Page 184 - Atlas of Small Animal CT and MRI

174 Atlas of Small Animal CT and MRI

hematomas appear hyperattenuating to brain paren- to hematoma formation, or from vascular occlusion,
chyma on unenhanced CT images and will have variable resulting in hemorrhagic or nonhemorrhagic infarction.
unenhanced T1 and T2 intensity on MR images, depend- Vascular occlusion (ischemic infarction) may be due to
ing on the age of the hematoma. 6,7 either in situ thrombus formation or obstructing emboli
originating elsewhere. Hemorrhagic ischemic infarction
Subdural hematoma occurs when the mural integrity of an occluded vessel is
Subdural hematomas are usually traumatic in origin, arise disrupted, secondarily leading to extravasation. It may be
in the potential space between the dura mater and the impossible to distinguish between hemorrhagic ischemic
arachnoid membrane, and typically occur as the result of infarction and hematoma resulting from vascular disrup-
venous sinus hemorrhage. Subdural hematomas are cres- tion, as both will have similar imaging features. Most
cent shaped, conforming to the convex surface of the brain infarctions are arterial in origin, and although stroke from
(Figure 2.4.7). Acute subdural hematomas will appear venous thrombosis is described in people, there are few
hyperattenuating to brain parenchyma on unenhanced comparable reports in veterinary medicine. The rostral
CT images with a gradual reduction in density over time. and middle cerebral and the striate and rostral cerebellar
They will have variable unenhanced T1 and T2 intensity arteries are the most commonly involved, and infarcts
on MR images, depending on the age of the hematoma. 6–8 involving the cerebrum, thalamus/midbrain, and cerebel-
lum have been reported. 13–17 Infarcts are described as ter-
Subarchnoid hemorrhage ritorial when they involve a major intracranial vessel and
Head trauma may cause bleeding into the subarachnoid lacunar when smaller penetrating vessels are obstructed.
space. Acute subarachnoid hemorrhage will appear Underlying causes for stroke include atherosclerosis,
hyperattenuating and will generally conform to the hypertension, and diabetes in people, although these have
convolutions of the cerebral cortex and the cisterns on not been confirmed as predisposing factors in veterinary
unenhanced CT images. Acute subarachnoid hemor- patients. 18
rhage will appear T1 isointense and T2 and FLAIR
hyperintense on MR images with a distribution similar Hematoma from vascular disruption
to that seen on CT. Intensity patterns will change with Hematomas from vascular disruption may occur as
chronicity (Figure 2.4.8). the result of vascular trauma or from spontaneous
hemorrhage, as may occur with rupture of an intracra-
Brain contusion and hemorrhage nial vascular malformation. Imaging characteristics will
Imaging features of brain contusion depend on the vary depending on the size, location, and chronicity of
combination of edema and hemorrhage in the affected the hematoma. Hematomas will generally appear as a
brain parenchyma. Edematous regions will appear hyperattenuating mass on unenhanced CT images, and
hypoattenuating (Figure 2.4.1) and focal areas of hemor- there may be evidence of contrast enhancement if active
rhage will appear hyperattenuating on unenhanced CT bleeding (acute) or neovascularization (chronic) is
images (Figure 2.4.7). Edema will appear T1 hypointense present. MR imaging features will generally follow the
and T2 hyperintense on MR images with hemorrhagic scheme outlined in Table 2.4.1, although age can be
regions having a T2* signal void and an otherwise ambiguous when multiple bleeding episodes occur over
variable appearance depending on duration since time. Secondary features of mass effect may include sur-
trauma. Edema and hemorrhage will increase brain rounding edema, midline shift, ventricular displacement
9,10
parenchymal volume, which can lead to midline shift, and compression, and sulcus and gyrus effacement on
ventricular compression, sulcal and gyral effacement, both modalities (Figure 2.4.9).
and brain herniation. Magnetic resonance angiography,
diffusion and perfusion weighted imaging, and diffusion Hemorrhagic infarction
tensor imaging can all be used to further characterize the Hemorrhagic infarctions may not be distinguishable from
extent of injury. 1,11,12 hematoma caused by vascular disruption (Figure 2.4.10).
Imaging features of hematoma described above are also
Vascular disorders applicable to hemorrhagic infarction.
Primary intracranial vascular disease is uncommon in
cats and dogs, as compared to stroke disorders in people. Nonhemorrhagic Infarction
Stroke occurs when blood flow to the brain is disrupted, CT imaging features of nonhemorrhagic infarction may
causing ischemia and eventual brain cell death. Stroke is be subtle and include focal or regional hypoattenuation
caused by either spontaneous vascular disruption, leading from edema and variable, but often minimal, mass effect.
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