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Chapter 4: Advanced Imaging: Intracranial Surgery 39

A B

Figure 4.12 Transverse T1‐weighted postcontrast (A) and sagittal T2‐weighted (B) MRI of a cat with a meningioma (arrows) rising from the meninges
overlying the osseous tentorium cerebelli. The superficial location of the mass with displacement of the overlying brain parenchyma away from the osseous
tentorium is best seen on the sagittal plane images. On the transverse plane images the extraaxial location cannot be determined but the mass shows marked
contrast enhancement typical of an extraaxial mass. Brain masses need to be evaluated on multiple imaging planes to allow correct anatomical
localization.

where displacement of the occipital lobes under the osseous tento- of a distinct, complex hypointense rim and bleeding of different
rium or caudal displacement of the cerebellum through the fora- durations within the lesion is suggestive of neoplasia. In some cases,
men magnum may be seen (Figure 4.13). There are many treatments repeat MRI to monitor progression of the lesion is required. The
for intracranial neoplasia and CT or MRI is required in most for presence of edema at a later stage, regression in size of the lesion,
accurate treatment planning. The postradiation MRI appearance of and failure to follow the expected evolution of a hematoma are
brain tumors has been described, including the necrosis that can be indicative of neoplasia.
associated with this treatment modality [101]. Large hematomas may show distinct fluid lines; if there is intra-
ventricular bleeding, then alterations in signal intensity (decreased
Intracranial Hemorrhage signal on T2W and increased on T1W and FLAIR images) and lay-
CT is exquisitely sensitive at detecting acute hemorrhage, which is ered fluid–fluid levels within ventricular CSF will be seen (see
evident as increased density due to attenuation of X‐rays by the glo- Figure 4.6). The presence of high signal on a T1‐weighted image
bin portion of blood [32,41]. The attenuation gradually decreases within the brain and very low signal on a T2‐weighted image are
until the hematoma is isodense at about 1 month after the onset. suggestive of recent hemorrhage. T2* GRE images are the most sen-
The periphery of the hematoma enhances with contrast at 6 days to sitive for showing small hemorrhages (which are hypointense) [54].
6 weeks due to revascularization. Until recently, CT was the pre- Hyperintensity on a T1‐weighted image is not 100% specific for
ferred imaging modality in human patients for determining the hemorrhage and may be seen with melanin, high protein, flow arti-
presence of hemorrhage in early stroke. Recent developments in facts, and paramagnetic effects (e.g., due to manganese) [104].
MRI mean that CT now offers little advantage apart from time of
acquisition in the diagnosis of stroke [102]. Cerebral Infarction
The appearance of hemorrhage on MRI is variable and depends Infarcts in the CNS of dogs are usually ischemic with little or no
on the age of the hemorrhage, pH, oxygenation, size of bleed, and hemorrhage. They are usually arterial and have characteristic imag-
magnetic field strength. T2* GRE sequences are the most sensitive ing findings [105]. CT images are frequently normal during the
for visualizing hemorrhage, which always appears as a signal void acute phase of ischemia; therefore the diagnosis of ischemic stroke
(black) (see Figures 4.6 and 4.11). On spin‐echo sequences, the using CT relies on the exclusion of “mimics” of stroke. Early CT
appearance depends on the degree of conversion of hemoglobin signs of ischemia can be subtle (Figure 4.15) and difficult to detect
(Table 4.1). even by experienced readers and include parenchymal hypodensity,
In small animals the main causes of intraparenchymal hemor- loss of gray–white matter differentiation, subtle effacement of the
rhage are coagulopathy, neoplasia, and trauma [54,55]. Subdural cortical sulci, and local mass effect [39].
and subarachnoid hematomas are rare in small animals in compari- MRI lesions are most obvious on T2‐weighted and FLAIR images
son with humans. Most brain hemorrhage seen in dogs and cats is where they are usually hyperintense. They have minimal or no mass
intraparenchymal [103]. Hemorrhage is commonly seen associated effect and are usually homogeneous, and sharply marginated with
with tumors but the MRI appearance is predominantly of a solid clear demarcation from adjacent parenchyma [106,107]. Gray mat-
mass rather than a hematoma. Hemorrhagic tumors are often com- ter is most severely affected and lesions are usually confined to one
plex masses with solid contrast‐enhancing parts (Figure 4.14). Lack vascular territory (Figure 4.16) [108].