Fig. 5.

Histograms of ${\mathrm{sO}}_2$ estimates in each voxel within the tube for (a) spectrally flat with mean ${\mathrm{sO}}_2$ of 90.3%, (b) spectrally varying background with mean ${\mathrm{sO}}_2$ of 91.6%, and (c) spectrally varying with depth-dependent fluence correction with mean ${\mathrm{sO}}_2$ of 88.8% obtained using linear inversion at 620/920 nm, 700/820 nm, and 700/820 nm, respectively; ${\mathrm{sO}}_2$ estimates for which the ratio $(\delta \mathrm{\Phi }/\mathrm{\Phi })/(\delta {\mu }_a/{\mu }_a)<0.01$ shown in red and $(\delta \mathrm{\Phi }/\mathrm{\Phi })/(\delta {\mu }_a/{\mu }_a)\ge 0.01$ in black. Notes: vertical line indicates true oxygenation of 90%; there are no ${\mathrm{sO}}_2$ estimates for which $(\delta \mathrm{\Phi }/\mathrm{\Phi })/(\delta {\mu }_a/{\mu }_a)<0.01$ for the spectrally varying background (uncorrected) due to severity of spectral coloring of fluence; note the scale change on vertical axis between flat and various backgrounds. These wavelength pairs yielded the most accurate ${\mathrm{sO}}_2$ estimates of all pairs examined. ${\mathrm{sO}}_2^{\mathrm{g}.\text{mean}}$ represents the mean of the ${\mathrm{sO}}_2$ estimates all voxels enclosed within the entire tube and is termed the global mean ${\mathrm{sO}}_2$.