Computed tomography, low-field magnetic resonance imaging and cross-sectional anatomy of the head in healthy rabbits

Purpose: To provide a detailed anatomical description of the head of normal rabbits by using computed tomography (CT) and low-field magnetic resonance imaging (MRI), simulating clinical conditions, and to compare it with a compilation of corresponding gross anatomical sections on transversal, dorsal and sagittal planes. Methods: The heads of 5 clinically normal New Zealand White rabbits, weighing about 4 kg and euthanatized for reasons not connected to our study, were submitted to examination with a 4th generation CT (PQ2000S Philips MD S.p.A., Monza, Italy) and low-field MRI (0,18 T, Vet-MR Esaote S.p.A., Genova, Italy) before decapitation. In CT study, to obtain a transverse section, each rabbit was positioned in ventral recumbency on CT couch, with its head extended on the neck, whereas to obtain a sagittal section, each rabbit was positioned as above, but with its head flexed at 90 degrees laterally on the neck. Dorsal images were obtained by placing each rabbit in dorsal recumbency, with its head perpendicular to the couch, and flexed at 90 degrees ventrally. Three-mm-thick slices were acquired on each plane with hard and soft convolution filter. For MRI study, the same rabbits were positioned in ventral recumbency on the MRI couch, with their heads placed inside a volumetric coil. Transverse, sagittal and dorsal 3-mm-thick T1-weighted images, T2-weighted images and Gradient Echo T1-weighted images were performed. CT and MRI transverse and dorsal sections were performed in two different planes: for transverse images single slices were obtained perpendicular to the hard palate and to the skull base respectively, for dorsal ones single slices were obtained parallel to the structure above. Immediately after MRI examination the rabbits were decapitated and their heads were positioned and frozen in an ice cube at −14°C until solid, and then sectioned at 6-mm-thick intervals by using an electric band saw, according to the same section plane applied for CT and MRI scans. Slab sections were immediately cleaned, photographed, and compared with corresponding CT and MRI images, to identify anatomic structures and match them with the different acquisition. For each cutting plane we gained a good overlap between the section obtained at the same level with different techniques (CT, MRI and cross sectional anatomy). Results: The bone-window CT images provided a good anatomic detail of dentition and hard structures of rabbit skulls, and the soft tissue structures were well valuated with the soft tissue−window. However, T1 and T2-weighted images provided detailed information regarding soft tissue and central nervous system structures, and Gradient Echo T1-weighted acquisition supplied adequate information about bone and teeth. Conclusions: The results of this study could be used as a guide for evaluating CT and MRI images of rabbits with head disorders. It is important to highlight that rabbits positioning both in CT and MRI studies were reproducible with anesthetized subjects, for clinical investigation also on dorsal and sagittal plane of acquisition.