Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

656 Small Animal Radiolo g y and Ultrasono graphy
Fig. 5-59 Longitudinal sonograms
of the left eye of a 7-year-old male
English Springer Spaniel with a history
of conjunctivitis and buphthalmos.
There is an irregularly shaped
hypoechoic mass in the retrobulbar
space. This may represent a tumor or
abscess. Diagnosis: Retrobulbar
squamous cell carcinoma.
defined. Distinction between tumor and inflammation is difficult. 34 Aspiration of lesions can
be guided by ultrasonography and can help in achieving a specific diagnosis. 167,169-171 Both
tumors and abscesses may be well defined or poorly defined (Figs. 5-58 and 5-59). Deformity
of the globe may be identified. In addition to permitting identification of the mass, an ultrasonographic
examination also helps guide needle aspiration or biopsy. 34
Retrobulbar foreign bodies can be identified. They usually are hyperechoic and have
shadowing deep to them. Hypoechoic zones may be identified if there is inflammation or
abscess formation around the foreign body.
C O M P U T E D T O M O G R A P H Y A N D M AG N E T I C R E S O N A N C E I M AG I N G O F
T H E O R B I T
CT has been used to evaluate the orbital structures of the dog. The infraorbital fat contrasts
with the extraocular muscles and nerves, and intracranial extension of an orbital mass can
be detected easily. 172,173 MRI also can be used for examination of the eye and orbit. Good
anatomical detail is produced. Ultrasonography is less expensive, is more readily available,
and provides better intraocular information but inferior information about surrounding
bony structures. All of these techniques are superior to contrast orbitography.
C O M P U T E D T O M O G R A P H Y A N D M AG N E T I C R E S O N A N C E I M AG I N G O F
T H E B R A I N
The brain can be evaluated using either CT or MRI. Technique selection is based mainly on
availability; however, MRI usually is preferred for evaluation of the human brain. There are
many references describing both techniques and these should be consulted if imaging of
the brain is desired. 174-177 High-contrast landmarks, such as cerebrospinal fluid and bony
structures, are important in identifying brain structures using CT. CT is useful in identifying
neoplastic and inflammatory brain diseases in dogs and cats. 177 It provides information
relative to lesion size, location, and character and defines the relationship between brain
lesions and normal structures of the calvarium. Similar changes were noted in tumors and
inflammatory lesions. These changes included enlargement or asymmetry of the ventricles,
midline shift of the falx, edema, focal changes in opacity of the brain (both before and after
contrast administration), periventricular contrast enhancement, and ringlike enhancement
of brain lesions. Only one of the inflammatory lesions described was multifocal.
MRI provides more anatomical information regarding the brain. Differentiation
between white and gray matter is superior in MRI when compared with CT. Differentiation
between neoplastic and inflammatory lesions is difficult.
Radiographic contrast agents often are administered during the CT examination, and
paramagnetic contrast agents are used during MRI examinations to enhance identification
of lesions. Both radiographic contrast and paramagnetic contrast agents can cross a disrupted
blood–brain barrier and this will increase the visibility of many lesions.