Last month’s column described a case in which diagnosis and treatment was drastically changed by use of computed tomography (CT). CT deserves more discussion because it has revolutionized our ability to detect subtle hard- and soft-tissue abnormalities of the maxillofacial region. Why CT rather than magnetic resonance imaging (MRI) when making a diagnosis of maxillofacial disease? There are occasions where I might choose MRI over CT, but that is the exception rather than the rule. It all depends on whether we are more interested in soft tissue or bone. MRI provides amazing detail of soft tissue, but not as much information about bone. When something is abnormal in the maxillofacial region, we often are interested in both soft-tissue structures and the bone. CT gives us detailed information about the bone surrounding dental structures (alveolar bone) and the bones that comprise the temporomandibular joint (TMJ). Soft-tissue detail seen with CT might not be as impressive as MRI, but the use of an IV contrast agent can provide further information about the soft tissues during a CT scan. CT for Tumor Evaluation Recently, a 10-year-old Labrador retriever was presented to me with a 2-centimeter mass arising just caudal to the right maxillary second molar. The patient seemed to be a good surgical candidate until we did a CT scan and saw that the visible portion of the mass was just a tip of the iceberg. Conversely, CT sometimes helps us realize that curative-intent surgery is possible when we initially thought it to be unlikely. Figure 1 shows an example of this: a mastiff presenting with right-sided exophthalmos and the inability to open its mouth wider than 4 centimeters. The CT scan showed what was eventually diagnosed as a multilobular osteochondrosarcoma arising from the right ramus. Utilizing the 3-D reconstruction software, a zygomectomy, ventral orbitectomy and caudal mandibulectomy allowed us to obtain clean margins. CT also is helpful in maxillofacial oncology for assessing the deeper lymph nodes of the neck that are not readily palpable. Though often referred to as the mandibular lymph node, multiple mandibular lymph nodes usually are present bilaterally and are readily palpable within the bifurcation of the lingual and facial veins, just rostral to the mandibular salivary gland. Other lymphocentrums of the head include the retropharyngeal and parotid lymphocentrums. Cross sections of the largest node of the head and neck—the medial retropharyngeal node can be seen on the axial slices of the CT scan. If indicated, aspiration of the medial retropharyngeal may be done via CT guidance or ultrasound guidance after CT. Paying attention to the status of the medial retropharyngeal lymph node may be important since other nodes drain to the retropharyngeal node,1 and a focus on the mandibular lymph node alone may underdiagnose metastasis. One study found that only 54.5 percent of oral tumor cases with metastasis to regional lymph nodes had metastasis that included the mandibular lymph node.2 CT is helpful in preoperative screening of distant metastatic disease in patients with maxillofacial tumors. A study compared the effectiveness of thoracic CT versus three-view thoracic radiographs in the ability to diagnose lung metastases. This study found that CT was more sensitive than three-view chest radiography for detection of pulmonary nodules. Of the dogs that had lung metastasis seen on CT, only 17 of 21—81 percent—had pulmonary nodules detected on radiographs.3 Another Way: CBCT What about cone-beam CT scanners (CBCT) in dentistry and oral surgery? What is the difference between CBCT and CT? Rather than a collection of thin, closely spaced slices obtained with a fan-shaped beam of the conventional CT, CBCT has an X-ray generator and detector that rotates around the patient, capturing data with a cone-shaped beam rather than a thin, fan-shaped slice. Both modalities can be used to create very helpful 3-D reconstructions of the areas of interest, but can CBCT scanners provide increased detail of dentoalveolar structures? One study compared CBCT’s diagnostic image quality with that of 64-multidetector row CT sagittal slices. Four blinded evaluators compared CBCT and 64-multidetector row CT images. Image quality was scored as it related to the anatomy of dentoalveolar structures. Trabecular bone, enamel, dentin, pulp cavity, periodontal ligament space, lamina dura and “overall impression” were scored. Images acquired with CBCT were found to be significantly superior in image quality when compared to images acquired with 64-multidetector CT in all categories.4 CT is an important modality, be it conventional CT, CBCT or other permutations that are sure to follow in upcoming decades. Even if your practice does not offer this technology, some hospitals can provide outpatient imaging for your patients in addition to full referral for both workup and treatment of challenging cases. We are very lucky to have this diagnostic modality to allow us to rule in or out the variety of maxillofacial diseases in our patients. References Belz GT, Heath TJ. “Lymph Pathways of the Medial Retropharyngeal Lymph Node in Dogs.” J Anat. 1995; 186(3):517-526. Herring ES, Smith MM, Robertson JL. “Lymph Node Staging of Oral and Maxillofacial Neoplasms in 31 Dogs and Cats.” J Vet Dent. 2002; 19(3):122-126. Armbrust LJ, Biller DS, Bamford A, Chun R, Garrett LD, Sanderson MW. “Comparison of Three-View Thoracic Radiography and Computed Tomography for Detection of Pulmonary Nodules in Dogs with Neoplasia.” J Am Vet Med Assoc. 2012; 240(9):1088-1094. Soukup JW, Drees R, Koenig LJ, Snyder CJ, Hetzel S, Miles CR, Schwarz T. “Comparison of the Diagnostic Image Quality of the Canine Maxillary Dentoalveolar Structures Obtained by Cone Beam Computed Tomography and 64-Multidetector Row Computed Tomography.” J Vet Dent. 2015; 32(2):80-6. Dr. John Lewis practices veterinary dentistry and oral surgery at NorthStar Vets in Robbinsville, N.J. Originally published in the March 2017 issue of Veterinary Practice News. Did you enjoy this article? Then subscribe today!
