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conventional radiograph is made with a م۵
stationary x-ray source and displays a_two-
dimentional image of a part of the body. such
images are often called plain or projection views.
©The principles of projection geometry describe
the effect of focal spot size and relative position
of the object and image receptor on image
clarity ,magnification and distortion.
© Clinician use these principles to maximize image
clarity,minimize magnification and distortion
and localize objects in the image field.
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2 مس تس
۱ Image resolution and
sharpness
© Sharpness measures how well a boundary
between two areas of differing radiodensity
is reveled.
© Image spatial resolution measures how well
a radiograph is able to reveal small objects
that are close together.
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FIGURE 6-1 hots aging odfeet places on
the fol ot (ed) et in oe of shapes on he
tndgoph, The des of the imoe changes fam a fgh
ما هذا ملم سيا abe in te ce fon ge of
coonel, enn oe, One eo lnge fr spt size
resin avid zone of wanes cpa wi smal
foc spot sie on he ih, which esata shaper mone
(baron ze of sharps)
‘Small focal spot
Unsharpness~9i4---
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خب ۱7
© There are three means to maximize image
sharpness:
1.use as small an effective focal spot as
practical
® Dental x-ray machines should have a effective
focal spot of 0.4 mm because this greatly
adds to image clarity.
© Alarger angle between target and long axis
of electron beam decresed heat per unit
of target prolonging tube life larger
effective focal spot loss of image clarity
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FIGURE 6-2 she ong ofthe tet becames coe
۵ papentot othe lg is of he econ bor ها
shown on the ri the au fc spot becomes sal,
hich decease heat dspton db بعلا The moe
‘err cg obo decease th eect لس po
te, inoesg the supe of ta sing Inge
Electron beam
Fg). objoct ———+ <i}
Effective
{ocal spot
Image
receptor
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2.increse the distance between the focal spot
and the object by using a long open-ended
cylindre
© A longer distance between the focal spot and
the object minimize blurring by using
photons whose path are almost parallel
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FIGURE 6-3 lnzesng the dstunce betwen te focal st ond the oc
res nan mage wh nse shapes nd ss monica of hoje.
asm on height
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: 3.minimize the distance between object and the
image weeeptor
minimizing the
divergenge of the x-ray photons enhanced
image sharpness
{ \
FIGURE 6-4 sce اهاط jt a emg a
] hopes cd eines mgicin oh oie ass ne
0
Image
receptor
۱5 1۵۱ 5 و
ae ی
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Image size distortion
®Image size distortion(magnification)is the
increase in size of the image on the
radiograph compared with the actual size of
the object.
©The divergent path of photons cause
enlargment of the image.
® Magnification results from the _ relative
distance of the focal spot to image receptor
and object to image receptor.
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FIGURE 6-3 leas the dstence between the foc spot nd the ojet
resus in on ioge wth nese shapes nd les اه مه موه
os en onthe ght
Image
receptor
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FIGURE 6-4 ها همه dunce beac the objet on mag repo
intense th shops on es in ess magico of the objet see on he
et
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Image shape distortion
®Image shape distortion is the result of
unequal magnification of different parts of
the same object. this situation arises when
not all parts of an object are at the same
focal spot to object distance.
©The physical shape of the object often
prevent its optimal orientation and results
in different appearance of the image on a
radiograph compared with the true shape.
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°To minimize shape distortion the
practitioner should make an effort to align
tube, object and image receptor
1.position the image receptor parallal to the
Jong axis of the object
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Image
receptor
0 5 10
-5 Foreshortening of o radiographic image results when the central ray is per
image receptor but the object is not parallel with the image receptor.
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-6 Flongaton اه ۵ radiographic image results when the cent roy is perpen-
ject but nat tothe image receptor.
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® 2.orient the central ray perpendicular to the
object and image receptor
© Image shape distortion occures if the object
and image receptor are parallal but the
central ray is not directed at right angles to
each.
© This distortion is most evident on maxillary
molar views.if the central ray is oriented
with excessive vertical angulation the palatal
roots appear disproportionally longer than
buccal roots.
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FIGURE 6-7 The central ray should be perpendicular tothe long axes of both the tooth
nd the image receptor. fhe direction of the xvay beam is nt at right angles to the long axis
of the tooth, the appearance of the tooth is distorted, typically by apparent elongation of the
length of the palatal roots of upper molos and distortion of the relationship of the height of
the alveolar crest relative to the cementoenamel junction.
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arallaling and bisecting
angle technique
In bisecting angle» image receptor is not
parallal to the long axes-ef the teeth inherently
causes distortion
© The angle between teeth and receptor is specially
apparent in the maxilla and anterior mandible
© By directing the central ray perpendicular to an
imaginary plane that bisects the angle between
the teeth and image receptor the length of the
tooth on image correspond to the actual length
but these images display a distorted image of the
position of alveolar crest with respect to the CEJ
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كوي سل
Central axis of tooth
Imaginary bisector
FIGURE 6-8 Inthe biseingonletectiqu, th central ry is dred ot o ght onle
tothe imoginary plane that bisects the angle formed by the image receptor ‘ond the central axis,
ofthe object. This method produces on image that i the some length as the objet but resus
in some imoge distortion,
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© The paralleling technique minimize image
distortion but causes some image
magnification and loss of sharpness
© To overcome these limitations the
paralleling technique uses a relatively long
open ended aiming cylinder(cone) to increase
focal spot to object distance
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Central axis of tooth
FIGURE 6-9 In the porlleing technique, the central roy is directed at a right angle to
‘he central axes ofthe object andthe image receptor. This technique requires a device to support
‘the fim in position.
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Object localization
© The dentist may wish to use radiographs to
determine the location of a foreign object or
an impacted tooth within the jaw.
Three methods are frequently used:
© 1.examine two images projected at right
angle to each other.
©2.use the tube shift technique employing
conventional periapical views.
© 3.cone beam CT technique.
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The right angle or cross section technique is
best for the mandible. on a maxillary
occlusal view the superimposition of features
in the anterior part of the skull frequently
FIGURE 6-10 A, Periapical radiograph shows impacted canine lying apical to roots of
lateral incisor and fist premolar. B, Vertex occlusal view shows thatthe canine lies polatal to
the roots ofthe lateral incisor and frst premolar.
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©The second method used to identify the
spatial position of an object is the tube shift
technique.other names for this procedure
are the buccal object rule and Clark rule.
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FIGURE 6-11 The position of an object may be determined with respect to reference
structures with use of the tube shift technique. AA, A radiopaque object on the lingual surface
of the mandible (black dot) may appear apical to the second premolar. B, When onother
tadiogroph is made of this region angulated from the mesial, the object appears to have moved
‘mesially with respect to the second premolar apex (“same lingual” in the acronym SLOB).
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الج
0 © الات ©) Canine __ Lateral
Step 1 - Determine tube shift between exposures
—
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© The relationships can be easily memebered
bye the acronym SLOB.
© If the object in position does not move to the
reference object it lies af the same depth (in
the vertical plane)jas the reference object.
© This technique uses just as well when the x-
ray machine is moved _ vertically as
horizontally.
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Parallax in the horizontal plane .1
(0 -palatal side)
Ref. 2
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Parallax in the vertical plane .2
{Crown port SST
(palatal side- )
Opposite = Buccal
(labial side-)
(0۳8
Ref. 2
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© The dentist may have two radiographs of a
region of the dentition that were made at
different angles but no record exists of the
orientation of the x-ray machine.comparision
of the anatomy displayed on the images help
distinguish changes in horizontal or vertical
angulation.
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Similarly
the
angulation
of the beam
is incresed
vertically
the
zygomatic
process is
projected
occlusally
over the
-teeth
FIGURE 6-13 The poston of ho madly 2yonon peas naan جوم مط د
fhe clas can bali iting the elton ef vis A, Tha ri Baer
the mama poss Hes owt the patel ao ofthe Sst la. B, The rb
of the rome pes es past tthe palit et af et mola, This nea
npn of the east pees woo fo the pla at nds Hot who he
image n A. wos mage, te ber wes جوا موم تا fs peat hon when the
Image in wes ma. The same ceca on bm ced agent by exanning
he fos of the etka. The pol ot steed the dab 00 he rege
18 A, bot es Betoon te to lnc ne nage in B.
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Eggshell effect —
Plain’ images that project a_ three
dimentional volume onto a two dimentional
receptor may produce an eggshell effect of
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FIGURE 6-14 tap ot. A Rah of had oN ot ino یه مه موه مرو is
ua tiles B, Secu eit pox on. مه تسه ما رواب 6
pc a eh fog ul og nt ee sets og pepe
Tse la a cote ts tly ee fa, eae ne
itl on nis view The ede terre pam Hn on nid boo of
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© Photons traveling through the periphery of a
curved surface are more attenuated than
photons traveling at right angles to the
surface.
©This eggshell effect accounts for why normal
structures such as the lamina dura the border
of the maxillary sinuse and nasal fossa and
corticated surface of cysts and benign tumors.
© Soft tissue masses such as nose and tongue do
not show an eggshell effect because they are
uniform rather than being composed of dense
layer surrounding a more radiolucent interior.