Identification of Normal Horse Head Structures , with Special Reference to Paranasal Sinuses , by Anatomical Cross-section and Magnetic Resonance Imaging ( MRI )

| The purpose of this report was to describe the axial appearance of the normal horse head with special reference to the paranasal sinuses using magnetic resonance imaging (MRI) technique and anatomical cross sections. In order to do this, T1 (longitudinal relaxation time)-weighted, 1cm –thick images of horse cadaver heads (n=5) were captured using closed magnet of a 1-Tesla (field strength). Afterwards, the frozen heads were cross-sectionally sliced at seven planes (from the level of upper and lower incisors to the level of perpendicular plate of ethmoidal bone) according to MRI slices thickness, and then correspondingly compared with the MRI images. The obtained data (per each plane) properly defined the normal anatomical features of the normal horse head by both anatomical cross-sections and MR images. The MR images from this investigation provided a clear definition of normal horse head structures and were consistent with that of gross anatomical sections. The T1-weighted images showed that the mineral-rich tissues (bones and teeth) appeared dark (no signal), cartilages and muscles appeared grey (low signal intensity, hypointense), and fat (subcutaneous and within bone marrow) appeared bright (high signal, hyperintense). The air-containing sinuses and conchae appeared dark without signals. As this study demonstrates normal horse head structures with a particular attention to nasal cavity structures and its relation to five types of paranasal sinuses, it could be used as a landmark and reference to properly diagnose various surgical disorders of head region particularly paranasal sinuses that are the most common in horses.


MAtErIAlS And MEthodS
Cadaver heads (n=5) employed in this study were harvested from normal adult horses of mixed breeds (6-years ± 9-months-old).This study was performed at Faculty of Veterinary Medicine, Zagazig University, and all conducted procedures were fulfilled according to the institutional guidelines for regulation of the care and use of animals.The horses were euthanized due to incurable diseases unrelated to the head region.The heads were kept cooled and scanned within 4-hours after euthanasia.Each head was scanned using MRI machine (Philips, Intra, USA) of 1-tesla field strength of internal magnetom and human body coil.The T1 (longitudinal relaxation time)-weighted transverse plane produced by using short time to echo (TE) and repetition time (TR) was performed.After imaging, the heads were kept frozen at -15 ˚C and then cross-sectionally sliced by an electrical saw at the same scanning points of transverse plane used during MRI examination.The cross-sectional slicing was done at seven different levels: at the level of upper and lower incisors, the body of mandible, the first and second premolar teeth, the third premolar tooth, the molar teeth, the diastema, and the perpendicular plate of ethmoidal bone.Each obtained section was thoroughly cleaned and photographic image was captured, labeled and then compared for matching with its relevant transverse MR image to assure the corresponding anatomic features.

rESultS
The representative seven MR images and their relevant cross-sections of the normal structures of the equine nasal cavity and their paranasal sinuses from the level of upper and lower incisors to the level of perpendicular plate of ethmoidal bone are depicted in Figures 1, 2, 3, 4, 5, 6 and 7.It is clear from the figures that dorsal, middle, ventral and ethmoidal nasal conchae divide the nasal cavity into different air meatuses: dorsal, middle, ventral and common nasal meatuses.Dorsal nasal concha has straight fold, while ventral nasal concha has alar fold and basal fold.Both last conchae are organized rostrally as recess and caudally as sinus.Middle and ethmoid nasal conchae appeared small and undivided.The both maxillary sinuses are divided into rostral maxillary sinus and caudal maxillary sinus.The two frontal sinuses occupy the dorsal part directly below the frontal bone and are divided by a bony septum.Dorsal and ventral conchal sinuses occupy the caudal part of the dorsal and ventral nasal concha.Concho-frontal sinus is an extension of frontal sinus and communicates with the sinus of the dorsal nasal concha.The concho-frontal sinus communicates with the caudal maxillary sinus by an opening termed fronto-maxillary opening.Sphenopalatine sinuses are the smallest sinuses and located in the body of sphenoid ( Figures 1, 2, 3, 4, 5, 6 and 7).
Herein by MR imaging, we found that mineral-rich tissues (bones and teeth) appeared dark (no signal).Cartilages and muscles appeared grey (low signal intensity, hypointense), while fat (subcutaneous and within bone marrow) appeared bright (high signal, hyperintense).The air-containing sinuses and conchae appeared dark without signals.       .As well, by this approach, a clear visualization and identification of the complex and super imposed structures (as in case of head region) can also be achieved.It is well known that the MRI is a beneficial diagnostic tool for clear discrimination the soft tissues from the mineral-rich tissues and gas-containing sacs particularly of complex-structured areas (Arencibia et al., 2000).The head area is considered as a good example for studying MRI because its anatomical complexity (Pease et al., 2017).Superimposition of head structures and presence of skull hinder the specific interpretation in case of radiography and improper sonic waves penetration in case of ultrasonography, respectively (Park 1993;Weller et al., 1999).Previous study has reported that computed tomography assists in identification of para-nasal sinuses of cadaver heads of adult horses (De Zani et al., 2010).Additionally, these sinuses as well as brain have been assessed in a normal and diseased condition by anatomical and MR imaging in horse (Chaffin et al., 1997;Arencibia et al., 2000;Arencibia et al., 2001;Cavalleri et al., 2013;Manso-Diaz et al., 2015).Hence, it was worthy to expand the limited information about normal anatomical structures of adult horse head by MRI.Since MRI is described by its superior contrast resolution particularly for soft tissues, the various structures of horse head, as shown herein, were clearly detailed using T1-weighted MRI in parallel with the cross-section anatomy.MRI is essential to properly diagnose various head disorders such as tumors, cysts, ethmoid and hematoma (Manso-Diaz et al., 2015).Although the availability of using MRI in horses is yet limited, future designation of its open magnet to fit horse size may become available due to its essential role for diagnostic purposes in such worthy animal, horse.In the current study, we found that the T1-relaxation time is reversibly correlated with the signal intensity (Arencibia et al., 2000).Hence by using this type of scan, the higher intensities (fats), hypointensities (muscles and cartilages), and lower intensities (bones, and air-containing sinuses) structures could be clearly visualized.

concluSIon
We could concluded that this study might constitute as landmark to assist in the proper evaluation of the normal and abnormal structures of the equine head with a particular attention to paranasal sinuses by using MR imaging.

AcKnowlEdgEMEntS
This work was supported by Faculty of Veterinary Medicine, Zagazig University, Egypt.

Figure 5 :
Figure 5: Cross sectional anatomy (Panel A) and MR image