How We Do It
The reflection of light off of a material can tell us a lot about the material’s properties — if we interpret the information correctly. Unfortunately, this is easier said than done. We all see color differently and misinterpret the differences between pigments (absorption) and opacity/translucency (scattering). At Modulated Imaging, we are commercializing Spatial Frequency Domain Imaging (SFDI), a scientifically researched and published imaging technique that uses patterned illumination technology, and advanced computational methods, to take the guesswork out of optical characterization.
Light in Skin…
As we said, the reflection of light off of a material can tell us a lot about the material’s properties. For example, if you ever donate blood in a drive, a bag of fresh, healthy oxygenated blood is a bright red color. However, as blood loses oxygen, it takes on a duller, more purple hue. This is due to the difference in the absorption properties of hemoglobin when it is oxygenated (oxy-hemoglobin), or has become deoxygenated (deoxy-hemoglobin).
In our circulatory system, hemoglobin is responsible for oxygen delivery to our tissue. A pulse oximeter, a mainstay in every intensive care unit, analyzes the signature pulsatile signal in the arteries at two different colors of light to isolate hemoglobin signatures. This results in an excellent measure of the relative amounts of oxy- and deoxy- hemoglobin being delivered to the tissue. These values can then be used to calculate the arterial oxygenation (SaO2), which essentially conveys how the heart and lungs are functioning. But there is more to the story than this – oxygen has to be delivered and consumed by tissue at the capillary level. At this level, the color and intensity of light output is no longer a simple function of hemoglobin absorption, but instead a complex combination of hemoglobin absorption, light scattering (or turbidity) (collagen, tissue matrix), and other components (melanin, water, etc.). We see a similar phenomenon every day when particles and clouds in the atmosphere scatter sunlight, and so make the sky appear to different shades of color throughout the day.
This is where the principles of Spatial Frequency Domain Imaging (SFDI) can help solve our problem of accurately separating individual scattering and absorption elements. SFDI uses patterned light to quantify the other contributing factors in tissue – in skin these include scattering and melanin. In Figure 1 we illustrate how different patterns and wavelengths of light (blue vs. red) travel through the layers of skin.
While wavelength information gives us the right contrast to tell between hemoglobin parameters, our structured illumination technology is needed to understand light diffusion, and so measure these parameters objectively. Together, this enables us to measure the amount of oxy- and deoxy-hemoglobin, and report an accurate measure of mixed arterial/venous oxygenation in the tissue below the epidermis. With this, we are able to determine the true health status of your tissue. Furthermore, we are now able to image areas to localize areas of concern over a large area—an added benefit to our approach.
Figure 1. Light absorbs and scatters differently for every wavelength.
Structured illumination and detection is the key enabling concept behind SFDI. If we shine a certain pattern on the sample, the pattern will reflect differently depending on the properties. In Figure 2, we show the brightness and contrast of the patterns is very different between a hand, a piece of paper and some of calibration standards we use for testing. This change in appearance is captured in our algorithms to better understand the properties of any turbid (scattering) material – this includes biological tissues, agriculture samples, cloudy water, clouds…you name it! Furthermore, by using modern projector technology we can measure the spatial distribution of these properties over large areas without any scanning.
Our team has been publishing and presenting this work for last ten years. Recently, we have optimized the wavelengths and patterns for certain fields, but the technology is a tool that can be pointed at any application where turbidity is a challenge. Contact us if you have an application where SFDI can be useful.
Figure 2. The patterns change depending on the properties.