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Diagnostic Skills Index page
Interpretation of Normal Radiographic
Anatomy
INSTRUCTIONS: This is a descriptive
overview of normal radiographic anatomy. It is intended to be studied
as a combination of text and images. The text is below and the images
appear when you click on the underlined blue text. Don't be concerned
if the text dissappears when you click to see an image, it is still
there. If you resize the image window to the size of the image,
you will see the text again. When you are finished looking at the
image, you can return to the text by clicking the close box in the
upper left corner of the image window (Mac) or click the "x"
box in the upper right corner of the image window (Windows). You
should come back to this basic anatomy any time during the course
or after the course if you have questions.
A radiograph is a two-dimensional representation of a three-dimensional
structure. There is always superimposition of anatomical structures
and sometimes the projection of anatomy into areas where you wouldn't
expect it to be normally. You must reconstruct the anatomy in your
mind to understand what you are seeing and understand how the angle
of the x-ray beam can affect the location of structures on the film.
Being able to identify "normal" anatomy on x-ray images
is critical-otherwise, how would you recognize an abnormality?
Appearance of the "Normal" Tooth on Periapical Radiographs
X-ray images rely on the physical attenuation of the x-ray beam
to form an image on film. The denser the tissue, the more radiation
it will absorb. The more radiation absorbed by the tissues, the
less reaches the film. The more radiation that passes through the
tissue and reaches the film, the darker the area will be on the
image.
The most dense area of a normal
tooth is the enamel cap, which typically appears more radiopaque (white)
than the other tissues. The dentin is less dense and appears as a uniform grey area.
The junction between
the enamel and dentin is very distinct. The layer of cementum
on the root surface is nearly the same density as the dentin, thus
it is usually not apparent radiographically. The soft tissues of
the pulp
are much less dense than the other tooth structures and typically
appear radiolucent. In normal, fully-formed teeth the root canal
may be apparent extending to the apex of the root with a recognizable apical
foramen.
Supporting structures of the tooth that are visible radiographically
include the lamina dura, the alveolar crest, the periodontal ligament
space, and the cancellous bone. When the x-ray beam is projected
directly through the long axis of the lamina dura, it is seen clearly as a thin, white line.
If the beam passes through at an angle, the lamina dura may appear
more diffuse or not be visible at all. The radiographic appearance
of the alveolar
crest varies from a dense layer of cortical bone to a smooth
surface without cortical bone. The level of the bony crest is considered
normal when it is not more than 1.5 mm from the cementoenamel junction
of the adjacent teeth. The periodontal ligament space appears as a radiolucent space
between the root and lamina dura, beginning at the alveolar crest,
extending around the portion of the root within the alveolus, and
returning to the alveolar crest on the opposite side. The width
of the PDL varies from tooth to tooth, although it is typically
thinner in the middle of the root and wider near the alveolar crest
and root apex. Cancellous bone lies between the cortical plates
of both jaws and shows many small radiolucent pockets of marrow
which create the trabecular pattern we see on the film. The trabecular
pattern varies considerably from patient to patient and
even within the same patient.
The trabeculae
in the maxilla are typically small and form a dense granular
pattern while the trabecular
pattern of the mandible is larger and coarser.
Normal Maxillary Radiographic Anatomy
The borders of the maxillary
sinus are formed of thin cortical bone which appear as thin
radiopaque lines on periapical radiographs. The size of the maxillary
sinus varies considerably although the right and left sinuses are
typically symmetrical. The floor of the sinus is seen on periapical radiographs near
the apices of the molars and premolars, and may extend down as far
as the crest of the alveolar ridge, particularly in
edentulous areas. Radiopaque lines traversing the sinus
either horizontally or vertically are septae,
bony projections from the floor and wall of the antrum. Septae give
the sinus the appearance of being divided into compartments, although
this is not the case. The radiolucent compartments formed by the
septae sometimes mimic periapical pathoses.
Occasionally small bone nodules on the floor of the maxillary sinus
may imitate root tips. One way to differentiate the two is to look
for trabecular pattern; a nodule will show trabecular pattern while
a root tip will not.
The zygoma
appears as a U-shaped radiopaque line with the round portion superimposing
the area of the first and second molars. Depending on the angle
in which the x-ray beam passes through the zygoma, it will vary
in size, width, and definition.
The nasolabial
fold may appear as an oblique line traversing the premolar
region. The line of contrast is well-defined and the area of increased
radiopacity is caused by the superimposition of the cheek tissue.
This feature increases with age and can be used to identify the
side of the maxilla if the area is edentulous.
The medial and
lateral pterygoid plates lying immediately posterior to
the maxillary tuberosities have a variable appearance(another view of the pterygoid plates),
often not being visible at all. Typical appearance is a single radiopaque
shadow with no trabecular pattern. The hamulus
may be seen extending inferiorly from the medial pterygoid plate
and does show trabecular pattern.
Normal Mandibular Radiographic Anatomy
On periapical radiographs of the central incisors the mental fossa appears as a
radiolucent depression extending laterally from the midline and
between the alveolar ridge and the mental ridge. Due to the thinness
of the bone in the area, the mental fossa appears slightly radiolucent
compared to adjacent bone and may be mistaken for periapical disease.
The mental foramen
is seen on some periapical radiographs and has a varying appearance;
sometimes round or oblong, sometimes slitlike. Typically it is positioned
halfway between the lower border of the mandible and the alveolar
crest, in the region of the apex of the second premolar. It may
appear over the apex of a tooth, mimicking periapical pathoses.
A second radiograph from another angle will likely cause the appearance
of the foramen to shift in relation to the apex and confirm its
identity.
The mandibular
canal appears inconsistently and is seen as a dark linear
shadow with thin radiopaque borders. The canal extends radiographically
from the mandibular foramen to the mental foramen.
Nutrient canals
appear in a small number of patients as radiolucent lines extending
vertically from the inferior dental canal to the interdental space
between the mandibular incisors. Occasionally the canals may appear
as small round radiolucencies perpendicular to the cortex and can
be mistaken for pathology.
The mylohyoid
ridge appears as a radiopaque line running from the area
of the third molars to the premolar region, occasionally superimposing
the molar roots. The margin of the ridge is varies and is often
not well defined.
The submandibular
gland fossa is located below the mylohyoid ridge in the
molar area and appears as a radiolucent area with a sparse trabecular
pattern.
The external
oblique ridge is the continuation of the anterior border
of the mandibular ramus which disappears in the area of the first
molar. On periapical radiographs it appears superior to the mylohyoid
ridge, running nearly parallel to it. Radiographically it appears
as a radiopaque line with varying width, density, and length.
The coronoid
process is often seen in the molar region and appears as
a triangular opacity superimposed on the area of the third molar.
Trabecular pattern may or may not be visible.
Normal Panoramic Anatomy
The multiple superimpositions of anatomy on a panoramic radiograph
make the identification of normal anatomy difficult.
The mandible should be symmetrical, although poor positioning of the jaw can cause the mandible to
appear otherwise. The cortical bone should be continuous
from one condylar head down along the border of the mandible and
up to the opposite condylar head and of a consistent thickness.
The density of the bone should be fairly consistent throughout,
with an increase in opacity along the midline. The region of the
submandibular gland will appear more radiolucent. The external oblique ridge is visible as a radiopaque line
along the superior edge of the mandible. The mandibular canal and mental
foramen are typically seen, although the clarity varies.
The mandibular canal extends from roughly the upper third
of the ramus and follows the curve of the jaw down to the area of
the second premolar, where it terminates in the mental foramen.
The mental ridge is apparent in the center of the mandible,
between the foramen. The hyoid
bone often superimposes the mandible bilaterally in the
region of the angle of the mandible, extending horizontally towards
the ends of the film.
The nasal septum
should appear as a vertical opacity in the relative center of the
film beginning in the area of the hard palate and extending upward.
On either side of the nasal septum the inferior turbinates are seen, typically as oval opacities.
The nasal cavity
appears as a radiolucency between the turbinates and the nasal septum.
The borders of the maxillary sinus are seen as thin radiopaque lines. The
sinuses should be roughly symmetrical and comparable in density,
although they will appear more opaque in the region of the zygoma.
The zygoma
arises over the first or second molar area. The zygomatic arch begins at the inferior portion of the zygoma
and extends posteriorly to the articular eminence and glenoid fossa
(another view of zygomatic arch). The zygomaticotemporal suture
is often seen in the center of zygomatic arch and may be confused
with a fracture site.
The inferior border of the orbit is commonly seen in the upper third of the maxillary
sinus. Between the inferior medial border of the orbit and the turbinates
is the radiolucent ethmoid sinus.
The hard palate
appears as a horizontal radiopaque line above the maxillary teeth.
Depending on patient positioning, the hard palate will sometimes
superimpose the apices of the teeth. Often the floor of the nasal cavity is superimposed with the hard
palate.
The pterygomaxillary
fissure is seen as a tear-drop shaped area with a radiopaque
outline. Air trapped between the tongue and the hard palate creates
a radiolucent area beneath the hard palate in the oropharyngeal air space, superimposing the apices of the
maxillary teeth. If the radiolucency is dark enough, the apices
of the teeth may not be visible. To prevent this, the patient must
press their entire tongue against the hard palate throughout the
exposure.
Air passing through the trachea and maxillary sinuses (nasopharyngeal air space) also
appears a band of radiolucency, although it does not superimpose
normally erupted maxillary teeth. The soft tissue shadow of the
ear lobe
may be visible lateral to the rami. Superior to the ear lobe the
external auditory
meatus may be visible as an oval radiolucency. Occasionally
the styloid process
is visible in the area of the ear lobe and angle of the mandible.
Along the outer edges of the film a portion of the cervical
spine is often seen and does not superimpose the mandible
unless the patient has been poorly positioned.
Normal Occlusal Radiographic Anatomy
Maxillary
Occlusal
The median palatal suture appears as a thin radiolucent line
between the central incisors extending roughly from the alveolar
crest to the apices of the central incisors. The incisive
foramen is seen as an oval radiolucent area between
the apices of the central incisors. The nasal septum
is seen as a radiopaque area extending vertically down the center
of the image from the posterior of the image to the apices of the
central incisors. It is bordered on each side by the nasal fossa
which appear as radiolucent lines parallel to the nasal septum.
If the maxillary sinus appears in the image it is seen as a radiolucent
area in the posterior lateral aspect of the image.
Mandibular
Occlusal
The inferior border of the mandible is seen in a mandibular
occlusal radiograph. Typically the cortical plate can be
seen as a radiopaque line along the border of the mandible. The
mental ridge also appears as a radiopaque line parallel to
the cortical plate. The genial tubercle appears as a small
pointed protrusion extending downward from the cortical plate.
Skull Anatomy
Posterioranterior
Projection
The PA Skull view is used to detect abnormalities caused by trauma
or developmental delays, particularly in the mediolateral dimensions
of the skull such as asymmetric growth. It demonstrates the frontal
and ethmoid sinuses as well as the orbits, nasal anatomy, coronoid
process of the mandible, and a frontal view of the mandible.
Areas of cortical bone that are thinner than other areas may mimic
disease by appearing more radiolucent than the surrounding bone.
Typically, cortical bone is thinner in the area of the mandibular
incisive fossa and submandibular fossa. In the maxilla, the incisive
fossa in the globulomaxillary region appears radiolucent.
. The radiopaque outline of the orbit is seen bilaterally,
traversed by the petrous portion of the temporal bone
mid-orbit. The radiopaque innominate line traverses upward
from the upper outer portion of the orbit bilaterally. The radiolucent
frontal sinus is visualized between the upper inner area
of the orbits and may vary greatly in size and shape. The sphenoid
and ethmoid sinuses are seen as radiolucent areas
located between the mid and lower inner portions of the orbit.
In the center of the image is the nose and its associated anatomy:
inferior nasal turbinates, nasal septum, and the nasal turbinates.
The nasal septum appears as a vertical radiopaque line floored
by the radiopaque nasal fossa and hard palate and sided by
the middle nasal turbinates superiorly and the inferior
nasal turbinates laterally.
Radiolucent areas lateral to the orbits are the mastoid air cells.
Just below the mastoid air cells begins the radiopaque coronoid
process of the mandible which extends downward to the mandible.
The mandibular canal is visible as it runs most of the length
of the rami, ending in the mental foramen at the area of
the second mandibular premolar. The shadow of the cervical
vertebrae begins in the area of the nasal anatomy and extends
downward past the mandible to the bottom of the film, where they
are seen much more clearly.
Lateral Skull
This view is used to demonstrate the skull and facial bones, visualizing
the hard palate, nasopharyngeal soft tissues, and paranasal sinuses.
In orthodontics this view is used to assess facial growth.
This view shows all of the sinuses which appear as a large cluster
radiolucent areas. The frontal sinus is the smallest and
most superior, with the maxillary sinus below it, bordered
by the hard palate which runs consistent with the floor
of the maxillary sinus. The ethmoid and sphenoid sinuses
are visualized posterior to the maxillary sinus. Adjacent to the
posterior wall of the maxillary sinus is the pterygomaxillary
fissure which appears triangular in shape and slightly radiopaque.
The pterygoid plates are posteriorly adjacent to the fissure.
The soft palate is an ill-defined radiopaque area between
the hard palate and the mandible.
Other radiolucent areas include the oropharyngeal airspace
running from the oral cavity to the trachea, and the nasopharyngeal
air space running from the maxillary sinuses to the trachea.
The cervical vertebrae are visible and well-defined. The
mastoid air cells appear as a slightly radiolucent area in the
skull above the process of the second vertebra.
Waters View
This view is particularly useful for evaluating the maxillary
sinuses which appear radiolucent. Other radiolucent structures
seen include the ethmoid air cells, orbits, nasal turbinates,
frontal sinus, and nasal cavity. Radiopaque structures
seen include the nasal septum, body of the zygoma, frontal process
of the zygoma, zygomatic arch, coronoid process of the mandible,
and crista galli.
Submentovertex
(SMV) View
Radiolucent structures demonstrated on this view include the ethmoid
sinus, mastoid air cells, and maxillary sinus. Radiopaque
structures demonstrated include the base of the skull, mandibular
condyles, coronoid process of the mandible, lateral and medial pterygoid
plates, nasal septum, and zygomatic arch. Structures
of a medium density that are commonly seen include the carotid
canal, foramen spinosum, foramen ovale, and foramen magnum.
Normal Radiographic
Anatomy of the Tempomandibular Joint
A panoramic image of the TMJ typically includes 2 views of each
condyle; one with the mouth wide open and one with the mouth closed.
Radiopaque anatomy demonstrated typically includes the mandibular
condyle, articular eminence, soft palate, and maxillary tuberosity.
Radiolucent structures seen include the maxillary sinus, external
auditory meatus, and pterygomaxillary fissure.                          
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