Page 122 - Feline diagnostic imaging

120 7 Normal Cross-sectional Anatomy of the Eye and Orbit

Corneal epithelium

Corneo-scleral limbus.
Stroma

Endothelium-
Descemet’s m.

Figure 7.10 Normal cornea anatomy. Ultrasound biomicroscopy image of the central cornea in a cat. The cornea has two parallel
lines with an anechoic echogenic center (stroma). The corneal stoma reveals an internal reflectivity that is lower than that found in
the more irregular collagen distribution of the sclera. This difference allows localization of the corneoscleral junction.

appearance of the optic nerve in A‐mode evaluation con-
sists of a double‐peaked sharp spike [7].

7.2 Computed Tomography
and Magnetic Resonance Imaging

The cross‐sectional imaging techniques of CT and MRI
have improved the diagnosis and management of ocular,
orbital, and neuroophthalmic conditions [41–43]. The orbit
in adult cats is approximately 24 mm wide × 26 mm high.
CT has been used to evaluate anatomic differences between
brachycephalic and mesencephalic cats [44,45].
Magnetic resonance imaging is considered superior to CT
for most neuroophthalmic indications, with better assess-
ment of both the intra‐ and extraorbital optic nerve [4,42,46].
Figure 7.11 Normal anterior chamber B-mode using a 35 MHz The advantages of CT over MRI include shorter data acquisi-
transducer. The anterior chamber depth can be measured from
the inner corneal surface (arrow) to the anterior lens capsule tion time, decreased slice thickness and greater spatial
(double arrow). Source: Courtesy of Dr Victoria Espejo. resolution, more precise imaging of cortical bone, soft tissue
mineralization, and acute hemorrhage, and the ability to
muscle is medium‐high reflective on A‐scan and less echo‐ image when magnetic foreign bodies are present [4,10,42].
dense than surrounding orbital soft tissue on B‐scan [39]. The objective of this section is to describe CT and MRI
The intraorbital portion of the optic nerve can be visual- anatomy of the feline orbit and ocular globe. The feline
ized as a distinct, homogeneous cone‐shaped, elongated orbit is formed by six bones; frontal, lacrimal, maxillary,
hypoechoic area in the medioventral part of the orbit and zygomatic, palatine and sphenoid bones [6,44].
surrounded by hyperechoic retrobulbar fat [7,10,40]. B‐
scan is used to evaluate general topography of the retrobul- 7.2.1 CT Eye and Orbit: Normal Anatomy
bar optic nerve; additionally, it can show the relationship
of the optic nerve to normal structures such as the globe The normal CT appearance of the cat’s eye and orbit is
and extraocular muscle [40]. The B‐scan examination is illustrated in Figures 7.17 and 7.18. The retrobulbar fat
usually performed with a medium gain setting and com- provides excellent image contrast on CT images and
parison with the contralateral optic nerve should always facilitates the identification of anatomic structures,
be performed at a similar gain setting [40]. The normal including the globe, extraocular muscle, optic nerve, and