Imaging II: Computed Tomography

Take Home Message— When oral examination and radiographs give little or questionable evidence to support a diagnosis, computed tomography (CT) is by far the best advanced imaging modality to reveal dental and craniofacial pathology.

I. Introduction

Diagnostic imaging is a critical tool for complete evaluation of oral health and disease in the horse. Even the most advanced oral exam using endoscopic techniques will not be able to determine the health of adjacent hard and soft tissue structures and the reserve crown and root located within the alveolus. Some equine teeth that appear normal on examination of the clinical crown can be the cause of regional sinusitis, impressive apical pathology, and fistula formation. Diagnostic imaging can help identify the tooth/teeth responsible for current clinical signs as well as provide additional information regarding the health of otherwise clinically normal teeth. Many times incidental pathology can be identified during imaging studies allowing the veterinarian to diagnose the current problem and to warn the owner of developing pathology.

Radiology is widely used, and the combination of view variety (extraoral and intraoral) and improved imaging systems has led to a renaissance in the practitioner’s ability to diagnose pathology with this modality. With the appropriate tools, adequate views, and experienced eyes, the majority of dental cases can be diagnosed with a complete oral examination and dental radiography. When radiography is not enough, advanced modalities can be utilized to provide additional information. CT has proven to be the best next step in advanced imaging to identify maxillofacial pathology as it provides accurate images in cross sectional and multiplanar views. Both soft and hard tissue algorithms with or without contrast provide ample information. Three-dimensional reconstructions are particularly useful for maxillary and mandibular fracture repair. CT is utilized mainly for equine dental and sinus pathology when examination and radiographs give little or questionable evidence to support a diagnosis and when pathology margins need to be determined for treatment planning. In addition, in mouths where multiple dental lesions exist and it is not clear which diseased tooth is most likely the cause of clinical signs, CT can help identify the pathologic tooth responsible and provide the clinician with the information necessary to create a long-term, staged treatment plan to address oral pathology. The main downside to CT in North America is that to the author’s knowledge there is no standing CT available; therefore, all horses must undergo general anesthesia. If the potential gain from CT imaging outweighs the risk of general anesthesia, this advanced imaging modality is by far the current gold standard for advanced dental imaging.

II. Discussion

Computed tomography in the horse is currently the best advanced imaging modality for dental pathology. In the third edition of Equine Dentistry, Drs. Simhofer and Boehler have written a comprehensive review that details the technical principles of CT and the appearance of normal and abnormal dental tissue.¹ Please refer to this review for detailed information and images. In the author’s experience, the vast majority of dental cases can be diagnosed with a thorough oral exam and high quality dental radiographs.² When exam and radiographs do not leave the practitioner confident about a diagnosis, it is time for CT. CT provides excellent images of hard tissue and diagnostic images of soft tissue. The use of contrast, different algorithms, multiplanar views and three-dimensional reconstruction all work together to provide the practitioner with a diagnosis and confidence in treatment planning. Usually, pathology resulting in distinct clinical signs will be clearly evident on CT.³ Newer machines are producing images with such detail that the veterinarian and radiologist have to be cautious of over interpreting minor abnormalities and focus on major pathology. Very little literature has been published describing the newer high-detail, multiplanar images, and some questions still remain about normal versus abnormal findings in some cases. The minor findings become more of an issue in cases with vague clinical presentations (e.g. discomfort with biting, plays with mouth, shakes head, carries head inappropriately) and a normal oral examination.

CT delivers the images necessary to identify the exact locations of pathology and the tooth/teeth associated with it. This can be particularly important in chronic sinusitis cases when radiographs may be inconclusive of tooth involvement. After years of evaluating CTs in only the cross sectional plane, the author has become particularly fond of multiplanar views. Cross sections are two dimensionally limited and three-dimensional reconstructions can be limited depending on the facility, technical acquisition of images, equipment limitations, and the radiologist processing the images. Multiplanar views have opened up a new world of three dimensional imaging and treatment planning for the surgeon. Borders of cysts, masses, thickened tissue, reactive bone, fracture placement, periodontal disease, diastema formation, infundibular carious lesions, apical pathology, and tooth fracture can all be triangulated with this capability. This enhances the veterinarian’s ability to diagnose abnormalities and plan for optimal treatments.

Most equine dental CT images are acquired first in a bone window with slice thickness ranging from 4mm to 10mm. This spacing results in roughly 3-4 cross-sections per tooth which is adequate for diagnosing major pathology, but sometimes will completely miss smaller lesions. Slice thickness ranging from 1 to 2mm provides more information per tooth (5-7 slices/tooth) and should be used to examine teeth in detail. To emphasize the importance of smaller slice thickness and multiplanar views imagine a 3-mm wide periodontal defect in a diastema, a 1-mm congenital malformation along the full length of the crown, or a 2-mm diameter oronasal fistula. All can cause significant clinical signs and yet be invisible or near invisible on a routine 4 mm slice thickness cross sectional view, but these defects can be easily identified on coronal and sagittal multiplanar views. Needless to say, the temporomandibular joint should always be evaluated with decreased slice thickness and multiplanar views.

Since words can never provide as much information as pictures when reviewing CT images, the majority of the presentation will focus on normal and abnormal findings and case presentations. Please see the references below for further images and reading.^4-6

References

1. Simhofer H, Boehler A. Computed Tomography. In: Easley J. Dixon P, Schumacher J ed. Equine dentistry 3^rd ed. London: Saunders Elsevier, 2011;221-229.
2. Barakzai S. Dental Imaging. In: Easley J. Dixon P, Schumacher J ed. Equine dentistry 3^rd ed. London: Saunders Elsevier, 2011;199-221.
3. Tucker RL, Farrell E. Computed tomography and magnetic resonance imaging of the equine head. Vet Clin North Am Equine Pract. 2001;17(1):131-44.
4. Windley Z, Weller R, Tremaine WH, Perkins JD. Two- and three-dimensional computed tomographic anatomy of enamel, infundibulae and pulp of 126 equine cheek teeth. Part 1: Findings in teeth without macroscopic occlusal or computed tomographic lesions. Equine Vet J. 2009;41(5):433-40.
5. Windley Z, Weller R, Tremaine WH, Perkins JD. Two- and three-dimensional computed tomographic anatomy of enamel, infundibulae and pulp of 126 equine cheek teeth. Part 2: Findings in teeth with macroscopic occlusal or computed tomographic lesions. Equine Vet J. 2009;41(5):441-7.
6. Morrow KL, Park, Arceneaux B, et. al. Computed tomographic imaging of the equine head. Vet Radiol Ultrasound. 2000;41(6):491-7.