From inspection of the CT image (slice #94), the maxillary sinus is generally clear (i.e., air-filled) and presumed normal. The modeled photon overlay (Fig. 3) reveals that the majority of detected photons have exited from the cheek area over the maxillary sinus. We have observed similar NIR patterns of illumination in the clinical NIR images of healthy, clear sinuses. The distribution of escaping photons in the cheek area is mainly symmetric; the brightest intensity is generally under the eyes and along the nose. There are a few intensity bright spots at locations in the image. For example, a few bright spots in the nasal area also correspond to small dark regions in the CT image. The nose and mouth are also air-filled cavities and do not strongly attenuate the light. However, there are also bright spots in the upper corners of the nose by the eyes. These spots may be evidence of sampling the ethmoid sinuses, which are network of small cavities between the eyes and nose. We also investigated the effects of changing the tissue optical properties (absorption and reduced scattering) in two different ways. First, we changed the bulk optical properties of the tissue constituents (e.g., bone) to estimate their impact on the final transillumination images. In other words, we changed the value of soft tissue absorption. Second, we spatially altered the optical properties (i.e., absorption and reduced scattering) of the tissue constituents according to a Gaussian distribution, rather than assume a single value for each tissue constituent. In other words, the value of the optical property in a given tissue component (e.g., bone scattering) was not assumed to be the same throughout, but rather it was varied throughout the region. In both cases we varied the absorption and scattering of the tissue constituents by about 20%, but found that the impact of changing the optical properties on the transillumination images were minimal (e.g., the original findings in the images were not altered by the change in optical properties).