A method includes determining a permeability metric of vascular tissue of interest based on a first time enhancement curve and second time enhancement curve corresponding to a first contrast material and a second contrast material flowing through the vascular tissue of interest and generating a signal indicative thereof. A plurality of light scattering dots 24 are formed in each of individual areas A1-A10 resulting from division of the at least one surface 12c of the light guide substrate 12 into a plurality of areas. Under one embodiment, the deleted interpolation language model parameters are formed using fractional counts.