decreased and (1−g) increased, increased. As and (1−g) increased—and therefore increased— increased. All these results were expected, but the coordinate variation in the and values of these plots was also apparent. The central thick circles show the approximate peaks of the histograms for the wildtype and mutant. Figure 6b shows simulated data using the peak values of and g cited above for the wildtype and mutant. These and g values were used in Eqs. 3, 6 to yield and , then used in Eq. 2 to yield . Then variation was added to , where , and is a vector of random numbers between 0 and 1 such that each position received its own unique fluctuation. This procedure added variation to the simulated values of . Such variation may simulate variation in the optical properties of the tissue or noise imposed by laser speckle. Finally, the versus was fit by a straight line with -intercept and slope . The resulting and were converted to and g using the analysis grid shown in Fig. 5, and the values were calculated. This procedure yielded Fig. 6b. The variation in the simulated results with added variation appeared similar to the variation seen in the experimental results. The source of variation is an ongoing topic of investigation.
From the data summarized in Fig. 6, the oim mutation appeared to decrease the anisotropy of scattering of the skin tissue from 0.81 to 0.46, which corresponds to less forward-directed scattering, presumably due to the failure of fibrils to aggregate into fiber bundles as large as the wildtype. The smaller size scale of the collagen fibers yielded more isotropic scattering. The scattering coefficient itself was only slightly increased by the mutation, from .
In summary, the rCSLM images allowed an assessment of the optical consequences of a single mutation that affected the collagen fibril assembly. The mutation caused a decrease in anisotropy g, with only a slight increase in the scattering coefficient . The distinct changes in and g could be separately determined. The behavior of an ensemble of local sites could be documented, and the statistical variation encountered could be considered.
This work is preliminary in nature. These are our first studies with these wildtype and mutant skin samples. More work is needed before drawing firm conclusions about the effects of the mutant. However, this study illustrates the utility of the method and some of the considerations in applying the analysis.