MANDIBLE: INTRODUCTION AND GENERAL PRINCIPLES
General Examination Technique
The entire mandible must be imaged even, for instance, if the temporomandibular joint (TMJ) is the only stated clinical area of interest. Treatment planning will almost always have to take into account the occlusal plane as well as the integrity of the entire mandible and its relationship to the maxilla and skull base attachment at the coronoid fossa.
Mandibular computed tomography (CT), cone beam volume computed tomography (CBVCT), and magnetic resonance imaging (MRI) must include the axial and off-axial acquisition and/or viewing as a routine. Multidetector computed tomography (MDCT) and CBVCT have made it simple to obtain a volume data set that can be routinely reconstructed as well as viewed with multiplanar reformation (MPR) tools (Figs. 96.1 and 96.2).
Any study of the mandible and facial region must be viewed and/or filmed in both soft tissue and bone windows. Other extracranial pathology may present with mandibular dysfunction, and either mandibular or other extracranial disease may extend intracranially. The soft tissues, including the brain, are not adequately evaluated when viewed on images done with a bone algorithm. This is a major shortcoming of CBVCT (Fig. 96.1H) since its low-contrast resolution is inherently very poor, thereby rendering its use primarily for osseous and dental structures of the maxillofacial complex.
Standard MDCT data sets should be obtained with 0.50- to 0.75-mm sections. Contrast is used with CT depending on specific indications. The most common applications of CT for uncomplicated mandibular and facial studies are done without intravenous contrast unless central osseous disease is suspected of soft tissue invasion. Specific CT protocols are presented in Appendix A.
CBVCT data can be acquired at voxel resolutions of up to 80 mm for limited fields of view. Most CBVCT units offer isotropic voxel dimensions ranging from 125 to 400 mm. MPR tools can be used to reconstruct two-dimensional slices at desired thicknesses ranging from the lowest possible voxel dimension offered to any desired level (0.125 mm to >100 mm) (Fig. 96.1F,G). Acquisition times range from 10 to 40 seconds using a pulsed or continuous exposure. The radiation burden is significantly lower compared to conventional CT. Metallic artifacts are significantly reduced in the reconstructions.
MRI should be done with 3-mm sections and a field of view of 12 to 16 cm. All three orthogonal planes might be obtained, but axials and coronals usually suffice. T1- and T2-weighted images must be obtained, preferably in both planes. Fat suppression by frequency-selective techniques may be used but are predictably and severely degraded by susceptibility artifacts along the dental arches when unremovable dental hardware or restorations are present (Figs. 1.3, 1.4, and 3.1 and Chapters 1 and 3). Non–fat-suppressed T1- and T2-weighted images before and after contrast are a safer choice, although the examination becomes lengthened. Intravenous contrast is used in most cases. Magnetic resonance (MR) protocols are presented in Appendix B.
Pros and Cons
MDCT, CBVCT, and MRI are used for the advanced imaging of the mandible and facial region (Table 4.1). Plain films including panoramic and intraoral dental images remain a standard starting point for the study of the body of the mandible, dentoalveolar processes, and teeth. Advanced imaging studies are reserved for more complex clinical decision making. Kinematic studies are possible on 320 MDCT units.
MRI has essentially replaced arthrography for evaluating internal derangements of the TMJ. Kinematic studies of the articular disk are possible. It is used only adjunctively in other mandibular problems.
Ultrasound has essentially no role in primarily mandibular pathology; however, there have been some initial attempts at utilizing ultrasound for the TMJ complex with mixed results.
Angiography is used occasionally in both diagnosis and treatment settings. Catheter angiography remains necessary if angioarchitecture and temporal flow dynamics of a lesion are in question. If not, CT and/or MR angiography will usually suffice.
Radionuclide studies, particularly single photon emission computed tomography (SPECT) gallium and 99mTc bone scans, are useful in selected cases of infectious disease or facial/mandibular asymmetry such as temporomandibular condyle hyperplasia. Fluorine-18 2-fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET) is assuming some role in infectious diseases. Specific radionuclide technique depends on the indications; those are discussed in Chapter 5 and other specific relevant chapters.
Specific Areas of Interest
Developmental abnormalities are studied primarily with CT because bone detail is critical to diagnosis and three-dimensional (3D) planning or craniofacial reconstruction. However, soft tissue algorithms with less beam hardening are best for 3D stereolithic model productions. These studies require CT to be done with as good a volumetric data set as possible at reasonable dose limits with little to no patient movement. Additionally, CBVCT data that are now being utilized in dentistry, including oral and maxillofacial surgery, orthodontics, periodontics, and oral and maxillofacial radiology, may now be utilized in specialized stand-alone software specific for their individual specialty needs.
MR may be used to look for associated brain anomalies or for concerns of intracranial extension or associations of congenital anomalies, especially when these are syndromic.
The evaluation of infectious and noninfectious inflammatory mandibular disease still often begins with plain films. Some patients with persistent, and especially worsening, symptoms after a course of conservative management requiring imaging will get a CT. Means of complying with reducing radiation exposure, such as CBVCT data volumes, should be considered in pediatric screening studies and multiple recurring CT examinations that may be required for certain postoperative follow-ups. Such indications might include maxillomandibular reconstructions, dental implants, and ongoing orthodontic therapy. CBVCT doses typically are ≤50% in comparison with MDCT studies, a factor producing increasing interest in CBVCT’s applications. Starburst artifacts are minimal and not as disruptive as an additional benefit (Fig. 96.1F,G).
The techniques used in the evaluation of patients who are potential candidates for major craniofacial reconstruction, possibly with allograft planning, require a high-resolution volume acquisition (Fig. 96.1I). Such a study must be done in compliance with equipment used for image-guided surgery if serving surgeons using such guidance systems. Such protocols are outlined in Appendix A.
Complex inflammatory cases posing a risk for associated abscess should be studied with standard MDCT during infusion of intravenous contrast (Appendix A).
Patients with benign and malignant tumors often present with signs and symptoms identical to those of dental inflammatory disease; therefore, these patients usually have had at least dental-type plain films, non–contrast-enhanced CT, or CBVCT by the time a tumor is confirmed by biopsy. A supplemental MRI may be useful. The MR in these cases is usually focused on clarifying specific issues that may critically alter patient management, such as the extent of extramandibular involvement and presence and extent of perineural spread. MRI or CT may be most appropriate for follow-up of treated cancers; however, with MRI, early bone invasion as a sign of recurrence or persistence may go unnoticed.
If MRI, CT, or the clinical evaluation suggests a vasoformative lesion or highly vascular tumor, angiography may be done to confirm this impression. In hypervascular lesions, embolotherapy may be used as a definitive treatment or adjunct to surgical removal. This possibility must be resolved before any dental procedure is initiated.
CT has replaced plain films for evaluating patients at risk or with complex injuries. These CT studies are done with volume acquisition and MPR viewing.