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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

:-:

l

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

15

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.

Ih -,

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.

17

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. ..

I,)

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.

19

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%.

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

21

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.

) )

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.

.

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.

VALID ITY

Validity (or) accuracy, is the extent to which in the absence of measurement

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).

\2

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.

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.

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'

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.

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).

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.

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.

.

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).

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.

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

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.

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.