Use of CT in the identification of equine cheek tooth disease
The equine skull presents one of the most challenging anatomical regions within equine diagnostic imaging. The hypsodont equine cheek teeth, elongated reserve crowns, structural complexity of the infundibulae and pulp systems, and the spatial relationship between maxillary cheek teeth and the paranasal sinuses render conventional two-dimensional radiography limited, even in experienced hands. Planar radiography is confounded by the superimposition of osseous and dental structures, restricting a clinician to a limited number of orthogonal planes, and this lacks the contrast resolution necessary to characterise subtle endodontic, periodontal, and alveolar pathology with confidence (Tietje et al.., Equine Vet J 1996;28:98–105; Townsend et al.., Equine Vet J 2011;43:170–178). Computed tomography (CT) has addressed these fundamental shortcomings and is currently considered the gold standard imaging modality for the investigation of dental disease.

Technical Principles and Acquisition Parameters
CT generates cross-sectional (tomographic) images through differential x-ray attenuation of tissues, reconstructed computationally with each pixel assigned an attenuation value in Hounsfield Units (HU). In this scale, pure water is designated 0 HU, air −1,000 HU, and cortical bone ≥1,000 HU, with the density of most soft tissues falling between 10 and 100 HU.

Multiplanar reformatting or reconstructions (MPR) of the topographic data in transverse, sagittal, and dorsal planes, together with three-dimensional volume-rendered reconstruction, enables comprehensive spatial evaluation of each tooth, the periodontium, adjacent alveolar bone, and the overlying (maxillary teeth) paranasal sinus compartments (Manso-Díaz et al.., Equine Vet Educ 2015;27:97–106). The varying densities (HU) of dental tissues - cementum, enamel, and dentine - and the lamina dura of the alveolus can then be differentiated, provided an appropriate algorithm and window width / level is utilised. A thin-slice (0.5 to 1.25 mm), bone window width and level setting is required for the interrogation of dental and osseous structures. However, evaluation in a soft tissue algorithm and window width / level is essential to characterise changes in the adjacent soft tissues, with an increased slice thickness often helpful (often around 2.5mm). In addition to facilitating visual tissue differentiation, HU values provide an objective measurement of region-of-interest (ROI) densities, allowing improved differentiation of ‘tissue composition’. Cementum, dentine, and bone exhibit overlapping attenuation ranges, of approximately 550–2,000 HU, but are distinguishable from pulp (−400 to +500 HU) and enamel (>2,500 HU), permitting identification of mineralised tissue deposition and structural defects. Post-contrast imaging is sometimes utilised for equine skull imaging. However, in practice, this is rarely necessary for dental disease assessment. This can be undertaken via intravenous injection or local instillation of iodine-based contrast media (i.e., iohexol) into fistulous tracts to improve differentiation of soft tissue alterations. Likewise, use of thin malleable metal probes can also be utilised for such cases to demonstrate thin or small tracts which could otherwise be challenging to follow.

Standing CT and Cone-Beam CT
The development of standing CT platforms, in which the deeply sedated horse is positioned on a surface suspended by compressed-air skates and passed through a stationary gantry, broadened clinical access to this modality by eliminating the requirement for general anaesthesia. Avoiding general anaesthesia makes standing CT a low-risk procedure, and more recently, a sliding gantry system has also been developed. Sedation is required to minimise patient movement, though with care not to over-sedate and induce “wobble”, which will degrade image quality and introduce motion artefact. Recently, cone-beam CT (CBCT) technology - employing a cone-shaped x-ray beam directed at a flat-panel detector - has been developed for the equine head. This has significant technical limitations in standing live horses, due to inherent low-power x-ray output and the susceptibility of volumetric acquisitions to be plagued with artefacts. Nevertheless, some literature does suggest that a radiological diagnosis or the exclusion of structural changes was achievable in 97% of cases. One prospective comparison study in 11 cadaver heads using a 64-slice MDCT scanner and a CBCT unit identified dental abnormalities in 122/468 teeth (26.1%) and 105/468 (22.4%) respectively, with near-perfect overall agreement (κ = 0.90). Agreement was κ = 0.95 for clinical crown abnormalities and κ = 0.93 for infundibular abnormalities, potentially suggesting CBCT as an alternative to MDCT for dental assessment (in cadavers), with a strong caveat that CBCT image quality is inferior to MDCT for soft tissue structure assessment.

Computed tomography V radiography
Multiple studies have established the significantly greater sensitivity of CT compared with conventional radiography in equine dental disease assessment. In one study comparing CT and radiography in 32 horses with dental disease, CT demonstrated a sensitivity of 100% and specificity of 96% for the diagnosis of dental disease. Pulpar and apical / periapical changes, highly indicative of maxillary cheek tooth apical infection, were present in all 32 examined teeth on CT but in only 17/32 teeth (53%) radiographically; with gross pulpar or apical abnormalities and histological periapical changes confirmed in 31/32 (97%) of extracted teeth on pathological examination (Liuti et al.., Equine Vet J 2018;50:41-47). These findings have also been supported by earlier work.

CT features of an abnormal cheek tooth
The diseased equine tooth on CT can be systematically categorised according to the anatomical segment(s) affected.

Endodontic (Pulp) Changes
CT can detect more subtle pulpar changes than radiography, including pulp horn irregularities, increased pulp volume with a heterogeneous internal density, and - most pathognomonic of infection - gas-attenuation within the pulp horns / common pulp chambers. This may also extend to gas presence within the adjacent periapical periodontal tissues, where negative HU values are consistent with gas; attributed to by-products of bacteria such as anaerobes within a necrotic pulp. In a cadaveric study of 30 abnormal cheek teeth, intra-pulpar gas was detected by CT in 19/28 (67.9%) apically infected teeth, alveolar bone ‘sclerosis’ in 20/28 (71.4%) and apical tooth root clubbing in 20/28, whilst periapical ‘halo’ formation was identified in 4/28 cases. The source of pulp gas may be bacterial activity, or in the context of infundibular caries-related or traumatic crown fractures / defects, may represent ingress of air from the oral cavity or infundibular cavities (Liuti et al.., Front Vet Sci 2018;4:236).

Apical / Periapical Changes
CT features of apical infection include widening of the periodontal space, loss or irregularity of the lamina dura, thickening / remodelling of the alveolar bone surrounding the apices, with or without osteolysis, blunting of the tooth roots (clubbing), periapical gas and root fragmentation. In some cases, periapical cementum deposition is also identified. A study of 49 horses by Bühler et al.. (Equine Vet J 2014;46:468–473) established that combined CT changes of the pulp, root, lamina dura, periapical bone, and periodontal space, in conjunction with the presence of a dental fracture, were reliable features to diagnose apical infection. A non-detectable lamina dura as an isolated finding should be interpreted with caution, as it was also identified as a solitary change in 76% of tooth roots from horses without clinical signs of dental disease, possibly reflecting the inherently thin nature of the structure and resolution limitations. Periapical sclerosis, clubbing of one or two roots, the degree of apical clubbing, and periapical halo formation demonstrated high sensitivities for apical infection (73–90%), though with only moderate specificity (61–63%); multivariable analysis identified severity of periapical sclerosis and extensive periapical halo formation as CT features most strongly associated with periapical infection.

Infundibular Disease
Infundibular changes are present with a high frequency in horses on CT examination and can be seen often in asymptomatic horses. A normal infundibulum appears on CT as a hypodense, linear, central structure; infundibula may contain gas in cemental defects that must not be mistaken for an infected pulp (different anatomic location). CT features of infundibular caries include hypoattenuation of the cementum, destruction of the infundibular enamel, and filling of the infundibular cavity with gas; the most advanced stage is characterised by linear hypoattenuation with a bulbous configuration at the apical extent segment. CT allows more thorough evaluation of the extent of infundibular lesions than radiography, particularly when restorative treatment is being considered; only 10% of infundibulae were found to be entirely “normal” on CT. The term infundibular hypoplasia has been proposed given that caries is invariably associated with occlusal exposure of developmental cemental hypoplasia.

Dental Fractures
A periapical infection grading system using HU measurement of endodontic, apical, and periapical regions has been applied to sagittal cheek tooth fractures; in 81 teeth from 49 horses. Apical/periapical infection was identified in 100% of midline sagittal fractures, 73% of buccal fractures, and 96% of fractures involving the infundibulae, with midline sagittal fractures significantly associated with secondary sinusitis (OR 5.92; 95% CI 1.67–20.83; p = 0.006), demonstrating the value of pre-operative CT in fracture cases.

Periodontal Disease
CT changes in periodontal disease range from focal, early findings (widening of the hypodense periodontal space beyond 1 mm), and focal lamina dura defects - to extensive food and gas pocketing with destruction of alveolar bone. In more advanced disease, marked widening of the periodontal space and thickening of the overlying alveolar bone are seen, with pockets of periapical gas with / without extension of soft tissue - food material or gas attenuation into the paranasal sinuses. Large defects can also lead to presence of oronasal or oromaxillary sinus fistulae.

Developmental and Neoplastic Abnormalities
CT is of considerable value in characterising odontogenic tumours - which, although rare, appear more frequently in equids than in other species CBCT-guided surgery has been described for removal of ectopic teeth, enabling real-time intraoperative navigation and minimally invasive approaches (Klopfenstein Bregger et al.., ECVS Proceedings 2023).

Conclusions
CT - particularly standing MDCT and, potentially in the future, CBCT - has transformed the diagnostic evaluation of equine dental disease by enabling high-resolution, three-dimensional, quantitative characterisation of the tooth, periodontium, alveolar bone, and paranasal sinuses without structural superimposition. HU-based characterisation, and capacity to detect intrapulpar gas / subtle periapical change enable diagnostic accuracy substantially exceeding conventional radiography.

References
• Bühler M, Fürst AE, Lewis FI et al.. Computed tomographic features of apical infection of equine maxillary cheek teeth: a retrospective study of 49 horses. Equine Vet J 2014;46:468–473.
• Casey MB, Tremaine WH. Gross, computed tomographic and histological findings in mandibular cheek teeth extracted from horses with clinical signs of pulpitis due to apical infection. Equine Vet J 2010;42:30–36.
• Henninger W, Frame EM, Willmann M et al.. CT features of alveolitis and sinusitis in horses. Vet Radiol Ultrasound2003;44:269–276.
• Liuti T, Smith S, Dixon PM. A comparison of computed tomographic, radiographic, gross and histological, dental, and alveolar findings in 30 abnormal cheek teeth from equine cadavers. Front Vet Sci 2018;4:236.
• Liuti T, Smith S, Dixon PM. Radiographic, computed tomographic, gross pathological and histological findings with suspected apical infection in 32 equine maxillary cheek teeth (2012–2015). Equine Vet J 2018;50:41–47.
• Manso-Díaz G, García-López JM, Maranda L, Taeymans O. The role of head computed tomography in equine practice. Equine Vet Educ 2015;27:136–145.
• Rowley KJ, Townsend NB, Chang YMR, Fiske-Jackson AR. A computed tomographic study of endodontic and apical changes in 81 equine cheek teeth with sagittal fractures. Equine Vet J 2022;54:541–548.
• Stieger-Vanegas SM, Hanna AL. The role of computed tomography in imaging non-neurologic disorders of the head in equine patients. Front Vet Sci 2022;9:798216.
• Tietje S, Becker M, Böckenhoff G. Computed tomographic evaluation of head diseases in the horse: 15 cases. Equine Vet J 1996;28:98–105.
• Townsend NB, Hawkes CS, Rex R et al.. Investigation of the sensitivity and specificity of radiological signs for diagnosis of periapical infection of equine cheek teeth. Equine Vet J 2011;43:170–178.
• van Zadelhoff C, van der Vekens E, Ehrle A et al.. Multidetector CT and cone-beam CT have substantial agreement in detecting dental and sinus abnormalities in equine cadaver heads. Vet Radiol Ultrasound 2021;62:425–434.
• Windley Z, Weller R, Tremaine WH, Perkins JD. Two- and three-dimensional CT anatomy of the enamel, infundibulae and pulp of 126 equine cheek teeth. Part 2: findings in teeth with macroscopic occlusal or CT lesions. Equine Vet J2009;41:441–447.