Transverse CT image of head of a horse with left-sided sinus disease, obtained at the level of the frontal sinus (FS). A small amount of hypoattenuating material is present within the left (L) caudal maxillary sinus (CMS) and ethmoidal sinus (ES) and there is a widened infraorbital canal (IOC). Frontal bone (FB) thickness was measured at a single site 1 cm dorsal to the junction between the medial aspect of the orbital rim and the caudal aspect of the nasal bone on the affected and contra-lateral sides (yellow lines).