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Concepts of Panoramic
Radiography
Theory of Panoramic Imaging
DHY 202 Clinical Radiology I
Dental Hygiene Department
William Rainey Harper College
Panoramic Imaging :
General Principles
• Employs scanography (slit beam) &
tomography
Panoramic Imaging :
General Principles
• Tomography: allows radiographing in
one plane of an object while blurring or
eliminating images from structures in
other planes.
• “Tomo” is Greek for section
• View sections or radiographic slices
Panoramic Imaging :
General Principles
• Tomography
– used extensively in medicine
– basis for CT (computed tomography) &
MRI (magnetic resonance imaging)
Panoramic Imaging :
General Principles
• Tomogram
– Client remains
stationary while xray
source & film move in
opposite directions in
a fixed relationship
through one or a
series of rotation
points. Rotation
points can be inside
or outside of the focal
trough
Panoramic Imaging :
General Principles
• Focal trough in
tomogram
– Or “plane of
acceptable detail,” or
“image layer,” is the
plane that is not
blurred on the
radiograph
Panoramic Imaging :
General Principles
• Focal trough in
pantogram
– Width & thickness
governed by many
factors
– Objects lying within the
focal spot are shown
clearly; objects outside
are blurred
Panoramic Imaging :
General Principles
• A panoramic
radiograph or
pantomogram is
produced using
curved-surface
tomography.
Curved
surface
Flat
surface
Panoramic Imaging :
General Principles
• Rotational panoramic
radiography is
accomplished by rotating
a narrow beam of
radiation in the horizontal
plane around an invisible
pivot point/axis
positioned intraorally.
• Film & tube travel in
opposite directions
around the client
Panoramic Imaging :
General Principles
• Client remains
stationary as xray tube
and film cassette-holder
(which are connected)
both rotate around the
client
Panoramic Imaging :
General Principles
• A vertical, narrow
beam is used
compared with the
larger, circular or
rectangular beam
used in
conventional
intraoral
radiography
Panoramic Imaging :
General Principles
• The pivot point/axis is called the
rotation center
• The center of rotation changes as the
film and tubehead rotate which allows
the image layer to conform to the
elliptical shape of the dental arches
Panoramic Imaging:
Projection in the Vertical Plane
• Vertical dimension unaffected by horizontal
rotation
• Vertical angulation same as conventional
intraoral projection
• Slight negative angulation; passes beneath
occipital area (-4 to -7 degrees)
Panoramic Imaging:
Projection in the Horizontal Plane
• Horizontal image affected by
horizontal rotation of the beam
• Xrays appear to diverge from
intraoral source but really originate
outside of the client
• Apparent intraoral source is called
the center of rotation
Panoramic Imaging:
Projection in the Horizontal Plane
True intraoral source &
focus of projection
Uniform magnification
Effective focus of
projection
Rotating beam
projected on
stationary film
Panoramic Imaging:
Projection in the Horizontal Plane
Rotating beam and
moving film has
proportions restored
•
Discrepancy in
horizontal versus
vertical magnification
eliminated by using a
moving film to equalize
the magnification in the
horizontal dimension
with the vertical
•
Film moves in direction
opposite to the
horizontal rotation of the
beam
Panoramic Imaging:
Principles of Image Layer
Formation
• Film placed on circular drum or a
moving flat cassette
• Horizontal magnification is reduced
to match vertical magnification by
adjusting speed of film in respect to
projection of beam
Panoramic Imaging:
Principles of Image Layer
Formation
• Vertical and horizontal
dimensions match only
when the object lies
within a particular
plane called the central
plane or sharply
depicted plane of the
image layer
Panoramic Imaging:
Principles of Image Layer
Formation
• The image layer is
called the focal
trough
Panoramic Imaging:
Image Layer/Focal Trough Defined
• A three-dimensional zone in which
structures are reasonably well-defined
• A zone in an object defined as containing
those object points depicted with
sufficient detail to be distinguished
• Determines where dental arches must be
positioned to achieve clearest image
Panoramic Imaging:
Principles of Image Layer
Formation
• Objects outside this sharply depicted
plane will appear distorted, fuzzy, or
may not be visible
Panoramic Imaging:
Width of the Image Layer
• Determined by
• Distance from center of rotation to
central plane of image layer
• Width of long, narrow slit beam
(the narrower the beam , the wider
the image layer)
Panoramic Imaging:
Position of the Image Layer
• Changes in film speed alter the
position of the image layer
• Increased film speed = image farther
away from rotation center
• Decreased film speed = image closer to
rotation center
• This is how the image layer is shaped
to center the jaws (anteriors narrower)
Panoramic Imaging:
Movement Pattern of the Xray
Beam
• Movement pattern of the xray beam
chosen to obtain a favorable
projection of the jaws
• Depending on the manufacturer,
number and location of rotational
centers differ
Panoramic Imaging:
Movement Pattern of the Xray
Beam
• Continuously
moving
rotation
center are
most popular
Panoramic Imaging:
Image Layer Analysis
• Objects closest to film will be
narrowed
• Objects closest or toward the source
will be widened
• Buccal objects projected lower
• Lingual objects projected higher
• Objects in the center of the layer are
magnified 20-30%
Panoramic Imaging:
Image Layer Analysis
• Less definition than than intraoral film
• More horizontal than vertical
magnification
• All objects, even those outside the focal
trough are projected onto the film, but
most are not seen
• Objects with the greatest density are
displayed in two places: intended image
and ghost image