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Multiplanar imaging-reformatting (MPR) has significantly increased the diagnostic accuracy and efficiency of the knowledgeable dental professional.
Reviewing the dental and maxillo-facial structures in all perspectives may reveal hidden aspects of relevant disease and may enhance diagnosis.
The novelty of the diagnostic tool (CBCT) and the unfamiliarity of the generated sectional images make knowledge of the anatomy mandatory.
Major anatomical structures, commonly seen in CBCT routine scans are reviewed as well as related pathology, including the para-nasal sinuses, neck and cervical spine, skull base and more.
Multiplanar imaging has offered an unparalleled diagnostic approach when dealing with an unknown entity (pathologic or not) that has stood as a diagnostic challenge. This concept is inherent to volumetric type of data (computed tomography [CT], cone beam computed tomography [CBCT], magnetic resonance imaging) and has offered the diagnostician the unique ability to generate images (sections) at different planes (flat or curved). Because a volume of data has been acquired and stored by CBCT, this data can be reformatted or realigned and several different types of images can be synthesized in any way the diagnostician requires,
thus eliminating the superimposition of the area or entity under investigation with other neighboring structures and allowing its assessment from all perspectives. With multiplanar imaging, the diagnostician/operator can re-create images in different planes (flat or curved) with very simple functions, increasing the diagnostic efficiency in the hands of the knowledgeable individual in an unparalleled way (Fig. 1).
Undoubtedly, multiplanar imaging, as provided by cone-beam imaging, is a novelty for the dental professional: most dentists and specialists, with only a few exceptions, are not familiar with diagnostic imaging in different planes, although they are keen on interpreting projectional images as those produced by traditional dental imaging modalities (intraoral radiography and panoramic radiography). Sectional images (tomographic images) reveal the spatial relationship of the various known anatomic structures in the maxillofacial region, which was more or less lost in projectional imaging.
In this article the appearance of several anatomic structures of the maxillofacial region as well as the head and neck region in general is reviewed; these structures are analyzed in all 3 basic tomographic planes (axial, coronal, sagittal). Additional reconstructed images may be used to view certain anatomic areas from all aspects. To review the anatomy of the maxillofacial region in a systematic way, the maxillofacial region is divided into smaller areas of interest. Emphasis in this topographic anatomic review is placed on areas that may demonstrate a higher incidence of occult disease. Emphasis is also on structures outside the dentoalveolar region. The anatomy of dental and dentoalveolar structures has been thoroughly reviewed elsewhere.
The paranasal sinuses are 4 pairs of air-filled osseous cavities that surround the nose and the orbits and that belong to the maxillary (maxillary sinuses), ethmoid (ethmoid air cells), frontal (frontal sinuses), and sphenoid (sphenoid sinuses) bones, respectively.
Paranasal sinuses are best assessed in coronal sections; in fact, coronal images are the most appropriate for the evaluation of anatomic structures that have a posteroanterior orientation. The maxillary, ethmoid, and sphenoid sinuses as well as the nasal cavity and certain structures in the skull base will be optimally imaged in these views. In this review, the anatomic structures of interest ventrally to dorsally (or anterior to posterior) are reviewed (see Fig. 21, Fig. 22, Fig. 23, Fig. 24, Fig. 25, Fig. 26).
At the level of maxillary premolars, the coronal images section through the frontal sinuses, the orbits, the anterior aspect of the maxillary sinuses, the ethmoid air cells, and the nasal cavity. The various anatomic structures of interest are addressed by means of importance to the dental professional (anatomic proximity to dental structures).
The nasal cavity is seen as a pyramidal-shaped air cavity that is divided in 2 distinct, fairly symmetric, noncommunicating air cavities by the nasal septum. Each one of them is separated further into smaller, blind (open) chambers by 3 elongated or arch-shaped osseous projections that originate from its lateral walls: these are the inferior, middle, and superior nasal conchae or turbinates, which border the inferior, middle, and superior nasal meatuses (chambers) (Fig. 2). Only the inferior concha is an independent facial bone; the rest are parts of the ethmoid bone. Although they are lined by 2 to 3 mm of mucosa, there are identifiable air passageways that guide the inhaled air to the paranasal sinuses. Shape alterations of the nasal chambers and the septum may affect the flow of air through the nose and may be associated with upper airway obstructive phenomena (ie, sleep apnea). In fact, a deviated nasal septum is a common cause of sleep apnea (see Fig. 2). The nasal turbinates may sometimes be pneumatized; in this case, instead of a dense bony process, the nasal turbinate is presented as an extension of the ethmoid air cells, filled with air, and surrounded by a thin corticated border. This phenomenon is considered an anatomic variant, and the pneumatized turbinate is best known as “concha bullosa,” and is more frequently seen in the middle nasal turbinates; its incidence ranges between 15% and 45%. Conchae bullosa may be inflamed because they are communicating with the ethmoid air cells; however, their presence does not seem to affect the pathogenesis of sinus inflammation (sinusitis).
Sometimes, conchae bullosa may contribute to upper airway obstruction because they may obliterate the air passageways in the nasal cavity (see Fig. 2).
Another prominent osseous canal is identified in the coronal sections through the anterior third of the nasal cavity: the nasolacrimal duct, which originates at the floor/medial wall of the orbit, opens into the inferior nasal meatus (Fig. 3), and drains tears form the orbit into the inferior meatus.
The maxillary sinuses are the largest among the various paranasal sinuses (Fig. 4, Fig. 5, Fig. 6). These air cavities belong to the maxillary bone. As all air cavities will be displayed as uniform dark or black because air is depicted as a very low density structure in computed tomography (CT or CBCT). The presence of any other appearance than black may represent pathologic abnormality in the air cavity. Almost pyramidal in shape, with the base of the pyramid being the medial wall or the wall that is shared with the nasal cavity and the tip of the pyramid being the zygomatic process of the maxilla (anterior end of the zygomatic arch). The other sides of the pyramid are the superior wall (roof of the maxillary sinus), which is shared with the orbit, the lateral wall, the anterior wall, and the posterior wall. CBCT images may provide a detailed evaluation of the integrity of the walls of the maxillary sinuses as well as the presence of disease in the air cavities. The posterior superior alveolar neurovascular canals may be sometimes be seen on the lateral wall of the maxillary sinus as a small, pinhead-size, low-density areas (in coronal images) running almost parallel to the floor of the sinus and turning cephalad (superior) in the premolar region (in panoramic reconstructions). Its diameter, as with all vascular canals, may provide information as far as it concerns its bleeding potential if injured during sinus grafting procedures (see Fig. 4).
The draining sites of the maxillary sinuses (maxillary sinuses' ostia) are likely to be visualized in coronal images toward the anterior third of the sinus cavities (from front to back) and may not be both identifiable in the same coronal plane. The ostium of the maxillary sinus is a small opening in the medial wall of the maxillary sinus (or lateral wall of the nasal cavity) toward the superior aspect, leading into the ethmoid infundibulum, a narrow passageway that opens into the middle nasal meatus; it is formed partially by the ethmoid bone (ethmoid bulla-superior) and a thin pointy osseous process on the lateral wall of the nasal cavity known as the uncinate process. The maxillary sinus and the anterior ethmoid air cells drain into the middle nasal meatus through the infundibulum.
The maxillary sinus ostium and the infundibulum are parts of the ostiomeatal complex, a broader anatomic unit that serves as the draining site of the maxillary, anterior ethmoid, and frontal sinuses. Slightly higher, in the same sections, the draining path of the frontal sinus is identified; this is known as the frontal recess. The narrow arch-shaped passageway between the ethmoid bulla and the middle turbinate just superior to the infundibulum is the hiatus semilunaris, named so because of its curved, almost semilunar shape, in the sagittal views (see Fig. 5). This hiatus semilunaris connects the ethmoid infundibulum to the frontal recess. The semilunar hiatus is the final segment of the drainage pathway from the maxillary sinus and ethmoidal infundibulum to the middle meatus.
The narrow and delicate nature of the above-mentioned draining sites makes them vulnerable to possible blockage when inflammation occurs in their vicinity. Moreover, their close proximity to each other renders them possible paths for spread of infection. Identification of the draining sites and assessment of their integrity is important in patients who will undergo maxillary sinus grafting procedures. Blockage of the draining site may prevent the aeration of the sinus cavity and result in accumulation of inflammatory products into the sinus (see Fig. 6). This fine and delicate anatomy of the ostiomeatal complex may be grossly altered if sinus surgery has occurred (Fig. 7).
The ethmoid air cells or sinuses are numerous, small, mostly square, air cavities that are separated by thin bony walls, grouped in 2 orthogonal prisms located on either side of the superior nasal cavities, and run parallel to the nasal cavities through their entire length (from front to back). They are bordered from the orbits with the lamina papyracea, a paper-thin osseous wall, and from the nasal cavities with the superior and middle nasal turbinates (Fig. 8). The anterior and middle ethmoid air cells drain into the middle nasal meatus, whereas the posterior ethmoid air cells drain into the superior nasal meatus (Fig. 9).
The sphenoid sinuses are the posterior-most air cavities and belong to the sphenoid bone. Their shape is similar to that of a truncated pyramid with its base being the bony roof of the nasopharynx; its roof, the sella turcica (pituitary fossa), and its lateral walls border the cavernus sinuses on either side of the body of the sphenoid bone. The sphenoid sinuses drain to the superior nasal meatus through a small opening in their anterior wall, the spheno-ethmoidal recess (Fig. 10).
Important anatomic entities, such as the optic canal, the foramen rotundum, and the vidian canal, are closely related to the sphenoid sinuses and will be addressed in later discussion in the anatomic review of the skull base (see Figs. 25 and 26).
The frontal sinuses are 2 funnel-shaped air cavities identified superior to the ethmoid air cells and the nasal cavities and belong to the frontal bone. They demonstrate a great deal of variation in shape and size. A septum, which is frequently deviated, separates the right from the left and asymmetry between the 2 is not uncommon. They drain into the middle nasal meatus through the frontal recess, a thin passageway, part of the ostio-meatal complex (discussed earlier).
The orbits are visualized toward the anterior third of the face as well as the ethmoid air cells, the most anterior part of which is sectioned at this level. These views are excellent for the assessment of the integrity of the osseous walls of the orbits and their borders. A short osseous canal seen originating from the floor of the orbit and directed inferomedially to the anterior wall of the maxillary sinus is the infraorbital foramen, which accommodates the infraorbital nerve (Fig. 11). Lamina papyracea, a paper-thin osseous diaphragm that belongs to the ethmoid bone and serves as the medial orbital wall, stands as the boundary between the orbit and the ethmoid air cells (see Figs. 6A and 8B).
Unfortunately, the soft tissue contrast of CBCT is inadequate for the assessment of the orbital content (eye globe, fat, and musculature of the eye globe).
Inflammation is by far the most common pathologic entity among the ones affecting the paranasal sinuses. Moreover, it seems that it is a frequent occult pathologic entity in CBCT scans prescribed for other diagnostic concerns.
A considerable portion of the patient's neck and cervical spine may be included in an extended field-of-view scan of the maxilla and mandible or of the mandible toward the inferior end of the scan. Axial images are best for the evaluation of the visible parts of the neck and cervical spine as well as the skull base, although a combination of reconstructed images in all 3 planes (axial, coronal, and sagittal) may be used to assess a region of interest fully.
Most times, the portion of the neck that is visualized in a CBCT includes the suprahyoid neck (above the hyoid bone). A few osseous structures and mostly soft tissue structures are present at that level (Fig. 12). The inferior border of the anterior mandible may be sectioned at this level and may be visualized toward the upper border of the image. The hyoid bone and the third or fourth cervical vertebra (these vertebrae are almost indistinguishable at that level in axial sections) are the only other bony structures seen toward the inferior end of the imaging volume. The sequential axial images of the neck are dominated by soft tissue structures, the visualization of which is generally limited because of reduced soft tissue contrast in CBCT images. In fact, soft tissue structures often blend together in the various reconstructed images, a fact that renders CBCT inadequate for the diagnosis of soft tissue pathologic abnormality. However, sometimes, soft tissues are identifiable and this may vary in different CBCT scanners.
The soft tissue structures identified in that level include the sternocleidomastoid muscle (SCMs) bilaterally, the geniohyoid muscles, as well as the submandibular salivary glands. Most of the time only the contour of these structures is discernible and, although their visualization is not adequate for possible disease involvement, they may be used as anatomic landmarks for orientation. In the center of the neck lies the only readily identifiable soft tissue structure: the airway; this is a semicircular, very low-density (dark) area bordered by the hyoid bone (ventrally) and the vertebral column (dorsally). The airway is separated almost in 2 halves by a soft tissue structure, crescent in shape (most often), the epiglottis.
Approximately at this level, the common carotid artery bifurcates into 2 main branches, the internal carotid and external carotid arteries, which supply the brain and face of each side, respectively. The most reliable reported landmarks to indicate the level at which bifurcation occurs are the C3/C4 level and the superior border of the thyroid cartilage of the larynx; however, variation is not uncommon (see Fig. 12). The blood vessel is accompanied by the internal jugular vein (the bigger blood vessel of the neck) and the vagus nerve to form the neurovascular bundle of the neck. The location of the bundle in the axial images of the neck, at the level of C3-C4, is posterolaterally to the airway and anteromedially to the sternocleidomastoid muscle (SCM) (Fig. 13). Superior to the bifurcation, the carotid artery branches are less distinguishable due their reduced diameter.
Despite that the major blood vessels of the neck discussed above are rarely distinguished from the rest of the soft tissues of the neck in CBCT images because of the limited soft tissue contrast, familiarity of the course of the blood vessel on the lateral neck is crucial to identify pathologic conditions inside or in the vicinity of the blood vessels, such as carotid artery atheromatosis, a pathologic condition in which calcified deposits (atheromas) accumulate on the internal wall of the blood vessel;
this gradually reduces the flexibility and functionality of the blood vessel. Carotid artery atheromatosis demonstrates a fairly high incidence rate in older age groups and has been associated with an increased risk of stroke. These calcifications most frequently occur within 10 to 15 mm of the bifurcation (above or below). Sometimes they have a clear tubular appearance that makes them more readily identifiable than not. Other times they look more like a cluster of calcifications in the region (Figs. 14 and 15).
Other types of neck calcifications may include calcifications in the thyroid cartilage complex, stylohyoid ligament calcifications, sialoliths, and tonsiloliths (Figs. 16 and 17). Some may resemble carotid artery calcifications; however, the appearance and location most of the times will assist in determining the origin of the calcification.
Apart from the visualization of the corresponding cervical vertebrae (depending on the level of the axial sections) and the mandibular bone, axial images of the floor of the mouth reveal minimal information about the soft tissue structures in the region (Fig. 18).
Sagittal images of the neck are best for the assessment of the cervical spine and the airway (Fig. 19A). The cervical spine is partially only visualized in CBCT scans (C1–C5). The normal (healthy) appearance of the vertebral bodies includes a fairly square body, a thin cortical outline, a cancellous component of homogenous density, and a fairly symmetric spacing between the vertebrae visible in the scan. However, pathologic abnormality associated with the cervical spine and other irregularities is not uncommon (see Fig. 19B). Often the above are incidental findings in scans that were prescribed for different reasons.
The airway is presented as a low-density (black), tubular-shaped structure, which may vary in width and lies just ventrally to the cervical part of the vertebral column. The position of the epiglottis, the laryngeal opening below the epiglottis, as well as the position of the tongue may have an effect on the diameter of the airway in several locations. CBCT images are very useful in the evaluation of the airway and the factors that may cause restrictions in the airflow in sleep-apnea cases (Fig. 20).
Shape alterations of the airway in the various levels may trigger alerts for possible disease. In fact, soft tissue pathologic abnormality around the borders of the airway may have an effect on the shape of the airway (Fig. 21).
Midface and skull base
The midfacial structures as well as the skull base are reviewed next in a series of axial sections (Fig. 22, Fig. 23, Fig. 24, Fig. 25, Fig. 26, Fig. 27). This review starts at the level of the floor of the maxillary sinuses and the base of the maxillary alveolar bone. Apart from the apices of the maxillary teeth, the hard palate, and the floor of the maxillary sinuses, the superior foramina (anteriorly) and the greater and lesser palatine foramina are visualized at that level. The superior foramina are the entrance of the nasopalatine canal and are located on the floor of the nasal cavity (inferior meatus); they host the nasopalatine nerve, which exists through the incisive foramen on the palatal aspect of the maxillary midline between the central incisor teeth. The greater and lesser palatine foramina serve as the passageways to the greater and lesser palatine nerves and vessels, which will run the hard palate from posterior to anterior just superior to the palatal roots of the maxillary molars on the soft tissue in a palatal mucosa (see Fig. 22). Their identification during palatal surgery and palatal flap elevation is important. Similarly to several other important anatomic structures, they will not always be visualized in CBCT scans.
The nasopharyngeal aspect of the airway dominates the center of the axial cuts of the midface. Its shape and size vary and may be affected by neighboring anatomic structures in the vicinity. A deep depression on the lateral walls of the nasopharynx bilaterally is the Eustachian tube, the tube that communicates and balances the air pressure between the inner ear and external ear (see Figs. 22 and 23). Just posterior to the Eustachian tube, separated only by soft tissue projection (torus tubarius), lies the pharyngeal recess or fossa of Rosenmueller. This region almost always will appear into the maxillary CBCT scans and it is imperative to be included in the evaluation. Further dorsally, an ovoid or ellipsoid structure is visualized toward the anterior aspect of the foramen magnum; this is the odontoid process of the axis (C2) (see Fig. 19A).
Several very important anatomic structures are identified posterior to the midface, in the skull base (see Fig. 24, Fig. 25, Fig. 26, Fig. 27; Figs. 28 and 29). These structures are the mandibular condyles, external auditory canals, mastoid processes (partially visualized), bilaterally, and the sphenoid sinus almost in the center of the axial image. Anteromedial to the mandibular condyles lie 2 important foramina: the foramen ovale (larger one) and the foramen spinosum (smaller one). The former hosts the third division of the trigeminal nerve (V3), the mandibular nerve, and the latter, the middle meningeal artery.
At the same level, simply by slightly changing the reformatting angle to make the sections more parallel to the skull base, additional very important anatomic structures will appear: one of the most important anatomic regions of the skull base is the pterygopalatine fossa (PPF), which is identified in contact with the posterior wall of the right and left maxillary sinuses. The PPF represents a major crossroad in the skull base: in the PPF open 2 large osseous channels: the Vidian canal (or pterygoid canal), which hosts fibers of the petrosal nerves, and the foramen rotundum, which carries the maxillary nerve (V2). With the PPF as a passageway, the middle cranial fossa communicates with the orbits (through the inferior orbital fissure), with the paranasal sinuses through the sphenopalatine foramen, the infratemporal fossa, and the nasal cavity. Through this crossroad, inflammation from the orbits, nasal cavity, sinuses, and oral cavity can be transferred into the middle cranial fossa and vice versa. The identification of the PPF and assessment of the integrity of its margins are absolutely necessary if this structure is demonstrated in the CBCT scan.
Just posterior to the foramen ovale and medial to the mandibular condyle lies the carotid canal, on either side of the skull base. The 2 canals converge toward the base of the sphenoid, where they pass close to the cavernous sinus before they ascend.
At the same level, almost in contact with the posterior border of the external auditory canals and medially to the mastoid air cells, the jugular foramina are located. Also known as jugular fossae (due to their large size), they are well-defined, wide, corticated canals that serve as the passage points for the ninth (glossopharyngeal), tenth (vagus), eleventh (accessory) cranial nerves as well as the jugular vein, among others. Variation in their shape and size as well asymmetry is not uncommon.
More cephalad axial sections (see Fig. 26) will show the orbits, the ethmoid sinuses, and the sphenoid sinuses. The posterior opening of the orbits at that level is the inferior orbital fissure, which communicates with the PPF as mentioned earlier.
The ethmoid sinuses are made up of numerous, small, thin-walled, air cells separated by the vomer bone (nasal septum) in the midline. Their complicated anatomy gave them the characterization of the ethmoid labyrinth. The larger sphenoid sinuses are located just posterior (dorsally) to the ethmoid sinuses. They occupy the base of the sphenoid bone (basisphenoid) and their thinned osseous walls are in contact with some rather important anatomic entities: the carotid canals (posterolaterally) and the foramina rotunda/PPF (anterolaterally). Anatomic variation and the presence of septations are often the rule rather than the exception.
The most superior (cephalad) axial sections will reveal the upper half of the orbits, the temporal fossa, and partially the middle and the posterior cranial fossae. The concavities seen toward the posterolateral orbital walls are the temporal fossae; they are anatomic depressions into the temporal bone and serve as the attachment point for the temporalis muscle (see Fig. 27).
At this level, the orbital apex (posterior opening of each orbit) appears to be splitting into 2 distinct openings: a lateral opening, which is rather wide and opens into the anterior cranial fossa (superior orbital fissure), and a narrower medial opening, which is longer and directed posteromedially toward the sella turica (optic canal). In fact, the 2 optic canals are converging toward the sella turcica, where they finally unite. The convergence point is the optic chiasma where the 2 optic nerves (content of the optic canals) cross each other's course.
The posterior clinoid processes (also identified at that plane) are 2 small osseous tubercles, which demonstrate a transverse orientation form the dorsal boundary of the sella turcica. They show a great deal of variation in shape and size, deepen the sella, and serve as attachment points (see Fig. 27).
The goal of the above anatomic review of the maxillofacial region was 2-fold: to shed some light into structures and anatomic landmarks that were “lost,” in some ways, in projectional imaging and discuss their location, course, and relationship with neighboring structures in 3 dimensions; and also to illustrate the effects of disease in known anatomic boundaries (canals, foramina, osseous cavities, soft tissue contours) that, sometimes, may be the only signs of developing pathologic abnormality.
The successful identification of an unknown entity lies in the ability of the observer to approach it from all perspectives, using fully the potential of the multiplanar imaging. It is strongly recommended that to take advantage of the CBCT images in full, the diagnostician should be able to understand and apply the concept of multiplanar reformatting to the highest degree. It is in our hands to reveal the information related to each diagnostic task. In other words, our diagnostic efficiency is based on our sound knowledge of anatomy and on our skills to retrieve relevant diagnostic information.
A comparison of maxillofacial CBCT and medical CT.
Atlas Oral Maxillofac Surg Clin North Am.2012; 20: 1-17