Radiological Evaluation of the Craniofacial Skeleton

Radiography

Radiographs of the skull at various angles allow visualization of the major osseous structures of the face and mandible. Plain radiographs are named based on the projection of the X-ray beam. The most common radiographs of the face in the trauma setting include: standard occipitomental (30 degrees OM or Waters view), posteroanterior (PA skull or Towne view), reverse Towne's and the true lateral skull ( Fig. 1.2.1 ). The submentovertex view, which requires hyperextension of the neck, is not obtained given the risk of a concomitant cervical spine injury seen in 2.2% of patients with maxillofacial fractures. Each projection of the face best demonstrates certain osseous features of the craniofacial skeleton ( Table 1.2.1 ). However, overlapping osseous structures limit evaluation of the entire craniofacial skeleton, especially the midface, sinuses, and skull base. Moreover, the patient must be able to cooperate with positioning to obtain the various projections. This is impossible with patients suffering from traumatic brain injuries associated with craniofacial fractures.

Panoramic Radiography

The panoramic radiograph (orthopantomogram or panorex) displays the entire maxillomandibular region on a single image ( Fig. 1.2.2A ). The term panoramic radiography is derived from panorama, “an unobstructed view of a region in every direction.” There are two different ways of obtaining panoramic views, which are defined by the location of the radiation source: intraoral or extraoral.

The intraoral technique requires a small X-ray tube be placed in the patient's mouth where radiation is directed through the jaws to expose a film placed just outside of the targeted facial anatomy. The first patient undergoing this procedure was reported by a German inventor, Horst Berger. The extraoral technique, which is the most popular method in use today, employs an extraoral source of radiation that rotates around the patient. To create a panoramic view, objects in front or behind the focal trough are blurred, leaving a sharp image of the entire mandible in one 2-dimensional plane facilitating the diagnosis of mandible fractures and dentoalveolar trauma ( Fig. 1.2.2B ).

Panoramic radiographs are fast and convenient with minimal training needed for the operating technician. Moreover, panoramic radiography provides the lowest radiation dose with broad coverage of maxillomandibular bones and teeth. This imaging modality is most advantageous for stable patients who cannot tolerate intraoral procedures or examination. The limitations of the radiographs include unequal magnification across the image, making linear measurements unreliable, as well as difficulty imaging both sides of the mandible when the patient has severe maxillomandibular discrepancies causing malocclusion. The tomographic technique of the exam also creates artifacts, including double images of the hyoid bone, cervical spine, and epiglottis. Ghost artifacts, which are higher than the true anatomic location on the opposite side of the image, are created by the upward inclination of the X-ray beam. These artifacts, in addition to the superimposition of overlying structures, limit the complete evaluation of the maxillomandibular structures, especially the mandibular condyle and coronoid process which overlap the mastoid prominence and midface, respectively.

Computed Tomography (CT)

In computed tomography, an X-ray tube rotates around the patient emitting a fan-shaped beam of X-rays that are detected by a row of detectors on the opposite side. Initially, the X-ray tube and detector rotated synchronously around the patient for one cycle then the table would move a small increment (i.e., 5 mm) for the next scan or slice image. This was known as the “step and shoot” method or so-called incremental scanners. In 1989, CT scanners acquired image data in a helical fashion where the X-ray tube and detectors continuously revolve around the patient while the table advances the patient through the gantry. This overlapping image data results in higher spatial resolution, improved multiplanar reconstructions, faster scanning and reduction in radiation dose. Multidetector CT scanners were introduced in 1998 beginning with a 4-detector row CT scanner and progressing to the current state-of-the art CT scanners with up to 320-detector rows allowing for multiple slices to be captured quickly and simultaneously, reducing exposure time and motion artifact. This also results in improved resolution and quality of the axial, reformatted, and 3-dimensional images of the face. Current technology allows data obtained via CT to be transmitted to the operating room for computer-assisted surgery, including presurgical planning and intraoperative navigation during the repair of craniomaxillofacial injuries.

In comparison to plain radiographs of the face, CT eliminates the superimposition of structures and provides high-resolution contrast of the osseous structures and soft tissues that can be viewed simultaneously in three orthogonal planes as well as 3D ( Figs. 1.2.3–1.2.7 ). This allows for complete evaluation of the face, skull base, cranium, and brain. In a large study, it was reported that the most common fractures were to the midface (71.5%) with supraorbital and frontobasal skull fractures present in 4.2% of the cases. CT is superior to plain radiographs when assessing the mandible, especially nondisplaced symphyseal fractures, given the overlapping spine on the AP Towne's view. CT has a higher accuracy, sensitivity, and specificity (90%, 90%, 87%, respectively) for condylar fractures when compared to panoramic radiographs (73%, 70%, 77%, respectively). CT has also been shown to detect additional fractures not seen on plain or panoramic radiographs, often leading to a change in operative management. In the setting of penetrating trauma, multidetector CT is the best imaging modality to assess the trajectory of the missile and injury to the adjacent soft tissues and vascular structures ( Fig. 1.2.8 ).