Page 45 - Zoo Animal Learning and Training

Chapter 4: Advanced Imaging: Intracranial Surgery 37

signal intensity on different pulse sequences allows identification of of administration of contrast media (e.g., contrast still in catheter,
properties of the tissue (e.g., fatty, cystic). With CT, tissue contrast leakage from catheter). The mechanism of contrast enhancement is
is largely due to differences in tissue density. Suspected lesions different on CT and MRI. CT contrast enhancement is the result of
should be cross‐referenced with different imaging planes and direct visualization of the contrast agent, whereas with MRI the
sequences as most genuine lesions are visible on more than one contrast agent is not directly visualized; enhancement occurs due to
plane. Partial volume averaging is common and can be mistaken for the effect of the contrast agent on the immediately adjacent tissues.
pathology (e.g., apparent defects of skull bones). The acquisition times of the images following contrast can affect
Assessment of brain images on CT and MRI is mainly based on interpretation of the underlying lesions but contrast enhancement
changes in symmetry (mass effect, loss of parenchyma), changes in cannot be consistently relied upon to detect similar pathologies
the ventricular system, and shape and size of the cerebellum and [89,90]. A technique described as dynamic contrast‐enhanced MRI
cranial spinal cord. CT may be of limited value for evaluation of has been useful in more specifically detecting histopathological
caudal fossa lesions, especially in larger dogs because of beam hard- abnormalities but it is still in its early assessment period [91,92].
ening artifact resulting in hypoattenuating streaks, which can
obscure pathology. It is important to recognize MRI alterations in
signal intensity (gray matter, white matter). CT images should be Brain Masses
evaluated for alterations in attenuation of tissues. Most brain masses are readily identified on MRI or CT
The extracranial soft tissues should be evaluated for: images [23,45,93,94]. They are often hyperintense on T2‐weighted
• muscle volume; MRI, and are associated with a mass effect: midline shift, and
• muscle signal; compression of ventricles and adjacent parenchyma [95]. Much of
• nasal or orbital lesions; the mass effect is often due to perilesional edema [94]. The presence
• lymph nodes; of edema is easier to appreciate on MRI than CT. On CT, edema
• changes in skull bones (loss of signal, erosion). results in reduced attenuation of the brain tissue. On MRI, edema is
Following administration of MRI contrast media (gadolinium hyperintense on T2‐weighted images, poorly marginated, usually
chelates), the images should be assessed to ensure that normal con- most severe within the white matter, and tends to follow the white
trast enhancement has occurred. With CT, the normal contrast matter tracts (especially the corona radiata) (Figure 4.9) [96].
enhancement pattern is similar to that seen on MRI, with contrast Differential diagnosis of brain masses is based on classification
evident in larger blood vessels and in tissues outside the blood– into extraaxial or intraaxial locations. Extraaxial masses arise from
brain barrier. The choroid plexuses, large veins, pituitary gland, outside the neuraxis (e.g., meninges, skull bone). Intraaxial
nasal mucosa, salivary glands, and trigeminal nerve ganglia/per- masses arise from within the neuraxis (e.g., glial cell tumors).
iganglionic vascular plexuses should all enhance in a normal animal Intraventricular masses are classified as extraaxial. The imaging
[47]. Failure of enhancement of a lesion may be due to lack of blood features of masses are nonspecific. A mass lesion is not necessarily
supply or intact blood-brain barrier but may also be due to failure neoplastic and other causes (e.g., granuloma, hematoma) should be

A B

Figure 4.9 Transverse plane T2‐weighted MRI (A) and postcontrast CT (B) of the brain of a Boxer with presumed glioma. While a mass effect is seen on
CT, the boundaries of the mass and the extent of the perilesional edema (arrows in A) are seen much better on MRI. The mass has resulted in a small focal
defect in the bone adjacent to the mass that is seen more clearly on CT (arrow in B).