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1. CEPHALOMETRICS- INSTRUMENTATION AND X – RAY GENERATION
PRINCIPLES
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 measurement of the dry skull from osteological landmarks called
craniometry, was then applied to living subjects so that a longitudinal growth study could
be undertaken. Since the measurements were taken through skin and soft tissue
coverage, their accuracy was questionable.
By the discovery of X-rays by Roentgen in 1895, a radiographic head image could
be measured in two dimensions, thereby making possible the accurate study of
craniofacial growth and development.
The credit of bringing the X – rays to the field of dentistry is given to C. Edmund
Kells. Soon after Roentgen announced his discovery in December 1895, Kells went to
work to make the capabilities of the X-ray available to the dental profession and thereby
forever changed the way dentistry would be practiced.
The measurement of head from the shadows of bony and soft tissue land marks on
the radiographic image became known as roentgenographic cephalometry.
(Krogman &Sassouni,1957)
HISTORY—
Cephlometrics like virtually all advances in healing arts is based on older methods.
Craniometrics was already being used to measure dried skulls, direct cephalometric
measurement was applied to external structures on the living and radiography was an
accepted clinical procedure. During the same period Pacini was also X-raying skulls in
Europe.
B. Holly .Broadbent merged those very different techniques to measure all three
dimensions of both internal and external structures of the heads of living subjects.
During 1920’s Broadbent refined the craniostat that was used to orient skulls for
measurement into a craniometer by the addition of metric scales. That proved to be the
1

2. first step in the evolution of the craniostat into a radiographic cephalostat. This direct
measuring instrument was later converted into a radiographic craniometer.
INTRODUCTION TO CEPHALOMETRICS—
Broadbent in 1931 introduced cephalometric radiography to overcome the
inappropriateness of earlier techniques in which the landmarks in the skull of the living
child had to be approached through the skin and soft tissue. Hofrath in Germany at the
same time developed his cephalometer independently.
At that time skull holders were used for craniometric studies . The first problem
Broadbent faced was to design and build a head holder along the lines of the skull holders
and second to find a means of recording precisely the craniometric as well as
cephalometric landmarks of the face and cranial base of the living head.
Keeping the Reserve craniostat as a basis, head holder was made and registering
of the internal landmarks of face and cranial base was made through perfection of a
roentgenographic technique that records these points accurately on the photographic film.
To test the accuracy of this method experiments were first made with skulls on a
specially constructed craniostat. The skulls were prepared by drilling a minute hole at
many of the internal and external cranial landmarks and inserting very small pieces of
lead that would register their exact position on the photographic film. Similar bits of lead
were placed on dental and facial points. The skulls were then clamped in the instrument
with the under surface of the upper side of the ear holes (external auditory meatus)
resting on the supports and the skull fixed in the Frankfort relation.
After the sites of the lead pieces were plotted in graphic projection in the sagittal
plane and their relationships defined by measurement, the skulls were x-rayed for the
lateral picture. Each skull was then rotated ninety degrees and measured in the frontal
plane, the graph made, and the frontal x-ray picture taken.
Superimposing the roentgenograms of the lateral and frontal projections on their
respective graphs, gave a measure of the technical precision and reliability of this
method.
2

3. Special Reserve Craniostat built for roentgenographic studies of skull
Two relations are necessary to produce two or more identical X- ray pictures of a skull.
1. Skull to the instrument .
2. Source of X-rays to the instrument.
Experimentally it was proven that most useful pictures were those made when the
path of the central ray coincided with the line joining the tops of the two ear supports
and the tube placed 5 feet or more from the middle of the craniostat.
The roentgenograms were measured with the aid of a Universal drafting machine
fitted with millimeter scales.
The head holder was designed on the working principles of the craniostat and built
for use in conjunction with the standard junior dental chairs, through the generosity of
Mrs. Chester. C. Bolton and her son Mr. Charles. B. Bolton.
The head holder was supported on a fixed base, above the child’s size dental chair
that has had the usual head rest removed. The chair does not come in contact with the
head holder but may be raised or lowered to permit comfortable adjustment of the child’s
head to the instrument.
The head rests on the upper most side of the calibrated ear rods, inserted into the ear
holes to allow centering of head. Then head is adjusted till the lowest point of the inferior
border of the left orbit is at the level of the top of the ear supports as indicated by the
orbital pointer.
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4. There was a front attachment which supported the frontal cassette and it also carried
a rest for the root of the nose.
The Head Holder with cassette in its place for lateral roentgenogram.
The head could not be rotated on a vertical axis, so two X- ray tubes, like one for
frontal and one for lateral picture were used. The resulting pictures register precisely the
desired craniometric landmarks of the cranial base and face in three planes of space.
Subsequent pictures at certain ages, in children revealed areas of non-growth in the
cranial base. These areas were used to precisely relate the pictures and measure changes
in the other parts.
So the areas in the cranial base that have not changed, offer a more precise basis
for relating tracings and consequently a more accurate method of measuring growth
and development in the living head. Therefore when we have an unchanged base
common to two or more subsequent pictures of the same child, like the area including
Sella Turcica and Nasion of this series, we superimpose them on these landmarks.
This roentgenographic method has the added advantage of disclosing changes, not
only of the teeth that have erupted, but it clearly shows the rate and amount of growth and
path of eruption of the unerupted teeth. With the opportunity to record the structural
changes along with means of measuring increase in size, we have a morphological as
well as a quantitive study.
The lateral cephalometric radiograph (cephalogram) itself is the product of a two-
dimensional image of the skull in lateral view, enabling the relationship between teeth,
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5. bone, soft tissue, and empty space to be scrutinized both horizontally and vertically. It
has influenced orthodontics in 3 major areas:
• In morphological analysis, by evaluating the sagittal and vertical relationships of
dentition, facial skeleton, and soft tissue profile.
• In growth analysis, by taking two or more cephalograms at different time
intervals and comparing the relative changes.
• In treatment analysis, by evaluating alterations during and after therapy.
X – RAY GENERATION
The basic components of the equipment for producing a lateral cephalogram are-
• X- ray apparatus
• An image – receptor system
• A cephalostat
X- Ray apparatus:
The basic apparatus for generating X- rays comprises of an X- ray tube, transformers,
filters, collimators, and a coolant system, all encased in the machine’s housing. The X-
ray tube is a high-vacuum tube that serves as a source of the x- rays. The 3 basic elements
that generate the x- rays are--
1. A cathode—a component of which is the filament that serves as source of
electrons.
2. An anode—(target) at which the beam of high speed electrons is directed.
3. Electrical power supply – through various circuits control tube performance.
Cathode – it is composed of 2 parts mainly
• Filament
• Focusing cup.
The filament, the source of electrons with in the X- ray tube, is a coil of Tungsten wire
about 0.2 cm in diameter and 1 cm or less in length. It is mounted on two strong stiff
wires that support it and carry the electric current. These 2 wires serve as a connection for
both high and low voltage electric sources. The filament is heated through incandescence
through a range of temperatures by varying the voltage (around 10 volts) across the
filament from a step down transformer in a low – voltage circuit.
5

6. The X –Ray tube with major component
The hot filament emits electrons at a rate proportional to its temperature by
thermoionic emission. The electrons lost by the filament form a cloud or space charge
about the filament and are replaced in the tungsten atoms from the negative side of the
high-voltage circuit, which is connected to one of the filament mounting wires.
Dental X-ray machine circuitry
In the figure; A is Filament step-down transformer ; B, filament current control (mA
switch; C, autotransformer; D, kVp selector dial (switch); E, high voltage transformer; F,
x ray timer (switch); G, tube voltage indicator (voltameter); H, tube current indicator
(ammeter); I, x- ray tube.
6

7. A milliamperage control, controls the flow of heating current through the filament,
thus thereby modulates the quantity of electrons that the filament emits, which inturn
controls the tube current and the number of X- ray photons subsequently produced.
Focusing cup: The filament is located in a focusing cup, a negatively charged concave
reflector of molybdenum. The focusing cup electrostatically focuses the electrons emitted
by the incandescent filament into a narrow beam directed at a small rectangular area on
the anode called the focal spot. Electrons move in this direction because of the strong
electrical field imposed between the cathode and anode.
Anode-
Anode is composed of a tungsten target and Copper stem. The purpose of the target in
an X- ray tube is to convert the kinetic energy of the electrons into X- ray photons.
Tungsten is usually selected as the target material because it represents an effective
compromise between the features of the ideal target material which are high atomic
number, high melting point and low vapor pressure at the high working
temperatures of an X-ray tube.
A target material with a high atomic number is best because it is more efficient for the
production of x- rays. High melting point is one of the major considerations in selection
of target material as 99% of kinetic energy of electrons is converted to heat. The low
vapor pressure of tungsten at high temperatures also precludes compromising the vaccum
in the tube at the high operating temperatures.
Thermal conductivity of tungsten is low, so it is embedded in a larger block of
copper, which dissipates heat. In addition, insulating oil may circulate between the
glass envelope and the protective tube housing. This type of anode is called stationary
anode.
Another method of dissipating the heat from a small focal spot is to use a rotating
anode. In this case the tungsten target is in the form of a beveled disc that rotates when
the tube is in operation. As a result of this arrangement the electrons strike successive
areas of the target as it rotates. This effectively widens the focal spot and distributes heat
over this expanded area. Such rotating anodes are not used in conventional dental x- ray
machines but may be used in cephalometric units and in medical x- ray machines.
7

8. Radiographic image quality is dependent in part on the geometry of the target. The
sharpness of the radiographic image increases as the size of the radiation source , the
focal spot, decreases. To take advantage of the benefits of a smaller focal spot, yet
effectively distribute the bombarding electrons over the greater surface of a larger target,
the target is placed at an angle with respect to the electron beam in the tube.
Target placed obliquely to the central ray.
The projection of the focal spot perpendicular to the electron beam (the effective focal
spot) will be smaller than the actual size of the focal spot. The use of an anode with the
target angulated such that the effective focal spot is smaller than actual focal spot size is
referred to as the Benson line focus principle.
Power supply-
The primary functions of the power supply is to provide
1. A current to heat the X –ray tube filament by use of a step-down trasformer
2. A potential difference between anode and the cathode.
The filament step down transformer reduces the voltage of the incoming alternating
current to less than 10 volts and its operation is regulated by the filament current control
switch, which adjusts the current flow through the low-voltage circuit and thus the
filament. This in turn regulates the heating of the filament and thus the quantity of the
electrons emitted. The electrons emitted by the filament travel to the anode and
constitute the tube current.
8

9. The output of autotransformer is regulated by Kvp selector dial, which select
varying voltages and it is applied to the primary of the high voltage transformer, which
controls the voltage between the anode and cathode of the X- ray tube. The high voltage
transformer provides the high voltage required by the x- ray tube to accelerate the
electrons and generate x rays. It accomplishes this by boosting the voltage of the
incoming line current to 60 to 100 Kvp.
Since the line current is an alternating current (60 cycles/ sec), the polarity of the X
– ray tube will alternate at the same frequency. When the electrons strike the focal spot
of the target, some of their energy converts to x- ray photons. X-rays are produced at the
target with greatest efficiency when the voltage applied across the tube is high. Thus the
intensity of x-ray pulses will tend to be sharply peaked at the center of each cycle.
During the following half (or negative half) of the cycle, the polarity of the AC
reverses and the filament becomes positive and the target negative. At these times
the electrons stay in the vicinity of the filament and do not flow across the gap
between the two elements of the tube. This voltage is called inverse voltage or reverse
bias. No x rays are generated during this half of the voltage cycle. Thus, when an x- ray
tube is powered with 60-cycle alternating current, 60 pulses of x- rays are generated each
second, each having duration of 1/20 second. This type of power supply circuitry, where
alternating high voltage is applied directly across the tube, limits X- ray production to
half of AC cycle is said to be self or half wave rectified.
A tube energized with a self-rectifying power supply must not be operated for
extended periods or the temperature of the target may reach the point of electronic
emission. If the target gets that hot, there is the possibility that during the negative half-
cycle the inverse voltage will drive electrons to the filament, causing it to overheat and
melt. The glass envelope may also be damaged if the electrons are driven in the wrong
direction by the reverse bias on the tube.
Some units have half wave tube rectification where the inverse voltage is
prevented from being applied across the tube during the negative half of the cycle.
Full wave rectification units are also used in some machines, that allows both
positive and negative phases to be utilized for X- ray production.
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10. Timer-
The timer completes the circuit with the high-voltage transformer. This controls the time
that the high voltage is applied to the tube and thus the time during which tube current
flows and x rays are produced. Before the high voltage is applied across the tube,
however, the filament must be at the proper operating temperature to assure an adequate
rate of electron emission. It is not practical to subject the filament to prolonged heating
at normal operating current.
Production of X- rays
The kinetic energy of electrons in the tube current is converted into X- ray photons
at the focal spot of an X-ray tube by two mechanisms
1. Bremsstrahlung radiation
2. Characteristic radiation
Bremsstrahlung radiation
Bremsstrahlung interaction,the primary source of an X-ray photons from an X- ray
tube , is produced by the sudden stopping or braking of the high speed electrons at the
target . The electrons are accelerated by the high voltage applied across the gap between
the filament and the target of the x – ray tube. When the electrons interact with the
electrostatic field of target nuclei of collide with nuclei, their direction of travel is altered.
This process of rapidly decelerating the high speed electron is called inelastic collision
and gives rise to Bremsstrahlung or braking radiation. This deceleration causes them to
lose some kinetic energy, which is given off in the form of photons of electromagnetic
radiation with an energy equal to that lost by the deflected electrons.
Bremsstrahlung interaction generates photons having a continuous spectrum of
energy. The reasons for this continuous spectrum are as follows:
1. The continuously varying voltage difference between the target and filament,
which is characteristic of half-wave rectification, cause the electrons striking the
target to have varying levels of kinetic energy.
2. Most electrons participate in many interactions before all their kinetic energy is
expended. As a consequence, an electron will carry differing amounts of energy at
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11. the time of each interaction with tungsten atom that results in the generation of an
x – ray photon.
3. The bombarding electrons pass at varying distances around tungsten nuclei and
are thus deflected to varying extents. As a result, they give up varying amounts of
energy in the form of Bremsstrahlung photons.
Characteristic radiation
It occurs when a bombarding electron displaces an electron from a shell of the
target atom, thereby ionizing the atom. So an electron from an outer shell (higher energy
level) occupies this vacancy, and gives off a photon, with an energy equivalent to the
difference in the two orbital binding energies. The energies of characteristic photons are a
function of the energy levels of various electron orbital levels and hence are characteristic
of the target atomic composition. Characteristic radiation is only a minor source of
radiation from an x- ray tube.
Image receptor system:
An image receptor system records the final product of X-rays after they pass through
the subject. The extraoral projection,like the lateral cephalometric technique, requires a
complex image receptor system that consists of an extraoral film, intensifying screens, a
cassette, a grid, and a soft- tissue shield.,
Films :
The X- ray image formed when X – rays pass through the patients head is recorded
by a film- screen combination enclosed in a cassette. Film size is usually 8X10” or
10X12” for some other purposes.
Basic components of the x-rays film are an emulsion of silver halide crystals
suspended in a gelatin framework 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
the metallic silver image. This is converted into a visible and permanent image after film
processing. The amount of metallic silver deposited in the emulsion determines film
11

12. density, whereas the grain size of the silver halide determines film sensitivity and
definition.
Intensifying screens:
They are used in pairs together with a screen film to reduce the patient’s exposure
dose and increase image contrast by intensifying the photographic effect of x- radiation.
X – rays are more readily absorbed by heavier atoms of materials. So the
intensifying screen is made of crystals of a high atomic no. material that absorbs X – rays
efficiently and convert it into light energy (fluoresce). This is absorbed by the light
sensitive radiograph, which is then processed to produce the radiograph.
The most commonly used screens are made of calcium tungstate,barium lead
sulphate crystals and are sensitive to X- rays generated by conventional dental X – ray
machines (60-90 Kvp).
More efficient rare earth intensifying screens using terbium activated gandolinium
oxysulfide and thalium activated lanthanum oxybromide can be used with special X- ray
film sensitive to the green spectral emission of rare earth screens. Calcium tungstate
screens emit blue light.
Films and screens may have fast, medium or slow speed depending on the crystal
size and thickness. High speed films and screens produce les detail and less sharp images
in radiographs.
Cassettes
They are light tight boxes used to hold the screens and film in intimate contact.
Cassettes contain two screens with a double emulsion film sandwitched between the
screens. Cassettes may be equipped with front and back screens with different speeds.
With the high speed screen in the back if a single emulsion film is used, only one screen
is needed in the cassette, but it may require more exposure.
Films should be handled carefully, by keeping them away from excessive
temperature or humidity. Rapid removal of the film from the cassette can produce
electrical discharges that can cause artifacts in the radiograph.
Grids
Scattered or secondary radiation causes the film to fog. It can be prevented by
placing a grid between the patient and the film. Grid consists of alternating strips of
12

13. radiopaque and radioluscent material. While the lead strips block some of the X- rays
coming from the tube, they effectively block the scattered rays that are traveling in
directions oblique to the X-ray beam.
The grid lines that appear in a radiograph can be avoided by moving the grid in a
direction that is at a right angle to the line during film exposure. Such a grid is called a
Bucky grid or a potter- Bucky diaphragm. Non –moving grid is called a stationary grid.
CEPHALOMETRIC RADIOGRAPHY--
Cephalometric radiographs can be made with conventional dental X- ray machine
used for making intra oral radiographs. These machines usually use a self rectified X- ray
tube with a stationary anode. 10 – 15 mA and 70 Kvp (peak kilovoltage). For
cephalometric purpose, the tube head of this type of machine is fixed to a stationary
device to direct the X- ray beam in a fixed position relative to the patient and film. With
medium speed film and screens, the exposure time is approximately 0.6 to 1.2 seconds.
X-ray generators capable of producing x- ray beams with great intensities of x-
radiation are available. These machines use either a 100 m A current or 100 Kvp a
rectified electric current to the tube and/or a rotating anode in the x- ray tube.
These facilitate the use of short exposure time ( in the region of 1/60th
of a second),
which can reduce motion unsharpness in the radiographic image.
Panoramic X – ray machines with the capability of aligning the tubes for
cephalometric radiography is also available.
PATIENT POSITIONING --
The patient is positioned differently on the X- ray beam for lateral, PA and oblique
views of the skull. Patient is in an upright position, either sitting or standing, with the X –
ray generator and film at a fixed height and a system for raising and lowering the patient
by using a motorized chair.
A cephalostat or head holder is used to stabilize the patient in a fixed position in
the X- ray beam. It basically consists of two ear rods that move simultaneously or
individually along the path of the central ray. The device holds the patient steady with the
13

14. central ray in the transmeatal axis. Many adjustments have been calibrated to standardize
patient position and caphalostats with measurement capabilities are called cephalometers.
Standardizing the Frankfort horizontal plane is accomplished on a cephalometer
with an orbital pointer. The pointer consists of a vertically adjustable horizontal rod that
is positioned at the patients’ orbitale. Another method is the use of a forehead positioner
located at Nasion. Some operators prefer to have the patients oriented to natural head
position, it is accomplished by asking the patient to look directly into the mirror image of
their own eyes.
To orient the patients mid saggital plane in a vertical position, a vertical line may
be placed on the mirror where the center of the cephalometric image is located as seen
from patient position.
Although the patient to source distance is standardized as 5 feet, the patient to film
distance may vary, thus varying the magnification. To calculate this, a radiopaque scale is
kept in the midsaggital plane and its magnification is measured.
Most cephalometers can be rotated through 360° on the vertical axis in contrast to
Broadbent – Bolton Roentgenographic Cephalometer, where two X- ray tube head
film holder systems were used at right angles to each other.
The orientation of the Frankfort plane around the transmeatal axis is important
because the superposition of different parts of the skull upon each other can occur with
different Frankfort plane positions it should also be standardized to make reliable
comparisions.
In dental cephalometric radiography, position of patients’ mandible is not fixed in
the cephalometer. Cephalometric radiographs are made with patients’ teeth in occlusion,
it can also be made in rest position or wide open position if desired.
THE THIRD DIMENSION
Clinical orthodontics is yet to fully utilize Broadbent’s contributions. He gave us a 3
dimensional analysis. But still in most clinical practices lateral roentgenographic view is
utilized. The lateral view is to work with and the patient is also much more recognizable
than in frontal (PA) view, especially with soft tissue enhancement. But it is not enough.
14

15. We treat in 3 dimensions, and the width dimensions that are visualized on the frontal
view are crucial in many cases. In these days of increasing awareness of the contributions
of muscular and esthetic function, we can no more afford to continue to close our eyes to
the information in the frontal view.
POSTEROANTERIOR (FRONTAL) CEPHALOMETRY
Malocclusion and dentofacial deformities constitute three dimensional conditions or
pathologies. Although all orthodontic patients deserve an equally comprehensive three-
dimensional diagnostic examination, assessment of posteroanterior cepalometric views
are of particular importance in cases of :
1. dentoalveolar and facial asymmetries
2. dental and skeletal crossbites
3. functional mandibular displacements.
The same equipment that is used for lateral cephalomeric projections is utilized, i.e.
a head holder or cephalostat, an x ray source, and a cassette holder containing the film.
The initial unit described by Broadbent consisted of a set up in which two x ray
sources with two cassettes were simultaneously used, so that lateral and frontal
cephlograms were taken at the same time.In this technique, the patient was placed with
the Frankfort horizontal plane parallel to the floor. The x ray source exposing the
cassette for the poteranterior cephalogram was 5 feet away from the earpost axis, behind
the patient, and the central x ray beam passed at the level of the Frankfort horizontal
plane and at a 90 degree angle to the beam of the lateral cephalogram. Although precise
three dimensional evaluations are possible using this technique, it has now been almost
abandoned since it requires a rather large equipment with two x ray sources.
Modern equipment uses one x ray source. A cephalostat that can rotated 90° is
used so that the patient can be repositioned, without any alteration in the Frankfort
horizontal relationship of the head to the floor, for taking the P-A cephalogram.
Maintaining this identical horizontal orientation from lateral to postero-anterior
projection is critical when comparative measures are made from one to the other.
(Moyers et al, 1988)
15

16. Natural head position as mentioned earlier, is a standardized orientation of the head,
which is readily assumed by focusing on a distant point at eye level. In using the natural
head position for poteroanterior cephalometric registrations, some practical problems are
encountered. The patients head is facing the cassette, which makes it difficult for the
patient to look into a mirror to register natural head position.
(Solow and Tallgren, 1971)
Furthermore, space problems make it impossible to place a nosepiece in front of
nasion to establish support in a vertical plane.
For better evaluation of patients with craniofacial anomalies that require special
attention to the upper face, the patient head should be positioned with the tip of the nose
and forehead lightly touching the cassette holder.
In cases of suspected significant mandibular displacement, the PA cephalogram
should be taken with mouth of the patient slightly opened in order to differentiate
between functional mandibular displacement and dentoskeletal facial asymmetry.
(Faber, 1985)
As far as exposure conditions and considerations are considered, more exposure is
necessary for PA cephalograms than for lateral views.
(Enlow, 1982)
QUALITY OF THE RADIOGRAPHIC CEPHALOMETRIC IMAGE
Image quality is a major factor influencing the accuracy of cephalometric analysis.
An acceptable diagnostic radiograph is considered in the light of two groups of
characteristics:
• Visual characteristics
• Geometric characteristics.
Visual characteristics
The visual characteristics – density and contrast – are those that relate to the
ability of the image to demonstrate optimum detail within anatomical structures and to
differentiate between them by means of relative transparency.
Density—
16

17. Density is the degree of blackness of the image when it is viewed in front of an
illuminator or view box. The radiographic density is calculated from the common
logarithm of the ratio of the intensity of the light beam of the illuminator striking the
image to the intensity of the light transmitted through the film.
As the x ray image is formed as a result of processing in which the silver halide
crystals in the emulsion of the film being exposed to the x rays are converted to the
metallic silver, the two main factors that control the radiographic density are-
1. The exposure technique
2. The processing procedure.
Exposure technique --
The exposure factors related to image density are-
• tube voltage (kilovoltage peak, kVp)
• tube current (milliamperage, mA)
• exposure time (second,S)
• focus-film distance (D)
The relationship of image density and these factors is expressed as an equation:
Density= (kVp X mA X S)/D
Processing procedure --
Film processing consists of developing , rinsing, 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 of the film by chemical
solutions. The image density is directly proportional to temperature of the developing
solution and developing time.
The size of silver halide crystals in the film emulsion determines the film speed.
A film with large grain size (high-speed film) produces greater density than a film with
small grain size.
Contrast-
Contrast is the difference in densities between adjacent areas on the radiographic image.
Factors controlling the radiographic contrast are:
17

18.  Tube voltage- the kilovoltage peak has the most effect on radiographic contrast.
When the kilovoltage peak is low , the contrast of the film is high, and the film
has short-scale contrast. 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. Various soft tissue shield has been
incorporated into the cephalometric system to remove secondary radiation,
including an aluminum filter, lead diaphragm and grid.
 Subject contrast – this refers to the nature and properties of the subject, such as
thickness, density, and atomic number.
 Processing procedure – the temperature of the developing solution affects image
contrast. The higher the temperature the greater the contrast.
Density and contrast are the image characteristics that are usually affected when
the kilovoltage peak is altered. However , only the radiographic density can be altered
without changing the contrast when the kilovoltage peak is constant and the
milliamperage –second is altered.
Geometric characteristics --
The geometric characteristics are-
1. image unsharpness
2. image magnification
3. shape distortion
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19. Radiographic image produced by divergent beam
X rays by their nature are divergent beams radiated in all directions. Consequently, when
they penetrate through a 3 dimensional object such as a skull, there is always some
unsharpness and magnification of the image, and some distortion of the shape of the
object being imaged.
The focal spot from which the x-rays originate, 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
a shadow of the object(the Umbra).
Image unsharpness--
Image unsharpness is classified into three types according to etiology, namely:
geometric, motion and material.
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20. Factors influencing the size of the penumbra.
Penumbra size decreases if the focal spot size decreases(B), the focus – film distance
increases (C), or the focus – film distance is increased while object-film distance is
decreased(D).
Geometric unsharpness is the fuzzy outline in a radiographic image caused by the
penumbra. 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 focal spot size and the object-film distance should be decreased and the
focus-film distance increased.
Geometric unsharpness is defined by the following equation-
Geometric unsharpness = (focal spot size X object-film distance)/focus-film distance.
Motion unsharpness is caused by movement of the patients’ head and movement of
the tube and film.
Material unsharpness is related to two factors.
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21. 1. it is directly proportional to the grain size of the silver halide crystals in the
emulsion.
2. it is related to the intensifying screens, which , although they can minimize x ray
dose to the patient, also result in unsharpness that is related to the size of the
phosphorescent crystals, the thickness of the fluorescent layer, and the film-screen
contact.
Image magnification
It is the enlargement of the actual size of the object. Factors influencing image
magnification are the same factors as those that influence geometric unsharpness (i.e. the
grain size of the silver halide crystals in the emulsion, and various features of the
intensifying screens).
Shape distortion
It results in an image that does not correspond proportionally to the subject. In the
case of a skull, which is three – dimensional object, the distortion usually occurs as a
result of improper orientation of the patient’s head in the cephalostat or 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 film and the midsagittal plane.
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.
Manufacturer provides pre programmed exposure factors consisting of mA, Kvp
and exposure time (S), which enable image density and contrast to be controlled when
object density and thickness are varied. The variations in the exposure factors depends on
the type of X- ray machine , target –film distance, the film-screen combination and the
grid chosen.
Tube current : theoretically there is a linear relationship between mA and tube output.
Thus the quantity of radiation produced by an x- ray tube (i.e. the number of photons that
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22. reach the patient and film) is directly related to the tube current and the time the tube is
operated.
Spectrum of photon energies showing effect of tube current
The quantity of radiation produced is expressed as the product of time and tube
current. The quantity of radiation will remain constant regardless of how mA and time are
changed if their product remains constant.
Exposure time: is the commonest factor to change, since altering it has greatest effect
on image density.
Spectrum of photon energies showing effect of exposure time
When exposure time is doubled, the number of photons generated is doubled but
the range of photon energies is unchanged. Thus the effect of changing time is simply to
control the quantity of the exposure (the number of photons generated).
Tube voltage: Altering Kvp not only affects image contrast but also the exposure time.
When kVp is increased the spectrum of energy range , as well as, the number of photons
produced at each energy value, and the average energy of the beam of photons will be
increased. Thus as the kVp is increased there is an increase in the energy of each electron
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23. has when it strikes the target. This results in an increased efficiency of conversion of
electron energy into x- ray photons, and thus in an increase in the
1. number of photons generated,
2. mean energy of the photons
3. maximum energy of the photons.
Spectrum of photon energies showing effect of tube voltage
This results from greater efficiency in the production of bremsstrahlung photons
when increased numbers of higher energy electrons interact with the target.
Image density and contrast can also be affected by film processing. When using an
automatic film processor, these factors are controlled by the temperature of the developer
and developing time.
Optimum temperature being 68°F and time 5 minutes.
Image sharpness and magnification are controlled by manufacturer and operator.
Manufacturer provides the most effective focal spot size, target film distance ,collimation
and filtration measures to produce maximum X- ray beams with best size and shape.
Filtration-
An x ray beam consists of a spectrum of x ray photon of different energies, but only
photons with sufficient energy to penetrate anatomic structures are useful for diagnostic
radiology. Those that are of low penetrating (long wavelength ) contribute to patient
exposure but not to the information on the film. Consequently , in the interest of patient
safety , it is necessary to increase the mean energy of the x ray beam by removing the less
penetrating photons. This can be accomplished by placing an aluminum filter in the path
of the beam.
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24. The aluminum filter removes many of the lower energy photons with little affect of
those that are able to penetrate the patient and reach the film.
The inherent filtration of the tube and its housing consists of the materials that x ray
photons encounter as they travel from the focal spot on the target to form the usable beam
outside the tube enclosure. These materials include the glass wall of the x ray tube, the
insulating oil that surrounds many dental tubes, and the barrier material that prevents the
oil from escaping through the x ray machines ranges from the equivalent of 0.5 to 2 mm
of aluminum.
Total filtration is the sum of the inherent filtration plus any added external filtration
supplied in the form of aluminum disks placed over the port in the head of the x ray
machine.
Collimation-
Collimation means to shape an x ray beam, usually by the use of metallic barrier
with an aperture in the middle collimation reduce the size of the x ray beam and thus the
volume of irradiated tissue within the patient from which the scattered photons originate.
Collimation thereby reduces patient exposure and increases film quality.
Diaphragm, tubular, and rectangular collimators are useful in dentistry. The diaphragm
collimator is a thick plate of radiopaque material (usually lead) with an aperture or
opening in it that is usually placed over the port in the x ray head through which the x ray
beam emerges.
Inverse square law--
The intensity of an x ray beam at a given point is dependent on the distance of the
measuring device from the focal spot. For a given beam, the intensity is inversely
proportional to the square of the distance from the source. The reason for this decrease in
intensity is that the x ray beam spreads out as it moves from the source.
24

25. Relation between the intensity of radiation and focus-film distance
Changing the distance between the x ray tube and the patient thus has a marked
effect on beam intensity, such a change will require a corresponding modification of the
kVp or mAs if the exposure of the film is to be kept constant.
USES—
Cephalometrics is not only a research tool. It is also useful for diagnosis, treatment
planning, prognosis, surveying the results of treatment, and for following or even
predicting growth. It is not confined to orthodontics, but can give valuable information to
the oral surgeon, plastic surgeon, prosthodontist, pedodontist, and speech pathologist.
Common clinical applications of Cephalometrics are:
 to evaluate dentofacial propotion and clarify the anatomic basis for a
malocclusion
 by means of cephalograms taken before, during and after orthodontic treatment it
is possible to recognize and evaluate changes brought about by the treatment.
 To predict changes that should occur in the future for a patient.
 Although cephalograms are not taken as a screen for pathology, but there is a
possibility of observing pathological changes on the cephalogram.
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26. LIMITATIONS—
• They give two-dimensional image of a three-dimensional object.
• There can be errors while developing cephalograms which can limit measuring
accuracy to 0.5 mm:
• Movement of the subject,
• Optical blurring (depends on the size of the focal spot),
• Grain size of film and intensifying screens.
This is why it is important to keep the subject-film distance as nearly constant as
possible. This is particularly true for linear measurements which are going to be enlarged
about 10 percent. Distortion enters in when landmarks are used which are not in the
midsagittal plane. The points nearest the film will be enlarged the least.
ADVANCEMENT IN THE INSTRUMENTATION
Bjork in 1968 designed an X- ray cephalostat research unit with a built in 5 inch
image intensifier that enabled the position of the patients’ head to be monitored on a TV
screen. It also allowed cephalometric X-ray examination of oral function on the TV
screen, which could also be recorded.
In 1988, Solow and Kreiborg, developed a multiprojection cephalometer, which
featured improved control of head position and digital exposure control. It uses laser
beams for head positioning.
Units have also been developed for roentgenocephalometric registeration of
infants.
Digital imaging in dentistry is a rapidly changing field. Within the last five years
new devices and computers systems have been introduced to record X-ray images and to
manipulate those images using a variety of image processing operations.
Combining radiology with telecommunications has produced teleradiography, the
transmission of radiographic images over telephone lines. In medicine, sharing images
with a colleague to whom you have referred a patient, or consulting with a colleague at a
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27. distant facility is now feasible. Applications in dentistry may become commonplace in
the future.
CONCLUSION --
Cephalometric radiographic techniques has advanced much on the solid basis put up by
Hofrath and Broadbent.
The cephalometric radiography has influenced orthodontics in 3 major ways
1. in morphological analysis, by evaluating. The saggital and vertical relationships
of dentition, facial skeleton and tissue profile.
2. in growth analysis- by comparing cephalograms taken at different time intervals.
3. in treatment analysis- by evaluating alterations during and after therapy.
But due to increasing awareness among patients about esthetic needs , functional
requirements has made it imperative to look seriously for getting 3 dimensional view and
frontal radiographic views which were largely being ignored till now, for providing better
patients satisfaction.
REFERENCES:
1. Orthodontic Cephalometer- Athanasios E. Athanasiou
2. Oral Radiology – Principles and Interpretation—Goaz &White.
3. Broadbent . B . – Angle Orthodontics 1, 45, 1931.
4. Sassouni. V. – AJO-41,735,1955.
5. Hofrath – Fortscher Orathhodontics 1:232-48, 1931.
6. Sollow B, Tallgren A.—Acta. Odontol. Scand, 597-607, 1971.
7. Pacini AJ, -- J. Radiology, 3:230-238, 1922.
8. Bjork A.—Am.J. Phys. Anthropology. 29:243-254, 1968.
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