Transcript Positioning of the Skull

Positioning of the Skull
By Prof. Stelmark
RADIOGRAPHIC ANATOMY
Skull
As with other body parts, radiography of the
skull requires a good understanding of all
related anatomy. The anatomy of the skull is
very complex, and specific attention to detail
is required of the technologist.
The skull, or bony skeleton of the head,
rests on the superior end of the vertebral
column and is divided into two main sets of
bones—the 8 cranial bones and the 14
facial bones.
CRANIAL BONES (8)
The eight bones of the cranium are divided into the
calvaria (skullcap) and the floor. Each of these two
areas primarily consists of four bones:
Calvaria (Skullcap)
1. Frontal
2. Right parietal
3. Left parietal
4. Occipital
Floor
5. Right temporal
6. Left temporal
7. Sphenoid (sfe′-noid)
8. Ethmoid (eth′-moid)
POSITIONING CONSIDERATIONS
Erect versus Recumbent
Projections of the skull may be taken with the patient in the recumbent or
erect position, depending on the patient's condition. Images can be
obtained in the erect position with the use of a standard x-ray table in the
vertical position or an upright Bucky. The erect position allows the patient
to be quickly and easily positioned and permits the use of a horizontal
beam. A horizontal beam is necessary to visualize any existing air-fluid
levels within the cranial or sinus cavities
Patient Comfort
Patient motion almost always results in an
unsatisfactory image. During skull radiography,
the patient's head must be placed in precise
positions and held motionless long enough for
an exposure to be obtained. Always remember
that a patient is attached to the skull that is
being manipulated. Every effort should be made
to make the patient's body as comfortable as
possible, and positioning aids such as sponges,
sandbags, and pillows should be used if
needed.
Except in cases of severe trauma, respiration
should be suspended during the exposure to
help prevent blurring of the image caused by
breathing movements of the thorax. This is
especially important when the patient is in a
prone position.
Hygiene
Cranial and facial radiography may require
the patient's face to be in direct contact
with the technologist's hands and the
table/upright Bucky surface. Therefore, it is
important that proper handwashing
techniques and surface disinfectants be
used before and after the examination.
Exposure Factors
The principal exposure factors for
radiography of the skull include the
following:
•Medium kV (65 to 85 kV film-based)
(70 to 80 kV digital radiography [DR]
and computed radiography [CR])
•Small focal spot <250 mA (if
equipment allows)
•Short exposure time
SID
The minimum SID with the image receptor in the
table or upright Bucky is 40 inches (100 cm).
Radiation Protection
The best techniques for minimizing radiation
exposure to the patient in skull radiography are to
(1) use good collimation practices, (2) immobilize
the head when necessary, minimizing repeats,
and (3) center properly.
Gonadal shielding
Generally, with accurate collimation, no
detectable contribution to gonadal exposure
occurs during radiography of the skull. However,
lead shields should be used to reassure the
patient
Digital Imaging Considerations
Guidelines for digital imaging (CR and DR) of the skull are:
1.Correct central ray angle and centering to body part and image
receptor. This provides for accurate post-processing of the image by the
image reader.
2.Close collimation. Improves image quality by reducing scatter and
secondary radiation to the highly sensitive digital image receptors.
3.Following ALARA principles (exposure to patient As Low As
Reasonably Achievable) in determining exposure factors (highest kV
and lowest mAs that will result in desirable image quality). An increase
in kV over film-screen imaging may be desirable, both for reducing
patient exposure and for improving image quality. (Sufficient mAs is
required to produce a high-resolution image.)
4.Post-processing evaluation of exposure indices (for assurance that
optimum quality image was achieved with least possible radiation to
patient). Examine the amount of kV and mAs used for a particular
exposure if the exposure index is above or below the recommended
range.
Skull Series
BASIC:
• AP axial (Towne method)
• Lateral
• PA axial 15° (Caldwell method) or PA axial 25° to
30°
• PA 0°
SPECIAL
• Submentovertex (SMV)
• PA axial (Haas method)
AP AXIAL PROJECTION: SKULL
SERIES
Towne Method
Pathology Demonstrated
Skull fractures
Technical Factors
• IR size—24 × 30 cm (10 × 12 inches), lengthwise
• Moving or stationary grid
• 70 to 80 kV range
• Small focal spot
Part Position
• Depress chin, bringing OML perpendicular
to IR. For patients unable to flex their neck
to this extent, align the IOML perpendicular
to the IR. Add radiolucent support under the
head if needed.
• Align midsagittal plane to CR and to
midline of the grid or the table/Bucky
surface.
• Ensure that no head rotation and/or no tilt
exists.
• Ensure that vertex of skull is in x-ray field.
Collimation
Collimate to outer margins of skull.
Respiration
Suspend respiration.
If patient is unable to depress the chin sufficiently to
bring the OML perpendicular to the IR even with a
small sponge under the head, the infraorbitomeatal
line (IOML) can be placed perpendicular instead and
the CR angle increased to 37° caudad. This maintains
the 30° angle between the OML and the CR and
demonstrates the same anatomic relationships. (A 7°
difference exists between the OML and the IOML.)
Central Ray
• Angle CR 30° caudad to OML, or 37° caudad to
IOML (see Note below).
• Center at the midsagittal plane 2½ inches (6.5 cm)
above the glabella to pass through the foramen
magnum at the level of the base of the occiput.
• Center IR to projected CR.
• Minimum SID is 40 inches (100 cm).
Structures Shown: • Occipital bone, petrous pyramids,
and foramen magnum are shown with the dorsum
sellae and posterior clinoids visualized in the shadow
of the foramen magnum.
LATERAL POSITION—RIGHT AND/OR LEFT
LATERAL: SKULL SERIES
Pathology Demonstrated
Skull fractures. A common general skull routine includes both
right and left laterals.
Part Position
• Place the head in a true lateral position, with
the side of interest closest to IR and the
patient's body in a semiprone position as
needed for comfort. • Align midsagittal plane
parallel to IR, ensuring no rotation or tilt.
• Align interpupillary line perpendicular to IR,
ensuring no tilt of head (see Note below).
• Adjust neck flexion to align IOML
perpendicular to front edge of IR
Central Ray
• Align CR perpendicular to IR.
• Center to a point 2 inches (5 cm) superior to EAM .
• Center IR to CR.
• Minimum SID is 40 inches (100 cm).
Structures Shown: • Superimposed cranial halves with superior
detail of the lateral cranium closest to the IR are demonstrated.
The entire sella turcica, including anterior and posterior clinoids
and dorsum sellae, is also shown. The sella turcica and clivus
are demonstrated in profile.
PA AXIAL PROJECTION: SKULL SERIES
15° CR (Caldwell Method) or 25° to 30° CR
Pathology Demonstrated
Skull fractures (medial and lateral displacement)
Part Position
• Rest patient's nose and forehead against table/Bucky
surface.
• Flex neck as needed to align OML perpendicular to
IR.
• Align midsagittal plane perpendicular to midline of the
grid or table/Bucky surface to prevent head rotation
and/or tilt.
• Center IR to CR.
Central Ray
• Angle CR 15° caudad and center to exit at nasion.
• Alternate with CR 25° to 30° caudad, and center to
exit at nasion.
• Minimum SID is 40 inches (100 cm).
Alternate 25° to 30°: An alternate projection is a 25°
to 30° caudad tube angle that allows better
visualization of the superior orbital fissures (black
arrows), the foramen rotundum (small white arrows),
and the inferior orbital rim region. CR exits at level of
nasion.
Structures Shown: • Greater and lesser sphenoid wings,
frontal bone, superior orbital fissures, frontal and anterior
ethmoid sinuses, superior orbital margins, and crista galli
are shown.
PA PROJECTION: SKULL SERIES
0° CR
Pathology Demonstrated
Skull fractures (medial and lateral displacement)
Part Position
• Rest patient's nose and forehead against
table/Bucky surface.
• Flex neck, aligning OML perpendicular to IR.
• Align midsagittal plane perpendicular to midline of
table/Bucky to prevent head rotation and/or tilt
(EAMs same distance from table/Bucky surface).
• Center IR to CR.
Structures Shown: • Frontal bone, crista galli, internal
auditory canals, frontal and anterior ethmoid sinuses,
petrous ridges, greater and lesser wings of sphenoid.
SUBMENTOVERTEX (SMV) PROJECTION: SKULL
SERIES
Warning: Rule out cervical spine fracture or subluxation
on trauma patient before attempting this projection.
Part Position
• Raise patient's chin and hyperextend the neck if
possible until IOML is parallel to IR.
• Rest patient's head on vertex.
• Align midsagittal plane perpendicular to the midline
of the grid or table/Bucky surface, thus avoiding tilt
and/or rotation.
Central Ray
• CR is perpendicular to infraorbitomeatal line.
• Center 1½ inch (4 cm) inferior to the mandibular
symphysis, or midway between the gonions.
• Center image receptor to CR.
• Minimum SID is 40 inches (100 cm)
Structures Shown: • Foramen ovale and spinosum,
mandible, sphenoid and posterior ethmoid sinuses, mastoid
processes, petrous ridges, hard palate, foramen magnum,
and occipital bone are shown.
PA AXIAL PROJECTION: SKULL SERIES
Haas Method
Pathology Demonstrated
Occipital bone, petrous pyramids, and foramen magnum,
with dorsum sellae and posterior clinoids in its shadow
Part Position
• Rest patient's nose and forehead against the
table/Bucky surface.
• Flex neck, bringing OML perpendicular to IR.
• Align midsagittal plane to CR and to the midline of the
grid or table/Bucky surface.
• Ensure that no rotation or tilt exists (midsagittal plane
perpendicular to IR).
Central Ray
• Angle CR 25° cephalad to OML.
• Center CR to midsagittal plane to pass through level of
EAMs and exit 1½ inches (4 cm) superior to the nasion.
• Center image receptor to projected CR.
• Minimum SID is 40 inches (100 cm).
Structures Shown: • Occipital bone, petrous pyramids,
and foramen magnum are shown, with the dorsum sellae
and posterior clinoids visualized in the shadow of the
foramen magnum.