Summary

Summary
	Numerous exogeneous fluororphores can be added to tissue to provide diagnostic information. In our work, we are predominantly interested in fluorescent molecules whicha are used in photodynamic therapy. Most of these agents absorb light throughtout the visible region and fluoresce in the red and near-infrared wavelengths. The focus of our work has been to develop fiber optic bundle technologies which allow quantitative and robust measurements of fluorescence from tissue. This ability to have an accurate signal which is not distorted by the tissue optical properties allows the use of fluorescence to quantify the fluorescent drug concentration in tissue.
Technological Development
	This fiber system is currently being further investigated in our laboratory. It is also being developed in a commerical form by Aurora Optics, Inc.
Numerical Simulation
	Computer simulations of the light propagation and collection in tissue can be achieved with a statistical simulation of photon scattering and absorption, using a geometry which is similar to that of the fiber itself, and combining this with interaction coefficients which are typical of tissue. This type of numerical study is called a Monte Carlo simulation, and has been discussed at length in our paper (Pogue and Burke, Fiber optic bundle design for non-invasive quantitative fluorescence measurements from tissue-simulating media. Applied Optics, 37(31) 7429-7436 (1998)). A visual example of this type of simulation is shown below, where the optical fiber is positioned immediately above a volume of tissue. Photons travel down from the fiber and enter the tissue, and there is a probability of scattering to occur given the average bulk scattering coefficient of the tissue. The angular distribution of scattering events is typically modeled with a Henyey-Greenstein phanse function, and the absorption probability is also defined. Shown below are three representative results of simulations where the excitation fluence within a 4 mm diameter square volume of tissue is shown in the top rows, for a 4 mm optical fiber on the left, a 2 mm fiber in middle and a 100 micron fiber on the right. Below each of these is the calculated image of the fluorescent fluence which is eventually captured by the same fiber. Note that as the diameter of the fiber decreases, so does the depth at which fluorescent light is sampled from. An important observation from these simulations is shown below. As the diameter of the fiber decreases below the typical scattering length of the tissue (often the scattering length in tissue is approximately 100 microns), then the average number of scatterings by fluorescent photons becomes less than one. Thus, if small fibers are used to sample the fluorescence, then the signal is not highly scattered and is much less dependent upon the absorption and scattering properties of the tissue. 4 mm fiber 2 mm fiber 100 micron fiber
In Vivo Implementation and Testing
	The signal measured from a fiber bundle with small fibers is predominantly linearly correlated to the concentration within tissues, and that the signal is not significantly dependent upon the tissue type which is being examined (see below). This is important because a single fiber bundle can then be used to sample fluorescence from many tissues without different calibrations needed for each. Also, we have found that this type of fiber bundle is predominantly sensitive to the cellular drug concentration, and not to the vascular (i.e. blood plasma) concentration. This phase of development is onogoing.