Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Abstract

Cross sectional imaging techniques for canine orbits.

Abstract

Ultrasound, magnetic resonance imaging (MRI) and tomodensitometry are all non-invasive imaging methods that offer good visualization of orbital structures. Ultrasound examination is indicated in cases involving opacification of transparent parts of the eye (e.g. associated with corneal oedema, hypopion, hyphaema or cataracts), enophthalmos, and abnormal position and retropulsion of the ocular globe. It can also be used for exploring the retrobulbar space. Ultrasound examination is readily available and does not usually require anaesthesia. Many ocular disorders can be examined using ultrasound, but it is does not offer high specificity when exploring retrobulbar processes, and it can be difficult to distinguish inflammatory processes from neoplasic processes. While ultrasound is generally considered the first intention method of choice, there are cases where the use of MRI or tomodensitometry is necessary. Ultrasound examination is not as useful as tomodensitometry or MRI when evaluating retrobulbar masses that extend beyond the osseous margins of the orbit or assessing osseous lysis. The presence of extensive masses in the orbit is a key indication for the use of tomodensitometry. This technique facilitates differentiation of inflammatory, vascular and tumoral lesions. Tomodensitometry permits evaluation of the spread of a retrobulbar mass and possible invasion of the sinuses or brain. It is also indicated in cases involving cranial trauma and is more sensitive than radiography in detecting fractures or foreign bodies. MRI offers high contrast images of soft tissues and accurate reproduction of anatomical details. This technique is less sensitive than tomodensitometry for appraisal of osseous structures and mineralization. Osseous lesions can be detected by MRI, but tomodensitometry is the method of choice for examining such lesions. MRI can detect minor invasions of the brain or sinuses, and is the treatment of choice when evaluating the optic nerve and optic chiasma. While MRI is not able to differentiate some foreign bodies in soft tissues, it can detect associated inflammatory reactions. The use of MRI is not advised where the presence of a metallic foreign body is suspected.