When Do We Need to Use a CT?

The ability to perform computed tomography (CT) scans of the equine head, including in standing patients, has proved hugely advantageous for both clinical and research purposes. This modality provides excellent contrast of bone, dental tissues and the air-filled sinuses and the nasal cavity; with the additional major benefit of avoiding superimposition of the complex head structures and allowing 3D multiplanar image reconstruction. As a result, CT has been shown to have high sensitivity and specificity for diagnosing disorders of the head. The short image acquisition times have allowed for modifications to facilitate image capture in standing sedated animals, eliminating the risks and costs of general anaesthesia.

Computed tomography has provided much useful information about the anatomy of the equine head, with numerous publications in the area, including descriptions of equine dental, nasal and sinus anatomy [1-4]. Studies using very high resolution (microCT – not possible in live equine patients) have been used to examine dental anatomy in finer detail, including the anatomy of the equine incisor and cheek teeth endodontic [5,6] and infundibular structures [7]. Additionally, numerous studies have reported the benefits of CT for diagnosing dental and sinonasal disorders and treatment planning [8-14].

However, CT is not without its disadvantages: moderately high radiation doses are applied during scanning, which should be considered for the patient and staff operating the machine. Not all animals stand well for scanning (requiring head placement within the CT scanner) and motion artefacts can make image interpretation difficult in some cases. The required CT and platform equipment are bulky and expensive, with resultant moderate costs (approximately £800–1000 per scan in the UK currently) and restricted availability. Similar to radiography, CT is not a functional imaging modality and thus some changes may be identified on CT scans that are of unknown duration and clinical significance, which can make decision making challenging in some cases.

As a result, it is sensible to fully consider the selection of cases for CT scanning. This author considers head CT diagnostically useful for the following broad categories of cases:

• Cases with equivocal diagnosis from other examinations such as radiography, oroscopy and/or rhinoscopy.
• Cases with sinusitis, to help rule in/out a dental cause.
• Cases requiring careful evaluation of endodontic health (e.g. clinical crown fractures), particularly if multiple teeth are involved.
• Cases with severe/advanced changes, e.g. abnormal anatomy or neoplasia requiring surgical planning.
• Cases with suspect temporomandibular or temporohyoid joint disease.
• Head-shakers.

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