Radiology and Scintigraphy: Techniques and Normal and Abnormal Findings

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1 Published in IVIS with the permission of the AAEP Close this window to return to IVIS Radiology and Scintigraphy: Techniques and Normal and Abnormal Findings Safia Barakzai, BVSc, MSc, Cert ES (ST), Diplomate ECVS Author s address: Division of Veterinary Clinical Studies, The University of Edinburgh, Easter Bush Veterinary Centre, Midlothian EH25 9RG Scotland. safia.barakzai@ed.ac.uk. Radiology The large proportion of unerupted tooth, which lies deep to the clinical crown, cannot be evaluated by clinical examination performed per os. Radiography is, therefore, an essential diagnostic tool for the complete evaluation of many equine dental disorders. Dental radiography is especially valuable for evaluating localised apical infection (which most commonly occurs in young horses which have long reserve crowns). It is also useful for evaluating dental fractures, dental-related tumours, diastemata, and supernumerary and displaced teeth. Excellent quality dental radiographs can be obtained in the standing, heavily sedated horse, and consequently, general anaesthesia is not required. Radiographic Techniques The x-ray machine used must be capable of allowing both vertical and horizontal movements of the x-ray beam. Exposure requirements are not high for equine dental radiography, especially if cassettes with rare-earth intensifying screens are used. The use of large (35x43cm) cassettes allows imaging of the entire cheek teeth rows and associated sino-nasal or mandibular structures. Radiation safety should be strictly adhered to when taking equine dental radiographs, and the primary beam should be collimated to include only the area of interest. Standard radiographic views include lateral radiographs, 30 o dorsolateral-lateral oblique radiographs (to view maxillary cheek teeth apices), o ventrolateral-lateral oblique radiographs (to view mandibular cheek teeth apices), and dorso-ventral radiographs to look for sagittal fractures and displaced teeth and in particular, to view the area of the ventral conchal sinus. Additional radiographic views, such as open-mouthed oblique radiographs (15 o ventrolateral-lateral radiographs to view the maxillary erupted crowns and o dorsolateral-lateral radiographs to view the mandibular erupted crowns) and intra-oral radiographs may also be indicated in some cases. The use of a small metal marker over a site of facial swelling or a metallic probe inserted into a sinus tract may help to ascertain which tooth is affected.

2 Normal Appearance and Age-related Variation in Radiographic Appearance The radiographic appearance of the equine CT, and particularly their apices, varies markedly with age of the horse, and an appreciation of the normal variations is required to enable proper interpretation of dental radiographs. Although radiography is a very specific diagnostic aid (95% specificity), it is not very sensitive (50% sensitivity) 1,2, and therefore, in early cases of periapical CT infection, even experienced clinicians may not be able to definitively identify lesions. Foals are born with 3 deciduous CT in each row, and these may be identified by their short, spicular roots. The dental sacs in the young horse are large, rounded, radiolucent structures with a striated pattern, which is due to partially calcified enamel. As a dental sac develops into a CT, its apical area appears as a round, radiolucent area with a wide periodontal space (eruption cyst). The lamina dura denta (a radiodense rim of cortical bone that lines the alveolus) is often not visible at the apex of developing teeth. As the horse ages and the CT erupt, the true roots (i.e. enamel-free areas) develop, and the apices change from rounded to pointed. The clinician should bear in mind that because the permanent CT of horses 1 to 4 years old are continuously developing and erupting, variation in the radiographic appearance of the apical region of the CT of young horses is normal. Radiographic Signs of Dental Disease Early periapical infection of the CT causes the periodontal space to widen and the lamina dura denta to thin or disappear. When periapical infection has been present for many weeks, the affected apex develops radiographically apparent lytic changes, especially if the tooth is mature and has well-formed true roots. These changes manifest as a periapical radiolucent 'halo' surrounding rounded or 'clubbed' tooth roots due to gross lysis/destruction of the root structures. If periapical infection is more chronic, a zone of radiodense sclerosis usually surrounds the periapical 'halo', due to new bone deposition around the lytic infected area. An external draining tract is common with periapical infection of a mandibular CT and sometimes accompanies infection of a rostral maxillary CT. Infection of a caudal maxillary CT is rarely accompanied by an external draining tract, unless the tooth is laterally displaced. If a draining tract is present, a blunt probe inserted into the tract often provides definitive evidence of which tooth is infected. Dental Sinusitis The rostral maxillary sinus (RMS) of the horse is usually positioned dorsal to the upper 08 s and 09 s (3 rd and 4 th cheek teeth) apices and is separated from the caudal maxillary sinus (CMS) by a complete bony septum. The CMS is usually positioned immediately dorsal to the upper 10 s and 11 s (5 th and 6 th cheek teeth) apices. Soft tissue densities may also be apparent in the sinuses if one of the caudal 3-4 maxillary CT has developed periapical infection. This may be due to a rounded, soft tissue granuloma or later, an encapsulated abscess developing over the infected apex. Fluid

3 lines may be apparent in straight lateral views of the sinuses, due to accumulation of liquid purulent material. In cases of dental sinusitis, as in other cases of chronic sinusitis, inflamed and hypertrophied sinus mucosa may cause general increased soft tissue density within the sinuses. Lavage of the sinuses, performed through a trephine hole, to evacuate exudate accumulated within the sinuses may allow for more accurate radiographic evaluation of the cheek teeth apices. Scintigraphy Although it is a very specific diagnostic tool, radiography has a low sensitivity for detection of equine periapical infection (i.e. high number of false negatives), and particularly so if dental infection is in its early stages. In such early cases, scintigraphy (which provides a functional image of bone) is a useful adjunctive diagnostic tool to radiography, being highly sensitive (95-96%), but of moderate specificity (79-86%). 2,3 It is also useful for differentiating dental sinusitis from primary sinusitis or sinusitis caused by other lesions, such as an encapsulated abscess or a sinus cyst. This differentiation is extremely important for appropriate treatment of horses with these disorders, because the consequences of extracting a healthy tooth may be serious. Scintigraphic Technique A dose of MBq 99m Tc-MDP/100kg bodyweight is administered to the horse via an indwelling jugular catheter. Typically, only bone-phase images are acquired, usually at 2-4 hours post injection, because pool or soft tissue phase images do not usually provide any additional useful information and considerably increase the radiation exposure of personnel. Images should be acquired using dynamic studies (e.g. 30 consecutive 2 second frames), because inevitable movements of the horse s head during the acquisition period which, even if small in magnitude, may cause blurring of lesions on a static study. Dynamic studies should then be motion corrected to produce a single corrected summated image. A 128 x 128 matrix usually produces the clearest scintigraphic images of the equine head. Ideally, counts should be over 200,000 for each image, but the amount of counts acquired depends on the dose of technetium administered, the uptake of technetium by the tissues, and the time-interval between injection and acquisition of the image. Lateral images are obtained with the gamma camera perpendicular to the floor and parallel to the sagittal plane of the head. Depending on the diameter of the gamma camera and the size of the horse s head, two lateral views centered at different postitions may be necessary to view the entire cheek teeth row and all of the paranasal sinuses. Dorsal views are obtained by positioning the camera parallel to the frontal bones. Oblique views may be useful for assisting lesion localisation. In horses suspected to have disorders affecting the hemi-mandibles, a ventral view may be obtained by positioning the camera underneath and parallel to the horizontal mandibular rami.

4 Normal Scintigraphic Anatomy Normal patterns of 99m Tc-MDP uptake in the equine head vary markedly between different age groups, due to the development and eruption of the permanent dentition and the accompanying active periapical bone remodelling. It is important to remember when assessing scintigrams of young horses, that areas of increased radionuclide uptake (IRU), should be bilaterally symmetrical when uptake is due to normal eruption of CT, whereas increased uptake is usually unilateral when it is due to disease involving a CT apex. Lateral views: these are often the most useful for identifying and localising periapical infection of individual cheek teeth. The reserve crowns of the cheek teeth appear as cold spots of reduced uptake of radionuclide, and are surrounded by zones of increased radionuclide uptake (IRU) corresponding to the alveolar and interdental (interproximal) bone. The erupted crown is often represented by an area devoid of radionuclide uptake. The ethmoturbinates, temporomandibular joints, and atlanto-occipital joint are normally associated with focal areas of markedly IRU. The ventral and caudal edges of the mandible and the zygomatic arches can also be clearly identified as areas of high metabolic activity. Dorsal views: IRU is seen in the alveolar and interdental bone associated with the cheek teeth, but identifying individual teeth apices with accuracy in dorsal scintigrams is often impossible. The ethmoturbinates are also clearly seen as areas of IRU, and a moderate amount of IRU may also be seen in the zygomatic arches and temporomandibular joints. Scintigraphy and Dental Disorders 99m Tc-MDP uptake associated with periapical infection is typically focal and intense, with IRU located over the apical region of the affected tooth. Region of interest studies performed on cases of periapical infection have shown IRU to be % greater than the same region on the normal, contralateral side when using right and left lateral views. If periapical infection is accompanied by secondary dental sinusitis, the focal intense uptake over the affected apex is surrounded by a diffuse region of moderately increased activity over the affected sinus(es). Care should be taken when interpreting scintigrams of horses with sinusitis. In one equine scintigraphic study, which included 15 cases of primary sinusitis, 9 horses had a focal area of moderate or marked IRU within the sinuses. This is important because if a focal area of IRU happens to be positioned over the apex of a cheek tooth, periapical dental infection can be falsely diagnosed. Careful, three-dimensional localisation of the focal area of IRU may help prevent such false diagnoses. Periodontal disease can cause areas of mild to moderate IRU on scintigrams of the equine skull. 2,4 Definitively diagnosing periodontal disease using scintigraphy can be difficult, however, because periodontal disease is often bilateral and multifocal and commonly affects old horses, whose cheek teeth are not clearly delineated during scintigraphy. Periodontal disease should be clinically evident from a thorough examination of the oral cavity, and the use of scintigraphy provides little additional benefit in diagnosis.

5 Summary Radiography is a very useful tool for identifying dental abnormalities, and it is likely to remain the most commonly used imaging modality for identifying abnormalities of the reserve crown and apices of equine CT. As computed tomography and magnetic resonance imaging of the equine skull become more accessible, these techniques are likely to eventually replace radiography. For the time being, however, in early cases of periapical infection, when radiographic changes may be very subtle, or in cases where radio-opaque sinus contents obscure the CT apices, scintigraphy can be valuable in determining which, if any, of the teeth is infected. Because of the major short- and longterm consequences of extracting a wrong CT, it is better to take a conservative approach in such cases if scintigraphy is not available, or if the combined results of these two imaging modalities are unclear. If there is any doubt as to whether a tooth needs to be removed, it is better to err on the side of caution by treating a horse suspected of having dental infection with systemically administered antibiotics and re-evaluating the horse radiographically 4-6 weeks later, when changes may have become more marked. References 1. Gibbs C and Lane JG Radiographic examination of the nasal and paranasal sinus regions of the horse. II: Radiological findings. Equine Vet J 1987 ;19; Weller R, Livesey L, Maierl J, et al. Comparison of radiography and scintigraphy in the diagnosis of dental disorders in the horse. Equine Vet J 2001;33; Barakzai S., Tremaine H, and Dixon P. Use of scintigraphy for diagnosis of equine paranasal sinus disorders. Vet Surg 2006;35; Archer DC, Blake CL, Singer ER et al. The normal scintigraphic appearance of the equine head. Equine Vet Edu 2003 ;15; Archer DC, Blake CL, Singer ER et al. Scintigraphic appearance of selected diseases of the equine head. Equine Vet Edu 2003;15; Further Reading 1. Barakzai S. How to radiograph the erupted (clinical) crown of equine cheek teeth. Clinical techniques in Equine Practice 2005;4; Barakzai S and McAllister H. Radiography and radiology of the respiratory tract In: McGorum, Robinson, Schumacher and Dixon eds. Equine Respiratory medicine and Surgery, Oxford:Elsevier, 2006; Klugh D Intraoral Radiography of Equine Premolars and Molars In: Proceedings. 49th Annual Convention of the American Association of Equine Practitioners, 2003 (online). 4. Weller R, Livesey L, Maierl J, et al. Comparison of radiography and scintigraphy in the diagnosis of dental disorders in the horse. Equine Vet J 2001;33;49-58.

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