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Normal Cross-sectional Anatomy of the Eye and Orbit
Eva M. Abarca

Hospital Ars Veterinaria, Barcelona, Spain

Over the last three decades, the development of cross‐ A‐scan and B‐scan echography are the types of ultrasound
sectional diagnostic modalities such as ultrasonography display systems most commonly used in ophthalmology
(US), computed tomography (CT), and magnetic reso- [13,14]. A‐scan ultrasound is a one‐dimensional acoustic
nance imaging (MRI) has greatly contributed to the display in which echoes are represented as vertical spikes
understanding and diagnosis of feline ocular and orbital from a baseline while B‐scan ultrasound produces a two‐
diseases [1–5]. dimensional acoustic section [14,15]. A‐mode echo indicates
Ocular US is often the first imaging technique selected to the internal composition of lesions [7]. Concurrent B‐mode
evaluate the eye and retrobulbar tissue in veterinary oph- and A‐mode ocular ultrasound evaluations can provide
thalmology [4,6–8]. CT and MRI can provide cross‐sec- complementary diagnostic information [10,16,17].
tional and three‐dimensional images of the eye so that the The use of color and Doppler ultrasound enables nonin-
spatial relationship between anatomic structures can be vasive estimation of blood velocity parameters [10].
assessed. This chapter reviews the normal findings of ocu- Resistive (RI) and pulsatility indices (PI) have been used to
lar and orbital structures in the cat (US, CT, and MRI) [9]. evaluate ocular vascular resistance in glaucoma and sys-
temic hypertension in humans and animals [18–20]. In
7.1 Ultrasonography normal cats, the long posterior ciliary arteries have been
consistently imaged with normal upper value of 0.72 for
ocular RI and 1.02 for PI [19].
7.1.1 Ultrasonographic Examination
The conventional 10 MHz B‐scan with a focal range of
There are two primary methods for imaging the feline eye: 3–4 cm is the most suitable ophthalmic transducer for the
the transpalpebral and corneal approaches. The corneal posterior segment and orbit evaluation [10,21]. Ocular
approach (Figure 7.1) is the preferred examination tech- ultrasound probes with higher frequencies between 20 and
nique and produces the highest‐quality imaging of the 35 MHZ called high‐frequency ultrasound and 50–100 MHz
globe and orbit [10]. It requires the application of a topical (ultrasound biomicroscopy or UBM) allow specialized
anesthetic such as tetracaine or propacaine [2,11]. A ster- examination of the anterior segment [22–24]. UBM offers
ile, water‐soluble acoustic gel such as methylcellulose is high‐resolution imaging (20–50 μm) and provides a nonin-
recommended [12]. vasive means of imaging the anterior segment structures at
Scanning of the eye using a transpalpebral approach is near microscopic resolution [23,25,26].
less desirable due to sound attenuation and artifacts The use of an eye cup was the most common method
[2,4,10]. However, this approach may be recommended if used in human and canine UBM [25,27,28]. However, due
there is a deep corneal ulcer, ocular perforation or recent to the relatively tight‐fitting eyelid of the cat, placement of
severe ocular trauma [2,3,12]. This approach requires that this eye cup was often problematic [23]. Newer UBM tech-
the hair be clipped to enhance contact between the trans- nology facilitates examination by providing handheld
ducer and eyelid [11]. transducers with a snap‐on water bath (Figure 7.2) [10].

Feline Diagnostic Imaging, First Edition. Edited by Merrilee Holland and Judith Hudson.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.