| | Use of Scintigraphy for the Diagnosis of Apical Infection of Equine Cheek TeethScintigraphy with 99mTechnetium Methyldiphosphate (99mTc-MDP) can detect changes in bone which precede radiographic changes, and this makes it a very useful imaging modality for the diagnosis of early periapical infection of the equine cheek teeth, where the results of radiography may be equivocal. It is a particularly useful technique for the differentiation of dental sinusitis from other causes of sinusitis. Bone phase images acquired 2 to 4 hours postinjection give the most useful information for the evaluation of dental and sino-nasal lesions. Images should be acquired in sedated horses using dynamic studies which are then motion corrected. Left and right lateral, dorsal, and ventral views are usually sufficient to image the equine skull fully; however, oblique views may occasionally be useful for lesion localization. Periapical infections of the cheek teeth typically result in focal and intense increased radionuclide uptake (IRU) located over the apical region of the affected tooth. In contrast, most cases of primary sinusitis show more diffuse and less marked IRU over the affected region(s). However, some cases of primary sinusitis do exhibit focal areas of moderate to marked IRU, and care should be taken to try and differentiate these from periapical infections by three-dimensional lesion localization.
Scintigraphy is unique among the imaging modalities because the images reflect active physiological processes rather than the structural features portrayed by radiography, ultrasonography, CT, or MRI. It involves the intravenous administration of a gamma ray-emitting radioisotope, which is bound to a tissue-seeking molecule. 99mTechnetium (99mTc) is currently the most commonly used radioisotope in the equine field, and is a meta-stable radionuclide which emits a gamma ray of 140 keV, with a physical half-life of 6 hours. The radioisotope is cleared at a fast rate from the blood and soft tissues and is incorporated selectively into bone in areas of resorption or formation.
Although scintigraphy has been used in equine orthopedics for many years, only recently have reports of the use of scintigraphy for the detection of disorders of the equine skull in large numbers of horses begun to emerge.1, 2, 3 The ability of scintigraphy, using 99mTc bound to phosphates, to detect changes in bone before they become radiographically apparent (because increased bone turnover usually precedes structural change) is one of the key advantages of this technique in the equine patient. Disadvantages of scintigraphy include the expense of setting up a gamma camera and appropriate software programs; licensing for the use, storage, and disposal of radioactive waste; appropriate stabling facilities which comply with radiation protection legislation; time required to isolate the patient (in most centers, horses are considered “radioactive” for 24-48 hours postinjection and cannot be handled) thereby delaying further diagnostic procedures or treatment; the requirement for technical expertise when reading scintigraphic images; and the risk of radiation exposure to personnel.
The complex structure of the equine skull can make radiographic images of this area difficult to assess due to numerous superimposed opacities, and early periapical infections are particularly difficult to detect radiographically.4, 5 In the more caudally positioned cheek teeth, where secondary sinusitis is common, infected teeth can be recognized with confidence in only 50% to 57% of cases.1, 4, 6 It was suggested by Lane and coworkers7 that the increase in bone density associated with maxillary osteitis superimposed on opaque sinus contents obscures the underlying dental structures and prevents detailed inspection so that subtle abnormalities may be missed. In many cases of sinusitis, the increased soft tissue opacity within the sinuses may simply be due to inflamed and hypertrophied sinus mucosa, which occurs secondary to any type of chronic sinus infection.6, 7 In contrast, scintigraphy has been shown to have excellent sensitivity (95.5%) and moderate specificity (86.4%) for detecting dental disease in the horse.1
Technique  Most equine skull scintigraphy is performed using the bone marker 99mTc-MDP. A dose of 1 to 1.5 GBq/100 kg bodyweight is administered to the horse intravenously, usually via a jugular catheter. Typically, only bone-phase images are acquired at 2 to 4 hours postinjection, as pool or soft tissue phase images do not usually provide any additional useful information and collection of such images considerably increases the radiation exposure of personnel.1, 8 The use of 99mTc-hexa-methylpropyleneamine(HMPAO)-radiolabeled leukocytes has been described for equine dental scintigraphy,1 but did not allow for positive identification of apical infections due to lack of anatomical resolution; additionally, this incurred considerable additional cost. Heavy sedation is usually required to allow close positioning of the camera to the patient and is achieved using a combination of an alpha-2 agonist (eg, xylazine, detomodine or romifidine) and butorphanol. Some horses are very sensitive to sounds generated by movement of the camera, and “plugging” the ears of such patients with cotton wool may reduce auditory stimuli. Blinkers may also be used to prevent movement away from the camera as it is brought into position close to the head, as long as they do not have metallic components which may attenuate the gamma rays. Similarly, a rope headcollar should be used to prevent artifactual “cold spots” from buckles and rings on regular headcollars. The horse’s head should be rested on a stool or similar object (Fig. 1), to minimize both movement induced by sedation and rotation in the sagittal plane. Images should be acquired using dynamic studies (eg, 30 consecutive 2-second frames) because there will be 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. Scintigraphic Views Lateral images are obtained with the gamma camera perpendicular to the floor and parallel to the head (Fig. 1). Depending on the diameter of the gamma camera and the size of the patient’s head, two lateral views centered in 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 (Fig. 2). Image quality may be enhanced when acquiring dorsal images by positioning a lead shield under the head to attenuate gamma rays from the neck and chest.9 Oblique views may be useful for assisting lesion localization.9 In horses suspected to have disorders affecting the hemimandibles, a ventral view may be obtained by positioning the camera underneath and parallel to the horizontal mandibular rami. Normal Scintigraphic Anatomy Age-Related Variations Normal patterns of 99mTc-MDP uptake in the equine head will vary markedly between different age groups due to the development and eruption of the permanent dentition and the active bone remodeling which accompanies this. The scintigraphic changes associated with eruption of the cheek teeth are shown in Table 1. It is important to remember when assessing scintigrams of young horses that areas of increased radionuclide uptake (IRU) which are due to normal eruption of a tooth should be bilaterally symmetrical, whereas disorders involving cheek tooth apices are most commonly unilateral. |
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IRU, increased radiopharmaceutical uptake. |
Lateral Views Lateral views (Fig. 3) are often the most useful for identification and localization of periapical infections of individual cheek teeth. The reserve crowns of the cheek teeth appear as “cold spots” of reduced uptake of radiopharmaceutical, and are surrounded by zones of increased radiopharmaceutical uptake (IRU) corresponding to the alveolar bone and interdental (interproximal) bone. The erupted crown and occlusal surfaces of the teeth are often represented by an area of absent radionuclide uptake. The ethmoturbinates can be identified as a region of IRU positioned dorsally and caudally to the 6th maxillary cheek tooth and are located within the frontal sinuses. The normal temporomandibular joints are also associated with focal areas of markedly IRU as are the atlanto-occipital joints. The ventral and caudal edges of the mandible and the zygomatic arch can be clearly identified as areas of high metabolic activity.1 Dorsal Views IRU is seen in(Fig. 4) the alveolar and interdental bone associated with the cheek teeth, but it is often impossible to identify individual tooth apices with accuracy. The ethmoturbinates are also clearly seen as areas of IRU immediately caudal and axial to the 6th maxillary cheek teeth. A moderate amount of IRU may also be seen in the zygomatic arch and temporomandibular joints. Ventral Views Greatest uptake(Fig. 5) of radionuclide is present in the alveolar and interdental bone of the mandibular cheek teeth. Moderate IRU can be seen in the region of the incisive plate of the mandible, at the caudal angles of the two hemimandibles and in the maxilla lateral to the mandibular arcades. Scintigraphic Abnormalities Periapical Infection Scintigraphy is most useful for diagnosis of periapical infection when used in combination with other diagnostic techniques, such as radiography, endoscopy, and, of course, clinical examination.1, 5, 10, 11 99mTc-MDP uptake associated with periapical infection is typically focal and intense with IRU located over the apical region of the affected tooth (Figure 6, Figure 7). Region of interest (ROI) studies performed on cases of periapical infection have shown IRU of 24% to 259% greater than the same region on the contralateral side when using right and left lateral views.3, 12 Because “strike through” may occur when comparing two lateral views, ROI taken from left and right sides on a dorsal (or ventral) view can show an even greater IRU% (as high as 700%12). If periapical infection is accompanied by secondary dental sinusitis, the focal intense uptake over the affected apex will be surrounded by a diffuse region of moderately increased activity over the affected sinus(es) (Fig. 8). After dental extraction, areas of IRU can be present for up to 24 months postoperatively, presumably due to continued remodeling of the dental alveolus.12 Differentiation of Periapical Infection from Other Skull Lesions Periodontal Disease Periodontal disease can cause areas of mild to moderate IRU within the alveolar and interdental bone on scintigraphy of the equine skull.1, 3, 12 However, because this disorder is often bilateral and multifocal and commonly affects older horses where the cheek teeth are often not delineated, it can be difficult to definitively diagnose this disorder using scintigraphy. Periodontal disease should be clinically evident from a thorough examination of the oral cavity, and therefore, there is little additional benefit from the use of scintigraphy in its diagnosis. Primary Sinusitis Horses with primary(Fig. 9) sinusitis may show variable patterns of IRU within the affected paranasal sinuses, but generally, IRU is more diffuse and less marked (6-300%3, 12) than is seen with periapical infection. It should be possible to identify the rostral and caudal maxillary and frontal sinuses individually on scintigrams based on anatomical location with respect to the cheek teeth and ethmoturbinates. Some cases of equine primary sinusitis exhibit focal area(s) of moderate to marked IRU (26-320% increase compared with contralateral side12) (Fig. 10). This is an important finding, because if these focal areas of IRU which are observed in cases of primary sinusitis happen to be positioned over the apex of a cheek tooth, a false diagnosis of periapical infection may be made. Careful three-dimensional localization of the focal area of IRU (as seen in the case in Fig. 10) may help prevent such false diagnoses in some cases. Skull Fractures Fractured bones (Fig. 11) often have a focal, intense increase in radionuclide uptake within hours of the fracture occurring due to exposure of hydroxyapatite crystals, which is followed by instigation of remodeling and callus formation within days to weeks.13, 14 When fractures are detected scintigraphically, they commonly cause between 50 and several hundred percent increased radionuclide uptake.15 IRU is variable in cases of skull fractures in horses. Stable fractures, such as those caused iatrogenically when creating sinus flaps or trephine holes, appear to be associated with little or no IRU.2, 12, 16 In contrast, skull fractures which are traumatic in origin are often associated with intense IRU, as is suturitis (eg, of the naso-frontal suture), a common sequel of equine skull trauma. If these focal areas of IRU which are observed in cases of skull fracture happen to be positioned over the apex of a cheek tooth, a false diagnosis of periapical infection may be made. Once again, careful three-dimensional localization of the focal area of IRU (as seen in the case in Fig. 11) may help prevent such false diagnoses in some horses. References 1.
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MEDLINE Department of Veterinary Clinical Studies, Royal (Dick) School of Veterinary Science, University of Edinburgh, Roslin, Midlothian, Scotland  Address reprint requests to Dr. Safia Barakzai, Department of Veterinary Clinical Studies, Royal (Dick) School of Veterinary Science, University of Edinburgh, Roslin, Midlothian, EH25 9RG, UK
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