Page 217 - Atlas of Small Animal CT and MRI

Infectious Inflammatory Disorders 207

Bacterial meningoencephalopathy Affected parenchymas appear T1 hypointense and T2
hyperintense on MRI images because of the presence
Intracranial abscess of vasogenic edema. Affected pachymeninges may be
Intracranial abscesses are caused by penetrating injuries, thickened and can appear T2 hyperintense. When
such as migrating foreign bodies and bite wounds; present, subdural or subarachnoid abscess will appear as
extension of ear and nasal infections; or from bacteremia a T1 hypointense, T2 hyperintense crescent adjacent to
or septic emboli. 8–10 Their location within or adjacent the affected region of the brain. Affected meninges
to the brain is dependent on the initiating cause, with generally intensely contrast enhance, although affected
the brainstem most often affected by extension of otitis brain parenchyma enhancement is variable and may be
media/interna. Because of the inflammatory nature of subtle (Figures 2.7.6, 2.7.7, 2.7.8).
the lesion and the often significant mass effect,
surrounding vasogenic edema is usually pronounced. Mycotic meningoencephalitis
Epidural or subdural fluid collections may be present,
particularly when the abscess is caused by penetrating A variety of mycotic species may cause central nervous
injury. Obstructive hydrocephalus may occur, depend- system disease. Although reports are few, Aspergillus,
ing on abscess location. Cryptococcus, Cladophialophora, and Coccidioides spe-
Intracranial abscesses usually appear as solitary space‐ cies are some of the organisms for which imaging features
occupying masses of variable size and with a hypoatten- have been described. 6,16–20 Prototheca species, blue–green
uating center on unenhanced CT images. Depending on algae, have also been a reported cause of infectious
21,22
the thickness of the abscess capsule, this may appear as a meningoencephalitis. In many patients, involvement
distinct iso‐ or mildly hyperattenuating lesion rim of the brain, spinal cord, and meninges is part of a mul-
surrounded by relatively hypoattenuating parenchymal tiple organ system infection. Young, female German
edema. Abscesses are generally intensely peripherally Shepherd Dogs are highly overrepresented, particularly
19
enhancing, and regional meningeal enhancement may for Aspergillus infections. Most mycotic CNS infec-
be present in some patients (Figure 2.7.3). tions occur through hematogenous spread, and they are
On MR images, abscesses are centrally T1 hypointense therefore often diffuse or multifocal and asymmetrically
(but higher intensity than normal CSF) and T2 hyperintense distributed. Infections may involve both brain paren-
because of the presence of abscess fluid and T1 hypointense chyma and meninges, and parenchymal lesions may take
and T2 hyperintense peripheral to the mass because of vaso- the form of abscesses, solid granulomas, or diffuse infil-
genic edema. Abscess contents appear hyperintense on trative lesions.
FLAIR sequences. A distinct T1 iso‐ to hyperintense abscess Fungal meningoencephalitis may have CT and MR
capsule may also be evident. Abscesses intensely peripher- features similar to those described for bacterial abscess
ally contrast enhance and regional meningeal enhancement and meningoencephalitis, although more heterogeneous
may be seen (Figures 2.7.4, 2.7.5). 8,9,11–14 Diffusion weighted contrast enhancement will be evident in solid and
imaging (DWI) typically reveals high signal intensity on the complex granulomas. Contrast enhancement of infiltra-
DWI map and low signal on the apparent diffusion coeffi- tive lesions may be diffuse and ill defined (Figures 2.7.9,
cient (ADC) map, indicative of restricted water diffusion 2.7.10, 2.7.11, 2.7.12, 2.7.13). 6,16–20
(Figure 2.7.5). This may be useful for differentiating
abscesses from necrotic brain lesions and mucinous intra‐ Protozoal meningoencephalitis
axial tumors, such as oligodendroglioma. Protozoal infection is a rare cause of encephalomyelitis,
but imaging features of CNS lesions have been reported
Bacterial meningoencephalitis for Toxoplasma in cats and Neospora, Leishmania, and
Intracranial bacterial infections may also present as a Acanthamoeba species in dogs. 23–27
diffuse or regional meningoencephalitis. 13,15 Because the veterinary imaging literature includes
Imaging features may be subtle or absent on only single and small series case reports, characteristic
unenhanced CT images. However, regional or diffuse features of protozoal CNS infection are not fully
parenchymal hypoattenuation from edema may be documented. However, imaging findings are generally
present, and pachymeninges may be visible if markedly consistent with those found with other inflammatory
thickened. Subdural or subarachnoid abscess may also brain disorders and include focal, regional, multifocal,
be evident as a hypoattenuating crescent adjacent to the or diffuse asymmetrical distribution in white and gray
affected region of the brain. Meningeal contrast enhance- matter; associated edema; variable meningeal involve-
ment may be obscured because of the adjacent dense ment; and heterogeneous to homogeneous contrast
calvarial bone (Figure 2.7.6). enhancement. In addition to the general characteristics,

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