errors in cephalometry _word
DESCRIPTION
orthodonticsTRANSCRIPT
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INTRODUCTION
A scientific approach to the scrutiny of human craniofacial patterns was
first initiated by anthropologists and anatomists, who recorded the various
dimensions of ancient dry skulls. The Measurements of the dry skull from the
osteological 1 andmarks, called craniometry, was then applied to living beings or
subject so that 'longitudinal growth study could be undertaken. This technique of
measurement of the head of a living subject from the bony landmarks located by
palpation or pressing through the Supra - adjacent tissues is called Cephalometry.
However the cephalometric method could never be wholly accurate as long as
Measurements were taken through the skin and the soft tissue coverage.
The discovery of X-rays by Roentgen in 1895 revolutionized the dental
profession. A radiographic head image could be measured in two dimensions,
thereby making possible the accurate study of craniofacial growth and
development. The Measurement of the head.
From the shadows of bony and soft tissue Landmarks on the radiographic
image become known as to roentgenographic cephalometry.
A Teleroentgenographic technique for producing a Lateral head Film was
introduced by Pacini in 1922. With this Method this size of the Image was
decreased by increasing the Focus- Film distance to 2m (78.7 in). But there was still some distortion because of head movement during the prolonged exposure
time.
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In the subsequent years the following authors such as Mac Gowen (1923),
Simpson (1923), Comte (1927), Reisner (1929) produced some type of radiograph
for CEPH measurements. None of the authors described and accurate method used
to take pictures and evaluation.
In 1931, Broadbent in the USA and Hofrath the Germany simultaneously
presented a Standardized ceph. Technique using a high-powered X -ray machine
and a Lead holder called cephalsotat (or) cephalometer.
According to broadbent, the patients, head was centered in the ccphalostat
with the Superior borders of the external auditory meatus resting on the upper
parts of the two ear rods. The nose clamp was fixed a t the root of the nose to
support the upper part of the face and the subject.
Tube - Film distance could be measured to calculate the Image magnification.
Then the 1968, Bjork designed our X-ray cephalostat research unit with a
built-in-S-inch Image Intensifier that enabled the position of the pt head to be
monitored on a T.V. screen. The pt head position in the cepholastat is also highly
reproducible.
Further more, this unit allowed the ceph x-ray examination or oral function
on the TV screen, which could also be recorded on a Video tape.
In the subsequent years the following authors such as Mac Gowen (1923),
Simpson (1923), Comte (1927), Reisner (1929) produced some type of radiograph
for CEPH measurements. None of the authors described and accurate method used
to take pictures and evaluation.
In 1931, Broadbent in the USA and Hofrath the Germany simultaneously
presented a Standardized ceph. Technique using a high-powered X -ray machine
and a Lead holder called cephalsotat (or) cephalometer.
According to broadbent, the patients, head was centered in the ccphalostat
with the Superior borders of the external auditory meatus resting on the upper
parts of the two ear rods. The nose clamp was fixed a t the root of the nose to
support the upper part of the face and the subject.
Tube - Film distance could be measured to calculate the Image magnification.
Then the 1968, Bjork designed our X-ray cephalostat research unit with a
built-in-S-inch Image Intensifier that enabled the position of the pt head to be
monitored on a T.V. screen. The pt head position in the cepholastat is also highly
reproducible.
Further more, this unit allowed the ceph x-ray examination or oral function
on the TV screen, which could also be recorded on a Video tape.
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More recently, the 1988 a Multi projection cephalometer developed for
research and Hospital environment was introduced by folow and kreiborg. This
apparatus featured Improved control of head position and digital exposure control
as well as number of technical operative innovations.
The development of such special units, especially for roentgeno
cephalometric registrations of infants has significantly contributed to the study of
the growth and development of Infants with craniofacial anomalies.
The Lateral ceph radiograph is the product of the two dimensional Image of
the skull in lateral view, enabiling to evaluate the relationship between the teeth,
bone, soft tissue, both horizontally and vertically. It has influenced orthodontist in
three major areas.
1. In morphological analysis, by evaluating the sagittal and vertical
relationship of dention, facial skeleton and soft tissue profile.
2. In growth Analysis, by taking two or more cephalograms at different time
Intervals and comparing the relative change.
3. In treatment analysis, by evaluating the alterations during and after
- Therapy.
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Pt undergoing orthodontic treatment have cephalometric x-ray taken in order to
provide diagnosis relating to their skeletal, Dental and cranial disharmonies,
Longitudinal radiographs are used to assess the growth and Therapeutic results.
In addition, the radiographs reveals pathologic condition such a pagets disease.
In order to minimize the pts exposure to radiations, High speed Film, more
each film screens, protective body drapes have been Introduced.
Although the need to minimize the radiation exposure, In universal, it is
particularly important for the pediatric pts, because they are more prone for
radiation introduced carcinogenesis.
The c ephalmotric radiograph introduced by orthodontist were widely used in
many phases 0 f dentistry. Hence it is a specialized d entofacial document which
permits qualification of changes that occurs due to treatment or growth.
The Awareness of health hazards of radiation is a valid concern.
Reduction of radiation needed to produce images of diagnostic quality is an
important goal. The introduction of "rare earth" methods of image intensification
in radiography has made this possible.
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TECHNICAL ASPECTS
The basis components of the equipment for producing a lateral cephalogram
an x-ray apparatus
an image receptor system, and
a cephalostat
THE X - RAY APP ARA TUS
The x-ray apparatus comprised an x-ray tube, transfonners, filters,
collimator, and coolant system. The x-ray tube is a high vacuum tube that serve as
a source of the x-rays. The three basic elements that generate the x-rays are a
cathode, an anode, and the electrical power supply. The cathode is a tungsten
filament surrounded by a molybdenum - focusing cup. The tungsten filament
serves as a source of electrons. It is connected to a low voltage circuit and to a
high - voltage circuit A step - down transfonner supplies the low - voltage circuit
with 10 V and a high current to heat the filament, is called thennionic emission. A
step-up transfonner supplies the high voltage circuit to create 65-90 kv. The
differential potential between the cathode and the anode accelerates the electron
cloud, which forms electron beams. The beams arc directed by the focusing cup to
strike a small target on the anode called the focal spot. Bombardment of this target
by the electrons produces the x-ray beam.
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The anode is stationery and comprises small tungsten block embedded III a
copper stem, which stops the accelerated electrons, whose kinetic energy causes
the creation of photons. Less than 1 % of the electron kinetic energy is converted to
X-ray photons; the rest is lost as heat. Although tungsten is a high atomic
substance necessary for producing x-ray photons, its thermal resistance is unable
to withstand the heat. Consequently, the copper stem acts as a thermal conductor.
This is an integral part of the coolant system, and it dissipates the heat into the oil
surrounding the x-ray tube.
X-rays are a forn1 of electromagnetic radiation; their frequency and energy
are much greater than visible light. X-rays are produced in an X-ray tube by
focusing a beam of high - energy electrons on to a tungsten target. They are able
to pass through a patient and on to x-ray film thus producing an image.
In passing through a patient the x-ray beam is decreased according to the
density and atomic number of the various tissues through which it passes in a
process known as attenuation. X-rays turn x-ray film black. Therefore the less
dense a material, the more x-rays get though and the blacker the film, i.e. materials
of low density appear darker than objects of high density.
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THE IMAGE RECEPTOR SYSTEM
An image receptor system records the final product of x-ray after they pass
through the subject. The extra oral projection, 1ike lateral cephalometric technique,
requires a complex image receptor system that consists of an extra oral film,
intensifying screens, a cassette, a grid and a soft tissue shield: The extra oral film,
which is either 8 inches x 10 inches or 10 inches x 12 inches is a screen fi 1m that is
sensitjve to the fluorescent hght radiated from the intensifying screen, Basic
components of the X-ray film are an emulsion or silver ha1ide crystals suspended
in a gelatin frame work and a transparent blue tinted cellulose acetate that serves
as the base.
When the silver halide. crystals are exposed to the radiation, they are
converted to metallic silver deposited in the film, thereby producing a latent
image. This is converted into a visible and permanent image after film processing.
The amount of metallic silver deposited in the emulsion determines film density,
whereas the grain size of the silver halide determine film sensitivity and definition.
Of all the original or primary beams that emerge from the x-ray apparatus,
only 10% have adequate energy to penetrate tissue and produce an acceptable
image on the film. The remaining 90% are absorbed by the irradiated tissuc and
emitted as secondary or scatter radiation. Since secondary radiation travels
obliquely to the primary beam and could cause fogging of the image, a grid
comprising alternative ratio opaque and radio lucent strips is placed between the
subject and the film to remove it before it reaches the film.
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The soft tissue shield is an aluminulll \\"l:dge that is placed over the c~tssclll'
or at the window of the x-ray apparatus in order to act as a filter and reduce O\er
penetration of the x-rays into the soft tissue profile. The thin edge of the shield is
positioned posteriorly over the bony area. while the thick edge is positIoned
anteriorly over the soft tissue area.
X-RA Y GRID
Used to reduce the amount of scattered radiation reaching the film and the{\
increases the contrast of the film and provide more detailed images or the
radiographic structures.
It consists of smal1 Lead strips aJTanged -+ paral1el to each other and in
convening pattern.
The pattern of the grid may be linear (or) crossed at 90 angles.
The strips are at increasing angle towards the x-ray beam known as focused
grid.
The grids effectiveness is determined by the ratio of the Length of the
strips themselves to the size of the space between the grid. The higher the grids
ratio -+ the higher will be the degree of scattered absorption and resultant Image
contrast.
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DISADVANTAGES
Faint radio - opaque patterns of the Grid will appear on the film Image.
The more Grid spaces between the strips per Inch ~ Less visible the grid Image.
It the grids moves slightly during the exposure it won't produce any visible
grid pattern. on the radiograph. This type of moving grid is known as Potter -
Bucky Grid.
X-RAY - CEPHALOST AT - FILM CASSETTE
The degree of magnification is determined by the ratio of the x-ray source
to object distance and the source to film distance. The larger the distance from the
source being imaged to the film, plane, the greater the magnifiation.
To minimize this effect the distance from the x-ray source to the Mid-
sagittal plane of the patient should be 5 FEET to ensure that the photons wi 11 travel
towards the object/film more parallel to each other, thereby reducing its
magnification.
The structure located near to the film will be magnified less than the
structure that are present near to the x-ray source(more).
The distance between the Midsagittal plane of the cephalostat and film
cassette should be 15cm.
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FILM/SCREEN COMBINATION
The extra-oral film is usually placed between the two-intensifying screens.
Under darkroom conditions. film is usually.p laced between the t wo-intensi fying
screens. Under darkroom conditions.
The ceph radiograph latent image is produced by primarily by Light from
the two screens than by the x-ray photos.
Tight contact between the screens-film = increases image sharpness.
The light-emitting screens are termed as "intensifying screens" because of
their ability to produce film images of proper density with less exposure l'nergy.
Intu111 it reduces the radiation dose received by patients.
Intensifying screens ---+ conventional (or) bluc cmitting scrccns cDatcd \\jth
calcium tungstate.
Rate earth screens are
coated with godolinium and lanthanum
emit green light
require only Yz of the x-ray energy that arc needed for conventional screens
It determines the Film Speed.
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New crystal technology has resulted in lattened, symmetrically shaped
silver halide crystals (Kodak's T -MA T Film) are more efficient than conventional
pebble shaped crystals. It will provide superior image details and sharpness while
retaining high-speed advantage.
The x-ray mi11iampere system should be reduced depending upon the speed
system used.
The remall1l11g 90% arc absorbed by the irradiated tissue and emitted as
secondary or scatter radiation. Since secondary radiation travels obliquely to the
primary beam and could cause fogging of the image, a grid comprising alternative
radio opaque and radio lucent strips is placed between the subject and the film to
remove it before it reaches the film.
The soft tissue shield is an aluminum wedge that is placed over the cassette
or at the window of the x-ray apparatus in order to act as a filter and reduce over
penetration of the x-rays into the soft tissue profile. The thin edge of the shied is
positioned posteriorly over the bony area, while the thick edge is positioned
anteriorly over the soft tissue area.
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F~ve principle densities arc recognized on plain x-ray films. They arc listed
here in order of increasing density.
1. Air/ gas: black
2. Fat: dark gray
3. Soft tissue / water: light gray
4. Bone: off-white
5. Contrast material: bright white
An object will be seen with conventional radiography if its borders lie
beside tissue of different density.
The x-ray photons emerging from the target are made up of a divergent
beam with different energy levels. The low energy photons are filtere~- out by
means of an aluminum filter. The divergent x-ray beam then passes through lead
diaphragm that fits over the opening of the machine housing and determines the
beams size and shape. Only x-rays with sufficient penetrating power are allowed
to reach the patient.
The relationship between the intensity of the x-ray beam and the focus film
distance follows the inverse square law, by which the intensity of the x-ray
inversely proportional to the square of the focus film distance.
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CEPHALOSTAT Consists of 2 ear rods Orbital pointer Nose pointer
by the two ear rods that are inserted into the ear holes so that the upper borders of
the ear holes rest on the upper parts of the ear rods. The head, which is centered in
the cephalostat, is oriented with the Frankfort plane parallel to the floor and the
midsagittal plane vertical and parallel to the cassette. The system can be moved
vertically relative to the x-ray rube, or the image receptor system and the
cephalostat as a whole can be moved to accommodate sitting or standing patients.
Vertically adjustable chairs are also used. The standardized Frankfort plane is
achieved by placing the infraorbital pointer at the patients orbit and then adjusting
the head vertically until the infra orbital pointer and the two ear rods are at the
same level. The upper part of the face is supported by the forehead clamp.
Positioned at the nasion.
If it is necessary for the cephalogram to be produced in the natural head
position, which represents the true horizontal plane, the patient should be standing
up and should look directly in to the reflection of his or her own eyes ifl a mirror
directly ahead in the middle of the cephalostat. In this case, the systcm has to be
moved vertically. To record the natural head position, the ear-rods are not used
for locking the patient's head into a fixed position but serve to place the medIal!
sagittal plane of the patient at a fixed distance from the film plane, and to assist the
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patient in keeping his ')I" her head in a constant position during expo:'lIlT.
However, the ear rods should allow for small adj ustments of the head to correct undesirable tilt or rotation.
The projection is taken when the teeth are in centric occlusion and the lips
in repose, (there should not be any peri-oral muscle strain) unless other
specifications have been recommended. The focus film distance should be usually
5 feet but different distances have been also reported.
QUALITY OF THE RADIOGRAPHIC CEPHALOMETRIC IMAGE
Image Quality is major factor influencing the accuracy 0 f cephalometric'
analysis, An acceptable diagnostic radiograph is considered in the light of two
groups as characteristics.
Visual Characteristics and
Geometric Characteristics
VISUAL CHARACTERISTICS
The visual characteristics - density and contrast are those that relate to the
ability 0 f the image to demonstrate optimum detail within anatomical structures
and to differentiate between them by means of relative transparency.
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DENSITY
Density is the degree of blackness of the image when it is viewed in front of
an illuminator or view box.
As the x-ray image is formed as a resu1t of processing in which thc sih'cr
halide crystals in the emulsion of the film exposed to the x-rays are converted to
metallic silver, the two main factors that control the radiographic density arc
The exposure technique
The processing procedure
EXPOSURE TECHNIQUE
The exposure factors related to image density are
Tube voltage
Tube current
Exposure time
And focus film distance
THE PROCESSING PROCEDURE
Film processing consists of developing, rinsing and washing, drying and
mounting the exposed film. An invisible image, produced when the silver halide
crystals are exposed to the x-rays, is altered to a visible and permanent image on
the film by chemical solutions. The image density is directly proportional to
temperature of the developing solution and developing time.
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The size of the silver halide crystal in the film emulsion determined the film
speed. A film with large grain size produces greater density than a film with small
gram SIze.
CONTRAST
Contrast is the difference in densities between adjacent areas on the
radiographic image. Factors controlling the radiographic contrast are:
Tube voltage - the kilovoltage peak has the most effect on radi-ographic
contrast. When the kilovoltage peak is low, the contrast of the film is high,
on the other hand, if the kilovoltage peak is high, the contrast of the film is
low, and the film has long scale contrast.
Secondary radiation or scatter radiation - the secondary radiation caused by
low energy x-ray beams decreases the contrast by producing film fog. The
amount of secondary radiation is directly proportional to the cross-sectional
area, thickness and density of the exposed tissues as well as the kilo voltage
peak. Several devices have been incorporated into the cephalometric
system to l' emove secondary l' adiation, including a n a luminum filter, 1 ead
diaphragm and grid.
Subject contrast - t his refers to the nature and properties a f the subject, such as thickness, density and atomic number. ..
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Processing procedure - the tempe_'ature of the developing solution affects
image contrast. The higher the temperature the greater the contrast.
Density and contrast are the image ch,aracteristics that are usually affected
when the kilo voltage peak is altered. However, only the radiographic density can
be altered without changing the contrast when the kilo voltage peak is constant and
the milliampere second is altered.
II GEOMETRIC CHARACTERISTICS
The geometric characteristics arc
Image unsharpness
Image magnification; and
Shape distortion
These three characteristics are usually present in every radiographic image,
owing to the nature of the x-ray beam and its source. X-rays, by their nature. arc
divergent beams radiated in all directions. Consequently, when they penetrate
through a three dimensional objects such as a skul1, there is always some distortion
of the shape of the object being imaged.
The focal spot, from which the x-ray originates, although small, has a finite
area, and every point on this area acts as an individual focal spot for the
origination of x-ray photons. Therefore, most of the x-rays emitted from the focal
spot are actually producing al shadow of the object.
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IMAGE UNSHARPNESS
Image unsharpness is classified 'into three type according to etiology
namely: geometric, motion and material. Factors that influence the geometric
unsharpness are size of the focal spot, focus-film distance and object film distance. In order to decrease the size of the penumbra, the object-film distance should be ,
decreased and the focus-film increased. Geometric un-sharpeners is defined by
the following equation.
Geometric un-sharpness = (focus spot size x object-film distance) / focus film distance.
IMAGE MAGNIFICATION
Image magnification is the enlargement of the actual size of the object.
Factors influencing image magnification are the same factors as those that
influence geometric unsharpness. The percentage of magnification can be
calculated by the equation: so, for example, if the focus-film distance is 190 em
and the object-film distance is 10 cm, the percentage of magnification of mid
sagittal structures in the lateral cephalogram will be 5.5%.
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SHARP DISTORTION
Shape distortion results in an image that does not colTespond proportionally
to the subject. [n the case of a skull, which is a three-dimensional object the
distortion usually occurs as a result of improper alignment of the film and central
ray. This kind of distortion can be minimized by placing the film parallel to the
midsagittal plane of the head and projecting the central ray perpendicularly to the I
film and the midsagittal plane. The lateral cephalogram is further distorted by the
foreshortening of distances between points lying in different planes and by the radial displacement of all points and structures that are not located on the central
ray.
FACTORS AFFECTING THE QUALITY OF THE IMAGE
Quality of the image is controlled by the manufacturer of the x-ray
equipment and by the operator. In general, the manufacturer provides pre
programmed exposure factors consisting of mill amperage, kilo voltage peak and
exposure time, which enable image density and thickness are varied. The
variations in the exposure factors depend on the type of x-ray machine, target-film
distance, the film-screen combination, and the grid chosen. Usually the
milliamperage setting does not exceed 10 mA, the kilo voltage is about 60-90 kV,
and the exposure time is not longer than k3 seconds. The grid ratio is 5: I, with 34
lines pcr centimeter. Quality of the image is controlled by the manufacturer of the
x-ray equipment and by the operator. In general, the n1anlll;\cturel plm ilks prc
programmed exposure factors consisting of milliamperage, kilo voltage peak and
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expo_,ure time, which enable image density and thid:ness are varied, The
variations in the exposure factors depend on the type of x-ray machine, target-film
distance, the film-screen combination, and the grid chosen. Usually the
milliamperage setting does not exceed 10 mA, the kilo voltage is about ()()-90 k\'.
and the exposure time is not longer than k3 seconds, The grid ratio is 5: I, with 34
Jines per centi meter.
The operator can adjust these exposure factors when subject densities as well
as thickness are altered, in order to maintain the overalJ image density of different
radiographs. The exposure time is the commonest factor to change, since by
altering, it has the greatest effect, especially image density. Altering the
mil1iamperage alone is not recommended, since the 15 mA range on dental X-rays
machine is too smaIJ to be varied and the difference in image density that can be
achieved by altering the milliamperage alone are almost undetectable. Altering
the kilo voltage peak affects not only image contrast but also exposure time, since
increased kilo voltage increases the number of photons as well as the amount of
secondary radiation. In order to reduce secondary radiation, exposure time has to
be reduced. Ani ncrease 0 f 1 5 k V necessitates a halving 0 f t he exposure time.
Therefore, in order to maintain image density and contrast of subjects with
different thickness and density, the miIJiamperage and kilo voltage have to
correspond with the type of film and intensifying screens recommended by the
manufacturer.
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Image density and contrast can also be atlcctcd by film processing. When
using an automatic film processor. density and contrast are both controllcd by the
temperature of the developer and by the d,eveloping time,
The optimum
temperature of the developer and developing time are 68"F and 5 minutes
respectively.
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SOURCES OF ERROR IN LATERAL CEPHALOMETRY
According to Moyers et al (1988), stated that ceph, radiography may be used
I. for gross inspection
2. to describe the morphology and growth
3. to diagnose anomalies
4. to forecast future relationship
5. to plan treatment
6. to evaluate treatment results.
Except the Gross inspections. All the fuctions are principally Governed
with the identification of landmarks and calculation of various linear and angular
variables.
If any constant conclusion has to be drawn from ceph. Data, it is equally
important to consider both validity and the reproducibility of the method used.
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VALID ITY
Validity (or) accuracy, is the extent to which in the absence of measurement
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The term reliabilit_. is used as a synonym for reproducibility but it is
sometimes also used in a broader sense that encompasses both validity and
reproducibility.
Errors of Ceph Measurements
The errors may be due to
1. radiographic projection errors
2. errors within the measuring system
3. errors in the landmark identification
Radiographic projection elTors
During the recording procedure, the convention radiograph is subjected to
Magnification
Distortion
Magnification:
The use of long focus object and short object - film distances has been
recommended to minimize the projective errors (Franklin 1952, Van Aken 1963).
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The re!atively long focus - film distance (more than 2xn cm) docs 110
significantly after the magnitude of the Projection en'or (Carlson 19()7, Ahlquist
eta!., 1986, 1888).
The use of Angular Measurements rather than linear measurements is a
consistent way to eliminate the impact of magni fication (Adam 1940). Because the
angular measurements remains constant regardless of the enlargement factor.
DISTORTION
Distortion occurs because of different magnification between different
planes.
Though the landmarks which are located in midsagittal plane are used,
some of the landmarks are affected by distortion due to their location in a different
depth of field. In this instance, linear and angular values are variously affected.
In case, where the linear distance are fore shortened, it can be compensated
if the relative displacement of the landmark from the Midsaggital plane is known.
For this purpose, A combination of lateral ceph, and frontal ceph is taken
(Broadbent 1931, m Savara et. aI., 1966). But the drawbacks are that only fcw
landmarks can be located on this kind of radiograph.
Projected angular measurements are distorted according to the laws of
perspective.
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Distortion of the Bilateral landmarks can be corr.pensated to some extent by
taking the midpoints of 2 points.
Bilateral structures in the symmetric head do not superimpose in a lateral
ceph. Because the X-ray beam expands as it passes through the head causing a
divergence between the Images of all bilateral structures (except) those along the central beam.
The lateral ceph tracing is inadequate to describe the head that is truly
asymmetrical (Grayson etal., 1984).
Misalignment or tilting of the cephalometric components (focal spot) the
cephalostat, and the film with respect to each other, as well as rotation of the
patients head in any plane of space will introduce another factor of distortion.
Malposition of the patient in cephalostat produces an symmetric distortion
for both linear and angular measurements on lateral eeph (baumrind and frantz
1971 )
The rotation of pt head upto SO does not produce significant error (or)
distortion. If it is more than SO _ Significant error (or) distortion in produced.
Therefore, it proper care in obtaining the radiographic records is taken, the
errors obtained during this phase, can be considered as negligible errors.
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Errors in Landmark Identification
It is considered to be the main source of cephalometric error.
If mainly depends upon the following factors
1. The quality of the radiographic Image
2. The precision of landmark definition and the reprouctibility of landmark
location.
3. The operator and the registration procedure
Quality of the radiographic Image
It is expressed in terms of
Blur
Sharpness
Contract
Notice
Sharpness It is the subjective perception of the distinctness of the boundaries
Blur of a structurc.
Is the distance of the optical density change betwccn thc bOlIIIt!;llll'
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It res:llt from 3 factors.
Geometric Un sharpness
Receptor Un sharpllcss
Motion Un Sharpness
Movement of the object, the tube (or) film during the exposure results in
Image Blur.
By increasing the current, it is possible to reduce the exposure time, thus
reducing the effect of movement.
Blur From thc scattered radiation can be reduced by using the grid at the
Image receptor end.
The major parameter which influence the Sharpness of the cephalogram is
the focus to film distance,
The voltage capacity Kv of the ceph equipments.
Contrast is the magnitude of the optical density differences between a structure
and its surroundings.
Ifplays a important vole in the radiographic Image quality.
Increases subjective perception of sharpness.
But excessive contrast leads to loss of details, oweing to blackening of
regions of low absorption.
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It is determined by
I. The tissuc being examined
2. the receptor
3. The level of Kv used.
The important parameters are
Film - cassette system
K v - Level used
Noise - refers to al1 factors that disturb the signal in a radiograph.
It is related to
1. The radiographic complexity of the region - this is known as noise of
Pattern, structure, (or) Anatomy.
2. Receptor MottIe - this is known as a quantum noise. It depends on the
sensibility and the number, of radio - sensitive grains present in the film.
Noise can be reduced by the use of cephalometrical laminography (Ricketts
1959). But in the conventional ceph it is unavoidable.
These types of errors can be Minimized by using the film of high quality.
The advantage of using digitized technology
1. enhances the sharpness, and contrast and reduce the noise.
2. Decrease in the radiation dose dour to the lower exposure times (Wenzcl
1988).
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Precision of Landmark Identification and
Reproducibility of Landmark Location
A clear unambiguous definition of the landmarks chosen is of the utmost
importance for cephalometric reliability.
Acc to Richardson 1966, Brumrjnd and Frantz 1971, Broach et. aI., 1981,
Cohen 1984, Methnke 1989. Some ceph landmarks can be located wjth 1110re
precision than other landmarks.
Geometrically constructed landmarks and landmarks identified as points of
change between convexity and concavity orten prove to be very unreliable (PM).
The radiological complexity of the region plays a important role, in making
some landmarks more difficult to ldenhfy.
Mjethke (1989) stated that the landmarks that can be located 1110re exactly
are incision superior Incisal, and jncision inferior incisal with a valve of mean X
and Y standard deviation as of 0.26 mm and 0.28 mm. The value up to 2.0 111111
were considered to be of acceptable reproducibility. 25% of the reference points
showed more than 2.0 mm (ie. Poor Precision).
Adenwa]]a et.a!., 1968, stead that the anatomic points and the condyle
cannot be located accurately and consistently on the lat ceph that is taken in the
closed position.
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Some landmar', are reliable in either Horizontal (or) vertical Planc--' depending on the
topographic orientation of the anatomic structures along which
their identification is assessed. (Brumrind and Franz 1971).
The validity of the Individual landmarks will also defend on the use of the
orthodontist is mak ing of them.
Baumrind and Frantz (1971) Pointed out that the impact of the error in
landmark location on the angular and linear measurements is a function of 3
variable.
1. The absolute magnitude of the error in landmark location.
2. The relative magnitude (or) the linear distance between the landmark
considered for that angular (or) liner measurement.
3. The direction from which the line connecting the landmark intercepts
their envelope or error.
The envelope is the pattern of the total elTor distribution.
Ceph landmarks have a non _ circular envelope of error, the advantage
error introduced in Linear measurement will be greater if the line segment
connecting them to another point intersects the wider part of the envelope.
WISTH AND BOE 1975, conducted a study to determine the reliability of
the ceph soft tissue measurements by analyzing comparable hard and soft tissue
measures, they concluded that the error of the landmark location were generally
the same.
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.
The errors in thc landmark for points or lines common to measurements can
generate misleading topographic correlations, which may obscure (or) exaggerate
a true biologic correlation (bjork and Solow 1962, Solow 1966, Houston 1983).
Errors in landmark identification c an be r educed if thc measurements arc
repeated and their values are averaged.
The location of the landmark is more accurate at the second time than at the
1st judgments (Miethke 1989).
More the replication _ smaller will be the total error.
More rcplication should be performed for the evaluation of the Individual
changes (Baumuind and Frantz 1971).
For specific landmarks, an application of alternative techniques of
radiological registration can minimize the error oflandmark (eg.) identification.
If the mandibular condyle is used as an important landmark in the ceph
analyses then the open mouth cephalogram should be taken and it should be
subsequently superimposed on the cephalogram taken in the centric occlusion can
provide thc accurate mcasurcmcnt. (Adcnwalla CUll., 1988).
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The operator and the registration Procedure
Acc to kvam and kraystad, Stated that the operators efficiency, alertness
and training and his (or) her working condition will affect the magnitude of the
ceph error.
Houston 1983, stated that the most important contribution to the
Omprovement in the landmarks Identification were depended upon the experience
and calibration.
A good method to reduce this error consists of calibration and periodical
recalibration test to establish specific confidence limits of reproducibility for each
observer.
When the serial records are being analyzed, it has been suggested that all
the records of one patient should be traced on the same occasion in order to
minimize the error variance within the individual observes.
Whilc tracing the scrial rccords of onc patient, if is better to usc template of
certain structures.
Ace to Houston 1983, after collection the ceph measurements should be
checked for wild values, Because sometimes it can be attributed to incorrect
identification of a landmark (or) misreading of an instrument.
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ERRORS IN GROWTH PREDICTION AND SUPER
IMPOSTIONT TECHNIQUES Growth prediction
Growth prediction is quite different because of the following reasons (or) factors.
I. The wi_e range of morphological differences. 2. The varying rates and the direction during the growth period.
3. The varying influence of modifying environmental factors.
4. The variation in thc timing of thc different arcas of activc growth.
5. The lack of correlation between the size of the facial structures at an early
age and the ultimate adult size.
Rakosi (1982) stated that the sources of error in growth prediction are
1. Variable growth rates in regional growth sites.
2. Growth pattern not being fully taken into account
3. the relationship of form and function.
Variable growth rates in regional growth Sites.
1. The mean annual rate of increase in the base of the maxilla between the age
8 - 14 is appro x 0.8mm compared 1.9 mm in the mandibular base.
2. During the same period, the growth ratio of S-N length to the mandibular
base ranges from 1: 1.35 to 1: 1.65 and that S-Ar to Ar-Go is approx I: 1.3
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Growth pattcrn not bcing filly takcn into account
1. Many method does not include the growth pattern and the paticnts are
assessed only relation to the population mean.
2. Growth rates vary quite differently for different growth types.
3. Horizontal growth changes are more predictable than the vcrtical growth
changes.
fb-r1Y' The relationship Of...fI:GHl and function
1. The inter - relationship of form and function is not taken into the
consideration.
2. For example, soft tissue influences in a patient with madibular
retrognathism can alter the tendency for compensatory proclination of the
lower incisor to a dysplastic retroclination (Melsen nd Athanasiou 1987).
The simplest method of prediction assumes that the growth will take place as a
linear expansion along the long axis of the structure being examined and that its
amount is quantified as average growth increments added progressively through
time. (Johnston 1975, Thompson 1977). The drawback of this mcthod is that the
individual variation is not taken into account.
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Individual prediction has been attempted by analyzing the existing t__cial pattern.
I. However the relationship of the existing facial dimension and of previous
growth changcs to future growth changes has not been found to be
predictive value. (Bjork 1955, Hixon 1972).
2. With some exceptions in children with extreme skeletal pattern (Nanda
1988).
Prediction of growth direction, particularly for Mandibular rotation has been
1. attempted in implant studies analyzing certain structural features (Bjork
1968).
2. anothcr study by thc mcthod of sutural growth prcdiction by (Bjork I ()(J3)
(42 children, 2 ceph taken, 4 years apart before & after pubertal growth
spurt)
Result: There was no absolute correlation between the scores of different
criteria and mandibular growth rotation during the four years of
observation.
The main elTor in the Growth prediction procedures is the lack of validity
of any method until now proposed, when it comes to the predication of the
Individual.
In the light of these results, it is even doubtful, whether thc ccph films
contain enough infomlation about the future growth, which will of predicative
valve.