Cephalometrics & x ray generation principles/orthodontic courses by indian dental academy


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Cephalometrics & x ray generation principles/orthodontic courses by indian dental academy

  1. 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. 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. 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. 3
  4. 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, 4
  5. 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. 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. 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. 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. 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. 9
  10. 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 10
  11. 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. 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. 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. 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. 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. 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. 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. 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 18
  19. 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. 19
  20. 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. 20
  21. 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 21
  22. 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 22
  23. 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. 23
  24. 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. 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. 25
  26. 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 26
  27. 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. 27