Page 116 - Feline diagnostic imaging

114 7 Normal Cross-sectional Anatomy of the Eye and Orbit
Placing a small amount of balanced salt solution (BSS) or [7,13,15,29]. When the horizontal (dorsal) axial scan is
eye irrigation solution on the vertex of the cover for sound performed, the marker is oriented toward the patient’s
conduction, the tip of the cover rests on the eye directly nose, which divides the globe into superior (dorsal) and
over the pathology for imaging. inferior (ventral) (Figure 7.4). The retrobulbar tissues can
The two basic probe orientations used to evaluate the be imaged through the eye or by placing the transducer
feline eye are vertical (sagittal) and horizontal (dorsal) caudal to the globe and orbital ligament [30].
axial scans; then selected oblique planes are used to deline-
ate a lesion better [1,3,10,12]. The vertical (sagittal) axial 7.1.2 Ultrasonographic Anatomy
scan is performed with the probe face centered on the cor-
nea and the marker oriented in the superior position, of the Normal Cat Eye
which divides the globe into nasal (medial) and temporal Detailed knowledge of the normal ultrasonographic anat-
(lateral), thereby displaying lens and optic nerve in the omy of the eye and retrobulbar space is essential to further
center of the echogram. The main advantage of this scan is identify and characterize ocular and orbital pathology
that it is the easiest to understand because it displays the (Figure 7.5) [4]. The ocular structures should be examined
lens and optic nerve in the center of the scan; however, in real time and in systematic order: cornea, anterior cham-
sound attenuation and refraction from the lens often ber, iris and ciliary body, lens, vitreous, retina‐choroid‐
hinder resolution of the posterior segment (Figure 7.3) sclera interface, and retrobulbar tissues [11]. For didactic
purposes, the globe has been divided into anterior and pos-
terior segments.

7.1.3 Eyeball (Ocular Globe)
The B‐mode image of the normal cat eye (Figure 7.6) is
characterized by a rounded, well‐delimited structure with
anechoic contents that represent the anterior chamber and
vitreous cavity [5,31]. A‐mode has been used to study feline
eye biometry in the domestic shorthair and mixed‐breed
cat [32,33]. Mean axial globe length (AGL), anterior chamber
length (AC), and vitreous cavity depth are 19.75 ± 1.59 mm,
4.66 ± 0.86 mm, and 7.92 ± 0.86 mm respectively [33]. Ocular
biometry of the Persian cat has been obtained using B‐mode
ultrasonography; head circumference in this breed showed a
positive correlation with AGL, AC, and lens thickness [34].
In the A‐mode image, the axial plane provides four major
Figure 7.1 B-scan ultrasound showing a transcorneal (axial)
approach in a cat. peaks that correspond to the cornea, anterior lens capsule,

Figure 7.2 Ultrasound biomicroscopic (UBM) examination technique in a cat (left), probe detail (right).