NOMENCLATURE AND DESCRIPTIVE ANATOMY OF THE OLFACTORY BULB FISSURE AND DEFINITION OF THE OLFACTORY BULB DIMENSIONS IN DOGS USING IN VIVO MRI.

Olfaction is one of the most important sensory functions in dogs, and one which strongly influences behaviour, including feeding, social interaction and reproduction. The olfactory bulbs are situated within the ethmoidal fossae of the cranial cavity, comprising part of the rostral cranial fossa and are bounded cranially by the cribriform plate of the ethmoidal bone. The nasal aspect of the cribriform plate is continuous with the ethmoidalturbinates, which are lined by the olfactory epithelium. The anatomical position of the olfactory bulb in relation to the brain has been shown to vary significantly with the cephalic index, becoming progressively more ventrally orientated with increasing degrees of brachycephalia. Correct identification of the olfactory bulbs and their related ethmoidalturbinates are also important in a number of neurological conditions, including neoplasia, hydrocephalus (with dilation of the olfactory bulb recess in obstructive hydrocephalus) and for harvesting of olfactory ensheathing cells from the nasal epithelium. A review of the anatomical literature from 1881 to 2012 describes a variety of terms for the grooves that separate the olfactory lobes from the rest of the brain; however there is no term to describe the groove separating the olfactory bulb from the brain. The aim of this study was to describe this anatomical feature by in vivo MRI and suggest olfactory bulb fissure as an appropriate descriptive term. MRI studies from 261 individual dogs, representing a wide spectrum of breeds, were reviewed to describe the anatomy of the olfactory bulb fissure. Dorsal and midline-sagittal plane T1- and T2-weighted images were reviewed for each case and a single image was selected from each of these imaging planes and sequences for each dog. All four images were not available in all dogs due to variation in the MRI studies and a total of 639 MR images were reviewed. The olfactory bulb fissure could be identified in 83% and 97% of T1- and T2-weighted midline-sagittal plane MR images respectively, and appeared as a line sloping from rostrodorsal to caudoventral, situated between the olfactory bulb and the frontal cortex. On dorsal plane MR images it could be identified in 86% and 95% of T1- and T2-weighted images respectively, and was evident as a line separating the olfactory bulb and frontal cortex, extending from rostromedial to caudolateral. In T1-weighted MR images the olfactory bulb fissure appeared as a hypointense line, while in T2-weighted images it appeared as a hyperintense line, confluent with the subarachnoid cerebrospinal fluid filled spaces. The olfactory blub fissure is proposed as a suitable descriptive term and can reliably be identified in the majority of T1- and T2-weighted dorsal and midline-sagittal plane MRI sequences. The olfactory bulb fissure is useful to define the position of the cribriform plate and to determine the olfactory bulb angulation, which has previously been reported to be an accurate and objective measure of the degree of brachycephalia in dogs.