Human tissue is a highly diffusive medium that does not permit easy detection of abnormalities beneath its surface. Rinaldo Cubeddu and associates from the Center for Quantum Electronics of CNR and the Physics Department of Politecnico (Milan, Italy) are investigating a method based on photon migration to image optical inhomogeneities, such as cancerous breast tumors, and determine their exact optical nature.
Determination of optical scattering and absorption coefficients using diffusion theory to interpret time-resolved measurements has enabled the researchers to produce a projection image that can aid in location of small optical inhomogeneities in highly diffusive media. It can also help in discriminating between scattering and absorption contributions. Such discrimination is important for predicting the type of tumor and distinguishing benign from malignant lesions.
Cubeddu describes looking for a tumor as similar to trying to find an object inside a glass of milk. "If one can see through a medium, it is transparent and light passes straight through it. When the light is blocked by an absorbing object, you can see the object," he says. But light does not pass straight through a diffusive medium, such as human tissue or milk. Some photons are absorbed by the object while others travel around it, scattered by the medium. Because there is no clear shadow, it is difficult to detect the object.
Optical techniques can penetrate a diffusive medium and detect different optical properties. Good analysis depends on the degree of penetration. When medium absorption is high, penetration is poor because light is completely absorbed by the first layers of the medium, which is what happens to ultraviolet (UV) and blue light in biological tissues. Longer-wavelength light, such as red or near-infrared, penetrates deeper, because less of it is absorbed, making it possible to detect photons transmitted through tissues several centimeters deep.
In order to distinguish between normal and pathological tissue, one must identify homogeneous and inhomogeneous areas, as well as the scale of variation of absorption and diffusion coefficients. To accomplish this, Cubeddu`s group developed a method based on the plot of several parameters derived from the same time-resolved data.
A synchronously pumped modelocked dye laser emitting at 650 nm was used to perform time-resolved transmittance measurements. Transmitted pulses were detected using an electronic chain for time-correlated single-photon counting, with an instrument transfer function of less than 50 ps. The incident laser pulses are delayed and their shape and amplitude are modified when they traverse a diffusive medium such as tissue.
Diffusion theory describes how photons propagate through turbid media and how the absorption and scattering properties of the medium affect the propagation. The best fit of the theoretical pulse shape to the experimental data allows estimation of the optical coefficients of the traversed medium when it is homogeneous. If the medium is not homogeneous, approximate values, averaged over the whole traversed volume, are determined.
Point-measurements were performed every 2 mm over a 60 × 60-mm area to construct images of tissue phantoms—test samples that simulate the geometric and optical properties of a compressed breast with a pathologic mass. For each measurement, a fit to the diffusion theory was performed to evaluate the average scattering and absorption coefficients. Also the number of photons arriving at various time intervals was evaluated to determine which ones are more sensitive to the optical properties. The number of early-arriving photons, for example, is determined primarily by the diffusive properties of the medium. The more diffusive the medium, the lower the number of early-arriving photons because scattering phenomena delay most of them.
The results showed that a combination of two parameters allows both detection of optical inhomogeneities and discrimination between diffusive and absorbing inhomogeneities. The fitted scattering plot allows scattering variations to be identified, while the intensity integrated over an interval set on the tail of the transmitted pulse classifies absorption changes.
Mammograms use x-rays for tumor detection. Cubeddu`s system of optical imaging can provide a complementary examination that could be repeated at any time because it is noninvasive. While x-rays are sensitive to density, that is, only to absorption, optical imaging is also sensitive to scattering, which may help in finding something undetectable by x-rays alone. Cubeddu says that this technology cannot be applied to a total body image because absorption and scattering are too high to allow such a deep penetration; however, it would be appropriate for detecting breast cancer.