The IN191 Blood Flow Meter defines a perfusion parameter from information contained in the optical spectrum of light remitted from the tissue. The actual measurement sampling volume or depth can only be determined by identifying precisely which blood vessels and erythrocytes have interacted with the remitted light, which in turn, is principally dependent on two parameters; namely the optical scattering and optical absorption coefficients of the tissue under observation. Since both of these coefficients are entirely dependent on the site of observation and perfusion of the microvasculature at the time of measurement, it is impossible to determine the actual sampling volume or depth at any tissue site.
Generally speaking however, we have estimated that for well-perfused tissue such as muscle, the mean sampling depth is in the region 0.5-1.0 mm with a concomitant sampling volume in the region 0.3-0.5 mm^3. For cutaneous measurements, the sampling depth is likely to be in the range 1.0 – 1.5 mm. These estimates have been obtained heuristically through many years of experience and are based on both in vitro observations and mathematical modeling of photon diffusion through 'imaginary tissues’ using Monte-Carlo techniques.