1. A method for optical imaging of a turbid medium present in a human subject, the method comprising: measuring reference intensities of light emanating from the turbid medium while an O 2 /N 2 gas mixture is supplied to the human subject and while the O 2 /N 2 gas mixture has a reference ratio of partial oxygen pressure (pO 2 ) to partial nitrogen pressure (pN 2 ); reconstructing a reference image of the turbid medium from the measured reference intensities; measuring contrast intensities of the light emanating from the turbid medium while the O 2 /N 2 gas mixture is supplied to the human subject and while the O 2 /N 2 gas mixture has a contrast ratio of partial oxygen pressure (pO 2 ) to partial nitrogen pressure (pN 2 ); reconstructing a contrast image of the turbid medium from the measured contrast intensities; and comparing the contrast image of the turbid medium to the reference image of the turbid medium, wherein the reference ratio is different from the contrast ratio.