1. A method for determining blood oxygen saturation in a vessel based upon a plurality of images of the vessel obtained in response to illumination of the vessel at different wavelengths, the method comprising: generating a plurality of background images based upon respective ones of the plurality of images of the vessel, wherein generating the background images comprises approximating an image of a tissue bed underlying the vessel; determining a plurality of transmittance images based upon respective pairs of the background images and the images of the vessel; and determining the blood oxygen saturation in the vessel based upon the plurality of transmittance images.