Advanced Medical Imaging for the Modern Dentist

Written by on January 30, 2015 in Insight - No comments

XrayWEBWhen the routine office dental X-rays show an unexpected or puzzling finding or do not provide a ready answer for a patient’s symptoms, today’s dentist has an array of medical imaging modalities at his/her disposal for diagnosis. Radiologists can facilitate detailed evaluation of dental related pathology with cross-sectional imaging.

Anatomically, the teeth are rooted in the maxillary and mandibular facial bones. The curved shape of the maxilla and mandible are challenging for routine X-rays because overlapping structures result in difficult visualization and inconsistent reproducibility. By flattening out the curvature of the mandible, the Panorex (panoramic) radiograph is the best X-ray view of the mandible to demonstrate fractures, bone lesions, temporomandibular joint (TMJ) dislocation and dentition.

Cross-sectional imaging modalities of computed tomography (CT) and magnetic resonance imaging (MRI) have significantly advanced dental imaging because of superior contrast resolution and unlimited capability for multiplanar reconstructions. There are distinct differences and advantages between the two modalities. CT has superior contrast resolution for bone and calcium and MRI for soft tissues. Helical or multidetector CT (MDCT) rapidly acquires an entire imaging data volume by multiple detectors spiraling around the body and generates highly detailed reconstructions in any plane. CT data acquisition is typically completed in 30 seconds or less within a breath hold and therefore, MDCT is suitable for the majority of patients including uncooperative patients and pediatric patients. Ionizing radiation in MDCT and magnetically induced spinning hydrogen protons in MRI constitute the physical basis for imaging. Therefore, radiation exposure is a concern for MDCT and safety of ferromagnetic devices, implants or foreign objects within a patient is a concern and possibly a contraindication for MRI.

Dental abnormalities are often incidentally demonstrated on MDCT of facial bones, paranasal sinuses and neck. Dental caries are seen as small round holes in the tooth enamel. Untreated periodontal disease progressing to periapical abscess appear as a small lucency at the tooth root. Supernumerary and impacted teeth and retained roots from tooth extraction are easily demonstrated.

Odontogenic infection is the most common cause of upper neck infection. Abscess in the cheek and under the tongue or mandible manifests as a fluid collection with a defined wall. Ludwig angina is cellulitis at the floor of the mouth causing airway narrowing and constitutes an emergency because of the risk of asphyxiation. Mandibular osteomyelitis (bone infection) may result from untreated infections and complicated or unrecognized fractures. Jaw osteonecrosis (cellular bone death) is a complication of chronic osteomyelitis related to odontogenic infection, radiation therapy for head and neck cancer and more recently, IV bisphosphonate therapy for bone metastases. Both osteomyelitis and osteonecrosis are demonstrated as lytic (decreased bone density) destructive areas on MDCT.

Endodontic dental implants have become widespread by significantly improving chewing function and facial aesthetics. These screw-type devices anchor teeth replacement appliances in the edentulous portions of the jaw.  Bone absorption is a sequela of tooth loss that may preclude proper implant placement and increase complications. MDCT is essential for dental implant pre-surgical planning. Specialized reconstruction software generates curved coronal (panoramic) and cross-sectional planes for tooth mapping, bone mineralization and measurements and location of adjacent anatomic structures such as the maxillary sinus floor and mandibular nerve canal.

TMJ’s are hinge and glide joints that open and close the jaw by articulation between the mandibular condyle and glenoid fossa of the temporal bones. The bowtie shaped articular disc sitting on top of the mandibular condyle cushions the condyle as it moves forward with the condyle during jaw opening. TMJ internal derangement is a common mechanical disorder in the general population.  Internal derangement is defined as abnormal position of the articular disc relative to the condyle and has a variety of causes including trauma, malocclusion, bruxism, stress and bone abnormalities. MRI is the only imaging modality that directly visualizes the disc and is therefore the reference standard for internal derangement. The disc shape and its position relative to the condyle is evaluated in the closed and open mouth positions. If the disc is displaced in the closed mouth position, it may or may not relocate (“recapture”) in the open mouth position. An audible click with or without jaw pain typically accompanies disc recapture in open mouth position. Non-recapture of the disc in the open mouth position signifies a more advanced stage with limited range of motion and no audible click. A stuck disc stays in a static position on closed and open mouth because of adhesions. Less common etiologies for TMJ disorders include inflammatory arthritis, osteoarthritis, crystalline deposition, trauma, and condylar anomalies.

Motor vehicle accidents and assaults are the most common causes of mandibular fractures. Because of muscle retraction, fracture displacement is usually medial or inward. Condylar fractures account for up to half of all mandibular fractures. TMJ dislocation may be traumatic or nontraumatic. Various nontraumatic causes include yawning, eating, dental treatment and oral intubation. MDCT multiplanar and 3D reconstructions are invaluable for fracture reduction and facial reconstruction planning.

In summary, the cross sectional modalities of MDCT and MRI greatly expand the demonstration of dental related anatomy and pathology.  The acquisition and expert interpretations of these studies by radiologists are instrumental in the service of modern dentists and their patients. Radiologists are key team players in maximizing medical imaging for improving patients’ dental health.

REFERENCES
Steinklein J, Nguyen V. Dental anatomy and pathology encountered on routine CT of the head and neck. American Journal of Radiology 2013; 201:843-853. doi:10.2214/AJR.12.9616.

Petscavage-Thomas J, Walker E. Unlocking the jaw: advanced imaging of the temporomandibular joint. American Journal of Radiology 2014; 203:1047-1058. doi:10.2214/AJR.13.12177.

Kaplan P, Helms C. Current status of temporomandibular joint imaging for the diagnosis of internal derangements. American Journal of Radiology 1989; 152:697-705.

Lomasney L, Steinberg M. Computerized imaging before patients undergo dental implantation. American Journal of Radiology 1999; 172:1439-1446.

By Marina Liem MD
Marina Liem MD is a board-certified diagnostic radiologist with a subspecialty in Neuroradiology. She has been practicing clinical radiology in community hospitals and outpatient centers in Florida, Pennsylvania and New Jersey and is also currently a freelance medical writer.

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