Methods and systems are described that examine tissue positioned between input ports and a detection port. At lease one source of a visible or infrared wavelength is provided that introduces electromagnetic radiation into the subject. The detection port is optically coupled to a detector that is connected to a detector circuit. Radiation intensities are selected for introduction at the input ports to define a null plane in the tissue. The detection port is positioned relative to the null plane. Radiation is introduced into the subject at the first input port and the radiation that migrates through the tissue is detected. The detector circuit stores a first detector signal corresponding to the first detected radiation. Radiation is introduced at the second input port and is detected. The first detector signal is subtracted from a second detector signal corresponding to the second detected radiation to obtain processed data.