Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

Chapter Five The Skull 621
The eye is examined easily using high-frequency transducers placed directly on the
cornea. 31-33 Although the eye can be examined through the eyelid, direct corneal contact
results in a superior image and is not damaging to the eye. An offset, a fluid-filled or tissue-dense
structure positioned between the transducer and the eye, can be used if examination
of the aqueous chamber is important. Electronic transducers have reduced contact
artifact that interferes with the image of the aqueous chamber, and therefore an offset may
not be needed to image the cornea and aqueous chamber. Topical anesthetic and sterile
lubricant are used for the ultrasonographic examination. Sedation is required in a few
cases, but manual restraint is sufficient for most examinations. Most animals do not resent
placement of the transducer directly on the anesthetized cornea. Although ultrasonographic
gel is a nonirritating substance, it is a good practice to flush the eye with sterile eyewash
after the ocular examination.
Both longitudinal and transverse planes are used for the ultrasonographic examination.
The anatomy of the aqueous chamber, lens, iris, vitreous chamber, and retina
can be demonstrated. A hyperechoic dot usually is seen on both the anterior and the
posterior surfaces of the lens capsule. The lens, aqueous, and vitreous should be anechoic.
In older dogs, echogenic debris may be observed within the vitreous chamber.
This appears to be a normal finding. The optic disc may be identified in some dogs as
an indentation in the posterior aspect of the globe. The normal retina is not identified.
Doppler velocity measurements of various ophthalmic arteries and veins can be
measured. 40,41
Color Doppler imaging has been performed. Vascular signals were observed in four
major areas and were thought to represent flow in the external ophthalmic, ethmoidal, and
posterior ciliary arteries and external ophthalmic veins. 42
The retrobulbar structures may be examined also through the globe, 34 although the
extraocular muscles cannot always be differentiated and the optic nerve cannot be
traced. In addition, retrobulbar structures may be evaluated by placing the transducer
caudal to the orbital ligament on the dorsolateral aspect of the orbit posterior to the eye,
the temporal approach. 43 Some ultrasonographers prefer the corneal contact method
for examining the retrobulbar space; however, we believe that the temporal view is
superior.
The open fontanelle at the frontoparietal suture has been used as an acoustic window
in dogs from birth to 3 or 4 weeks of age. Ultrasonographic examination of the
brain has been performed in older dogs with open fontanelles and in dogs with closed
sutures through a craniotomy. Both transverse and longitudinal scans may be obtained
and the detailed anatomy of the brain has been described. Dimensions of the lateral
ventricle could be measured; however, the third and fourth ventricles could not be identified
in normal neonatal dogs. A maximal height of the lateral ventricle of 0.15 cm was
reported for both neonatal and adult dogs. Lateral ventricles with heights greater than
0.35 cm were considered enlarged. Others have related the dorsoventral dimension of
the lateral ventricle to the overall dorsoventral dimension of the brain. The dorsoventral
dimension of the lateral ventricle should be 14% or less than the dorsoventral diameter
of the brain. 36-39
The lateral ventricles normally appear as slitlike anechoic areas. The third ventricle may
be identified depending on its size. It will be located on the midline and appears as an oval
central anechoic area, with a surrounding thin hyperechoic region that represents the
choroid plexus.
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
Both CT and MRI have been used for evaluation of a wide variety of diseases that affect the
skull and associated soft tissues. In many cases the information derived from these studies
far surpasses that obtained by standard radiographic studies. 44 However, standard radiography
may be superior in many situations (e.g., fractures, dental disease). Both CT and
MRI produce cross-sectional images of the head, and the information gained from the
studies is superior in most other situations. The major limitations are the availability of the
equipment and the expense of the examination. Most institutions or referral centers have
access to these techniques.