Page 176 - Atlas of Small Animal CT and MRI

166 Atlas of Small Animal CT and MRI

an expanded fourth ventricle, which is hypoattenuating other locations. 24–26 Uncomplicated arachnoid cysts
compared to adjacent brain parenchyma. Unenhanced have a thin unicameral membrane, contain cerebrospi­
21
MR imaging features are reported to be similar, with nal fluid, and conform to the margins of adjacent
enlargement of the fourth ventricle appearing T1 hypoin­ structures. 23,24,26 Although many quadrigeminal arach­
tense and T2 hyperintense (Figure 2.3.4). Concurrent noid cysts are clinically silent, large cysts can produce
hydrocephalus has been reported in one dog. 22 cerebellar and occipital lobe compression leading to
development of neurologic clinical signs. 24,27–29 The pres­
Diverticulation and cleavage disorders ence of intracystic hematomas has been reported and
may lend credence to the thought that some arachnoid
Diverticulation and cleavage disorders include complex cysts are traumatic in origin, as described in people. 30
anomalies, such as holoprosencephaly and septo‐optic On unenhanced CT images, uncomplicated intracra­
dysplasia, that occur early in development and involve nial arachnoid cysts have well‐defined margins, contain
not only the brain but may affect the face, cranial nerves, fluid isoattenuating to cerebrospinal fluid, and do not
and pituitary gland as well. Such anomalies are not well contrast enhance (Figure 2.3.7). On MR images, cysts
described in domestic animals since most affected are clearly extraaxial, contain fluid isointense with CSF,
animals likely die early in life. On both CT and MR and do not contrast enhance (Figures 2.3.8, 2.3.9). 24,26
images, these disorders will vary depending on the Arachnoid cysts containing blood or organizing hema­
nature and severity of the anomaly. 2
tomas may have variable attenuation on CT and variable
Malformations of cortical development T1 and T2 signal intensity on MRI. 30
Malformations of cortical development represent a
diverse group of developmental anomalies, including Epidermoid and dermoid cysts
microencephaly, pachygyria–polymicrogyria, lissen­ Epidermoid and dermoid cysts are rare and result from
cephaly, and schizencephaly. These anomalies may fea­ aberrant ectodermal cell migration and entrapment dur­
ture variable and often reduced brain volume, cortical ing neural tube closure. The most common locations are
convolution anomalies, and cortical clefts. On both CT the fourth ventricle and cerebellopontine angle. Clinical
and MR images, the appearance of these disorders will signs may result from obstructive hydrocephalus.
vary depending on the nature and severity of the anom­ Epidermoid cysts consist primarily of desquamated
aly, although disruption of the normal contours of the skin cells. These masses appear hypoattenuating to
cortex is a consistent feature (Figures 2.3.5, 2.3.6). 2 adjacent brain on unenhanced CT images and are T1
hypointense and T2 hyperintense on unenhanced MR
Nonneoplastic cysts images. 31,32 Epidermoid cysts would not be expected
to enhance unless ruptured, producing a peripheral
Arachnoid cysts inflammatory response (Figure 2.3.10).
Intracranial arachnoid cysts arise from the arachnoid Dermoid cysts are more complex, containing hair
membrane surrounding the brain, do not communicate follicles and sebaceous material that appear hypoatten­
with the ventricular system, and are thought to be uating on unenhanced CT images and T1 and T2
primarily developmental (although acquired cysts are hyperintense on unenhanced MR images because of the
suspected to also occur). Young, small‐breed brachyce­ lipid content. Fat‐suppressed T1 sequences may be used
phalic dogs are most frequently affected, although cysts to null the lipid signal to better characterize the lesion. 33,34
have also been reported in other canine breeds and in Similar to epidermoid cysts, dermoid cysts would not be
cats. 23–26 Arachnoid cysts most commonly arise from the expected to enhance unless ruptured, producing a
quadrigeminal cistern but will occasionally occur in peripheral inflammatory response. 33

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