PROJECTION GEOMETRY/ dental implant courses

PROJECTION GEOMETRY
 CONTENTS
1. Image sharpness and resolution
2. Image size distortion[magnification]
3. Image shape distortion
4. Object localization
5. Peripheral egg shell effect
INDIAN DENTAL ACADEMY
Leader in continuing Dental Education
www.indiandentalacademy.com

Image sharpness and resolution
Image sharpness measures how well a boundary
between two areas of differing radio density is
revealed.
Image spatial resolution measures how well a
radiograph is able to reveal small objects that are close
together.

When x rays are produced at the target in an x-ray tube
, they originate from different points and travel in
straight lines, their projection of a feature of an object
do not occur at exactly same location on a film.
As a result , the image of the edge of an object is
slightly blurred rather than sharp and distinct.

Photons originating at different points on the focal spot results in penumbra
Or zone of un sharpness on the radiograph .the density of the image changes
From high value to low value in the area of edge of enamel , dentin or bone

Three methods exist for minimizing this loss of image
clarity and improving the quality of radiograph
1. Use as small an effective focal spot as practical
2. Increase the distance between the focal spot and
object
3. Minimize the distance between object and the film

 SMALL FOCAL SPOT
Dental x -ray machines have a nominal focal spot size
of 1mm or less.
The size of the effective focal spot is function of the
angle of the target with respect to the long axis of the
electron beam
A large angle distributes the electron beam over a
larger area and decreases the heat generated per unit
of target area ,thus prolonging the tube life

This results in larger effective focal spot and loss of
image clarity
A small angle has greater wearing effect on the target
but results in smaller effective focal spot , decreased
un sharpness ,and increased image sharpness and
resolution
The angle of the face of the target to the central x -ray
beam usually is between 10 to 20 degrees

Decreasing the angle of the target perpendicular to the long axis of the
electron beam decreases the actual focal spot size and decreases heat
dissipation and there by tube life .it also decreases the effective focal spot size
,thus increasing the sharpness of the image .

 Increase the distance between the focal spot and the object by using a long open ended
cylinder.

 BY MINIMIZING OBJECT AND FILM DISTANCE

IMAGE SIZE DISTORTION
Image size distortion is the increase in the size of the
image on the radiograph compared with the actual size
of the film.
The divergent paths of photons in an x -ray beam cause
enlargement of the image on a radiograph.
By increasing the focal spot to film distance and
decreasing the object film distance minimizes the
image magnification.

IMAGE SHAPE DISTORTION
It is the result of un equal magnification of different
parts of the same object.
 To minimize the shape distortion
1. Position the film parallel to the long axis of the
object
2. Orient the central ray perpendicular to object and
film

Foreshortening of a radiographic image results when the central ray
Is perpendicular to the film but the object is not parallel with the film

 Elongation of radiographic image results when the central ray is perpendicular to the object but not
the film

BISECTING ANGLE TECHNIQUE

PARELLELING TECHNIQUE

Geometric
charactreristic
s
INFLUENCING
FACTORS
EFFECT OF
INFLUENCING
FACTORS
SHARPNESS FOCALSPOT SIZE FFOCALSPOT SIZE
FOCAL SPOT SIZE
.SHARPNESS
SHARPNESS
FILM COMPOSITION CRYSTAL SIZE
CRYSTAL SIZE
SHARPNESS S
SHARPNESS
MOVEMENT MOVEMENT
MOVEMENT
R SHARPNESS
SHARPNESS
 GEOMETRIC CHARACTERISTICS AND INFLUENCING FACTORS

GEOMETRIC
CHARECTERISTICS
INFLUENCING
FACTORS
EFFECT OF
INFLUENCING
FACTORS
MAGNIFICATION TARGET FILM
DISTANCE(TF
D
TFD
TFD
MAGNIFICATION
MAGNIFICATION
OBJECT FILM
DISTANCE
(OFD)
OFD
OFD
MAGNIFICATION
MAGNIFICATION
DISTORTION OBJECT FILM
ALIGNMENT
OBJECT AND
FILM PARALLEL
OBJECT AND
FILM NOT
PARALLEL
DISTORTION
DISTORTION

XRAY BEAM
ALIGNMENT
BEAM
PERPENDICULAR
TO OBJECT AND
FILM
DISTORTION
BEAM NOT
PERPENDICULAR
TO OBJECT AND
FILM
DISTORTION

OBJECT LOCALISATION
TECHNIQUES
It is a technique used to locate the position of a tooth
or object in the jaws
PURPOSE AND USE
It is two dimensional picture of a three dimensional
object.
It is used to locate
 Foreign bodies
 Impacted teeth
 Un erupted teeth

 Retained roots
 Root position
 Salivary stones
 Jaw fractures
 Broken needles and instruments
 Filling materials

 Two basic techniques are used to localize object
1. Buccal object rule
2. Right angle technique
 BUCCAL OBJECT RULE
 It is rule governing the orientation of structures
portrayed in two radiographs exposed at different
angulations
 One peri apical or bitewing film exposed using
proper technique and angulation.

 A second peri apical or bitewing is then exposed
after changing the direction of the x ray beam ,a
different horizontal or vertical angulation is used.
 After the two films have exposed and processed,
the radiograph are compared with each other
SLOB stands for
SAME : LINGUAL
OPPOSITE: BUCCAL

The position of an object may be determined with reference structures with
use of tube shift technique . In FIG A an object on the lingual surface of the mandible
may appear apical to the 2nd premolar. Another radiograph is made of this region
angulated from mesially ,FIG B shows that object appears to have moved mesially
with respect to the 2nd premolar
FIG A
FIG B

FIG A , shows an object on the buccal surface of the mandible may appear apical
to the 2nd premolar . When another radiograph is made of this region angulated
from the mesial FIG B , shows that object appears to have moved distally with
respect to the 2nd premolar
FIG A
FIG B

 RIGHT ANGLE TECHNIQUE
It is the another rule for the orientation of structures
seen in two radiographs.
One peri apical film is exposed using proper technique
and angulation to show the position of the object in
the superior-inferior and anterior-posterior
relationships
Next an occlusal film is exposed directing the central
ray at a right angle or perpendicular to the film .

The occlusal film shows the object in the buccal-
lingual and anterior – posterior relationships.
After the two films have been exposed and processed
,the radiograph are compared with each other to locate
the object in three dimensions .

In FIG A , periapical radiograph shows impacted canine lying
apical to roots of lateral incisor and first premolar . In FIG B, the
vertex occlusal view shows that canine lies palatal to the roots of
lateral incisor and first premolar.
FIG A FIG B

PERIPHERAL EGG SHELL EFFECT
Projection images , those that project 3- dimensional
volume onto a two-dimensional receptor ,may
produce peripheral egg shell effect
The top photon has a tangential path through apex of
the egg and a much longer path through the shell of
the egg than does the lower photon ,which strikes the
egg at right angles to the surface and travels through
two thickness of the shell.

 As a result , photons traveling through the periphery of
a curved surface are more attentuated than those
traveling at right angles to the surface.

REFERENCES
1. Oral Radiology Principles and Interpretations – White
and Pharoah edition 5.
2. Dental radiology principles and techniques- Joen
Iannucci Haring edition 2.

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