Use of computed tomography in diagnosis of equine dental and sinonasal disease by Sarah E Powell

ABSTRACT: Computed tomography |CT) is a form of cross-sectionat’ imaging. Techniques for a diverse range of equine orthopaedic and medical conditions, of both the head and distal limbs of adult horses and thorax and abdomen of foats, have been described over the last decade Recently, a few referral centres have installed a new system capable of scanning the head under standing sedation A CT scan enables precise evaluation of the teeth and their relationships to structures around them. As more CT scanners become available, these exquisite images may well become the gold standard of care in a number of equine diseases.

Radiography is based on the absorption of X-rays as they pass through the different parts of the animal. Depending on the amount absorbed by a particular tissue (such as cortical bone, medullary bone or soft tissue) a different quantity of X-rays will pass through the tissue and exit the body.

During conventional X-ray imaging (plain radiography), the exiting X-rays interact with a detection device (X-ray film or digital plate) and provide a two¬dimensional projection radiographic image of the tissues within the horses body.

Although similarly based on the variable absorption of X-rays by different tissues, computed tomography (CT) imaging, also known as ‘CAT scanning’ (computerised axial tomography), provides a different form of imaging known as cross-sectional imaging.

The origin of the word ‘tomography’ is from the Greek word ‘tomos’ meaning ‘slice’ or ‘section’ and ‘graphe’ meaning ‘drawing’. A CT imaging system produces cross-sectional images or ‘slices’ of anatomy, like slicing a loaf of bread.

The cross-sectional images can be manipulated or post processed in a number of ways to be used for a variety of diagnostic purposes in a diverse range of pathological conditions (Figure 1).

Figure 1: Two-dimensional images (left) can be used to detect fluid in the sinuses and damage to the roots of the teeth. Alternatively the slices' can be post-processed to form three-dimensional images (right), which is useful in some orthopaedic conditions

From humans to horses

With clear benefits over other imaging techniques when imaging complex anatomic areas, CT filtered down to the veterinary world in the early 1980s, but the machines built for human patients didn’t lend themselves well to use in horses. The modifications for use in equine patients were initially makeshift and cumbersome, but it was to prove useful in a range of conditions and D D Barbee and his colleagues at Washington State University first published the technique in the 1986 Proceedings of the American Association of Equine Practitioners.

The ongoing progress of CT technology is relentless and the result is a rapid turnover of CT machines in human hospitals. This has led to an abundance of equipment becoming available on the secondhand market, making the installation of a CT facility a viable prospect for larger equine veterinary hospitals. Techniques for a diverse range of equine orthopaedic and medical conditions, of both the head and distal limbs of adult horses and thorax and abdomen of foals, have been described over the last decade as CT affords us an unprecedented amount of information in complex conditions, which is unequalled by other means.

Until recently, it was necessary to anaesthetise horses that required a computed tomographic scan of any area of the body. However, in 2006, the late Alastair Nelson at Rainbow Equine Clinic, designed and installed the first CT system in the UK capable of scanning the head under standing sedation. The legacy that Alastair left is a dramatic increase in equine CT scans as the cost to the client and the risk to the patient has been reduced. Four centres across the UK have now installed versions of Alastair’s standing system, with more planned for the future, both in Europe and the USA.

Veterinary support staff vital

Scanning horses under standing sedation is not a simple procedure and requires a team of skilled veterinary support staff, in addition to the veterinary radiologist. It takes time and patience to position the horses correctly, the horse must stand squarely and be stable on all four limbs whilst under profound sedation (Figure 2).

Figure 2: A horse is positioned on the standing platform at Rossdales Equine Diagnostic Centre prior to undergoing a CT scan of the head. Achieving an optimal sedation level to enable the horse to accept the procedure whilst not being too ataxic to stand squarely on the inflatable platform, is critical to the success of the scan procedure

The standing systems vary slightly, but all require the horses head to move slowly through the gantry whilst positioned on the human CT table (Figure 3). This requires them to stand on a platform capable of moving without friction. The system at Rossdales Equine Diagnostic Centre utilises a platform attached to the human CT table, which moves along the floor on stainless steel runners via a system of air skates, inflated via a compressor system located adjacent to the scanning room.

Figure 3: The platform is connected to the human CT table, which moves through the gantry, pulling the horse through the bore of the CT machine as the X-ray tubes rotate around him. The sand bags over the horse's neck weigh him down to ensure his mandible is firmly in contact with the table, minimising patient movement during the scan

Carrying out a scan with minimal risk of injury to both horse and human requires handlers who are familiar with the system, experienced in sedation techniques and skilled at working with nervous horses. It is often the work of the veterinary nurse and imaging technicians during the positioning of the horses limbs and securing the head in the gantry that leads to the completion of a successful CT scan.

Sedation methods vary between centres, but usually consist of premedication with acepromazine, followed by a combination of butorphanol and detomidine. The use of diazepam may enable extremely nervous horses to be scanned successfully. Horses that cannot safely be scanned under standing sedation can be given a general anaesthetic and the images acquired with the horses recumbent on the modified GA table.

This rapid accumulation of data is proving invaluable for vets working in many specialities but particularly for those dealing with head, dental and sinus pathology, where CT has proved extremely useful. A CT scan enables precise evaluation of the teeth and their relationships to the structures around them, such as masses and fluid within the sinuses (Figure 4), without the overlay of anatomy, which so often complicates and obscures plain radiographs.

Figure 4: A single CT image or 'slice' through the head of a horse at the level of the orbit. Fluid can be seen filling the dental sinuses on the right side of the image, left side of the horse (arrows). This horse was suffering from an ethmoid haematoma, which was removed under sedation following the CT scan

Sophisticated images

One of the advantages of CT images is the post-processing capability, where images can be reformatted to view them in multiple orthogonal planes (Figure 5), three-dimensional reconstructions of bony structures and teeth (Figure 6), and even to reconstruct endoscopic views of the nasal passages, pharynx and larynx (Figure 7).

Figure 5: All images are acquired in an axial or transverse plane. The images can then be processed after the scan and reconstructed into orthogonal planes. Here, in a process called 'multiplanar reformatting’, a sagittal plane view of the cheek teeth has been created from the axial image data

Figure 6: The hundreds of CT slices can be processed to form three-dimensional reconstructions of bony structures, such as the skull seen here. These images are useful in the assessment of skull fractures, for example

Figure 7: This image is a virtual endoscopic reconstruction of the CT data. The virtual camera is positioned within nasopharynx, looking rostrally towards the nasal passages and nasal septum (white arrow). A mass is present in the soft palate (indicated by the black arrow), which was associated with a fracture of the hamulus (a small bony process on the base of the skull)

Post-processing of images is extremely useful in the investigation of skull fractures and conditions of the guttural pouches and hyoid apparatus. This extra information leads to definitive diagnoses and more appropriate treatment and, in many cases, the images serve to guide surgical treatment of a number of sinonasal conditions (Figure 8).

Figure 8: Here the surgeon is using CT images to accurately locate a sinus lesion in the sedated horse

Summary

CT will undoubtedly continue to establish its role as a diagnostic tool in equine veterinary imaging. The cost of the scan and risk of general anaesthesia (when necessary) is offset by the information the CT scan affords the clinician, which can be instrumental in arriving at a correct diagnosis, planning surgical or medical treatment and ensuring the best outcome for the case. As more CT scanners become available around the country, these exquisite images may well become the gold standard of care in a number of equine diseases.

Author

Sarah E Powell MA VetMB MRCVS

Sarah is a senior orthopaedic clinician at Rossdales Equine Diagnostic Centre, Newmarket, where she heads the 3-D imaging facility. She is particularly interested in advanced imaging techniques, a subject on which she publishes and lectures regularly. Sarah provides an image reading service and has a special interest in the use of MRI to detect stress injuries in racehorses.

To cite this article use either

DOI: 10.1111/j.2045-0648.2011.00111.x or Veterinary Nursing Journal Vol26 pp 400-403.

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