Extinction measurements in diffusing mammalian tissue with heterodyne detection and a titanium:sapphire laser

Total optical absorption in mammalian tissues is measured in the near infrared by the use of heterodyne detection and a Ti:sapphire laser. Because of the high sensitivity, directivity, and signal-to-noise ratio of the setup, we were able to detect coherent photons after attenuation by more than 9 optical densities. This method allows us to detect unscattered photons that are passing through more than 7 mm of various tissues such as brain, muscle, liver, skin, and fat selectively.