COMPUTED tomography (CT) and magnetic resonance imaging (MRI) are increasingly being used in veterinary diagnosis due to greater accessibility of the equipment, advances in treatment options and increasing owner expectations. However, CT and MRI scanning are complex and expensive procedures that warrant careful patient selection. They should be used to supplement, rather than replace, conventional diagnostic tools. This article describes the most important indications for CT and MRI in veterinary patients and indicates which technique is preferable, should a choice exist. It also discusses the basic principles of the two imaging modalities, together with their limitations. PRINCIPLES OF COMPUTED TOMOGRAPHY CT preceded MRI as a medical and veterinary imaging technique, although it is now less widely performed on veterinary patients. CT is essentially cross-sectional radi-ography, with image production being a function of the absorption of x-ray photons by the tissues. Computer enhancement of the electronically detected image produces much greater tissue definition than is achieved with conventional radiography, and post-processing tech-niques allow expansion of selected areas on a greyscale in order to emphasise either soft tissue or bone detail. The interpretation of CT scans is similar in principle to radiographic interpretation, with mineralised and bony material appearing radiopaque, fluid and soft tissue producing intermediate grey shades and fat being more radiolucent (see images overleaf). However, unlike radi-ography, differentiation between soft tissue and fluid is possible and internal soft tissue architecture is visible. In particular, CT is highly sensitive for subtle bone changes.
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