Page 65 - Atlas of Small Animal CT and MRI

1.4

Skull

Introduction segments may result in atlantoaxial instability or
subluxation causing spinal cord compression. CT and
The skull is made up of many bones that fuse as the MR imaging allow 3D visualization of the malformation
animal becomes skeletally mature. CT is an excellent itself as well as the effects on the spinal cord (Figure 1.4.1).
modality to depict and study the complex anatomy of Dogs should be positioned with care if instability is
the skull using multi-planar 2D images as well as 3D suspected.
renderings. The foramina of the skull, through which
vasculature and the cranial nerves exit, have been Atlantooccipital overlapping
described on CT and MR images. CT imaging of the Atlantooccipital overlapping is rostral malposition of
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skull is prone to beam‐hardening artifact because of the the atlas and axis resulting in compression of the
thickness of the bones, particularly the temporal bone cerebellum and kinking of the medulla oblongata. Since
region. Axial images tend to produce fewer artifacts than it is seen with other congenital anomalies, such as
helical images, especially in cats and smaller dogs. Chiari‐like malformation (see Chapter 2.3) and dens
The skull is generally symmetric along the sagittal plane, hypoplasia resulting in atlantoaxial instability (see
which can be used for comparisons of paired structures Chapter 3.1), it may be a consequence of other anoma-
during interpretation. However, variations of normal lies; however, it can also be seen as a sole abnormality.
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anatomy that cause asymmetry exist, especially in Syringomyelia, seen as a continuous or intermittent
asymptomatic cats whose frontal and sphenoid sinuses are T2 hyperintense fluid collection in the spinal cord
often unequally sized and whose nasal septum may be parenchyma, is associated with the chronic compres-
deviated. 2 sion. Fibrous bands dorsal to the atlantoaxial or atlan-
tooccipital junctions can also be seen with many of
Developmental disorders these disorders and contribute to the spinal cord
Occipitoatlantoaxial malformations compression.
Congenital occipitoatlantoaxial malformations are
rare in dogs; however, hypermotility or stenosis can Benign calvarial hyperostosis
cause severe neurologic compromise secondary to com- Benign calvarial hyperostosis has been described in
pression of the spinal cord. The occipital bone, foramen young Bull Mastiffs as a diffuse thickening of the bones
magnum, atlas, and ligamentous structures make up this of the calvarium, with some similarities to cranioman-
region. The spectrum of abnormalities includes hypo- dibular osteopathy. On MR images of one patient, the
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plasia of the occipital condyles, fusion of the atlas to the frontal bones were markedly thickened with hypointense
occiput, multiple separate centers of ossification, and T1 and T2 signal due to loss of normal marrow signal
malformation of the dens. The abnormally fused cranial and T2 hyperintensity of the surrounding tissues.
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Atlas of Small Animal CT and MRI, First Edition. Erik R. Wisner and Allison L. Zwingenberger.
© 2015 John Wiley & Sons, Inc. Published 2015 by John Wiley & Sons, Inc.
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