Oral Radiology Image Characteristics PDF

Oral Radiology IMAGE CHARACTERISTICS Part 1 OMD-711n Radiography in Dental Clinic  Objectives of Lecture By the end of the lecture ,the student should be able to :- 1-Choose and purchase the best X-ray machine suitable for a specific workload. 2-Identify the best e...

Oral Radiology IMAGE CHARACTERISTICS Part 1 OMD-711n Radiography in Dental Clinic  Objectives of Lecture By the end of the lecture ,the student should be able to :- 1-Choose and purchase the best X-ray machine suitable for a specific workload. 2-Identify the best exposure parameters during conventional imaging. 3- Demonstrate the significance of linking the chosen exposure parameters to yield an ideal diagnostic image IMAGE CHARACTERISTICS are : I-Visual II-Geometric Sharpness Density (Definition) Size Contrast Shape  I- Density It is the overall degree of darkening of an exposed film Primary Factors Secondary Factors 1. Subject thickness and 1. Exposure factors density. (amalgam, bone, etc.; as  mA object density increases,  Exposure time image density decreases)  kVp 2. Filtration 3. Collimation 2.Source- film distance 4. Film (speed – processing) Controlling factor: 1-Milliamperage (mA):  An increase in Milliamperage produces more x- rays that expose the film and, as a result, increases film density.  If the Milliamperage is increased, the film density increases and the radiograph appear darker.  On other hand, if the Milliamperage is decreased, the film density decreases and the radiograph appear lighter. 2- Exposure time:  An increase in exposure time increases film density by increasing the total number of x-rays that reach the film surface.  So, the film density increases, and the radiograph appears darker& vice versa.  The product of mA setting and exposure time (mA × s, or mAs) is often used as a single parameter to denote the total number of photons produced.  For instance, a machine operating at 10 mA for 1 second (10 × 1 = 10 mAs) produces the same number of photons when operated at 20 mA for 0.5 second (20 × 0.5 = 10 mAs). 3- Operating kilovoltage peak (kVp):  An increase in operating kilovoltage increases film density by producing x-rays of higher energy (more penetrating ability).  If the operating kilo-voltage is increased, the film density increases and the radiograph appear darker & vice versa.  Increasing the kVp increases the potential difference between the cathode and the anode, increasing the kinetic energy of the electrons as they move toward the target.  Increasing the kVp of an x-ray machine increases: The number of photons generated The maximal energy of the photons The mean energy of the photons 4- Subject thickness:  Fewer x-rays reach the film in a patient with an increased amount of soft tissue or thick, dense bones. As a result, the radiograph has less density and appears lighter.  Adjustments in operating kilovoltage, Milliamperage, or exposure time can be made to compensate for variations in the size of patients and subject thickness. 5- Subject Density:  Variations in the density & the amount of the subject (tooth structure, metal, composite, etc.) exert a deep effect on the image. As a general, film density (darkness) decreases when object density increases. 6- Source- film distance:  The intensity (I) of an x-ray beam (the number of photons per cross-sectional area) is inversely proportional to the distance (D). If the distance ↑ the intensity ↓ thus the density will decrease.  Changing the distance between the x-ray tube and the patient, such as by switching from a machine with a short tube to one with a long tube, has a marked effect. Such a change requires a corresponding modification of the kVp or mA to keep constant the exposure to the film.  Density (cont.)  Finally, the overall density of the film affects the diagnostic value of the film. Too Light Ideal Density Too Dark The x-ray tube Photo of an X-ray Tube Beryllium Window Leaded glass Target Focusing cup (filament located inside)  2. Radiographic Contrast  Contrast refers to the range of densities on a radiograph. It is defined also as the difference in densities between light and dark regions on a radiograph.  Thus an image that shows both light areas and dark areas has High Contrast.  In dental radiography, the terms (Short–Scale Contrast) and (Long-Scale Contrast) may be used to describe the appearance of a radiograph.  Short Gray Scale of Contrast  A dental radiograph that shows only two densities, i.e., areas of black and white.  It can also be described as having High Contrast, in which areas of black and white & easily differentiated from each other.  High contrast is best for caries detection. (WHY???)  Long Gray Scale of Contrast  A dental radiograph that have many densities or many shades of gray, has a long contrast scale.  Many shades of gray can also be described as having Low Contrast.  Best for periapical or periodontal evaluation. (WHY???) Low Contrast High Contrast  Contrast is influenced by: Primary Factors Secondary Factors 1. Exposure factors 1. Subject contrast  kVp 2. Filtration 3. Collimation 4. Film (contrast - processing) 1- kVp  Increasing the kilovoltage affects film contrast by increasing the average energy of the x-rays and by producing higher energy x-rays.  X-rays with higher energy are better able to penetrate tissue.  A 90-kVp setting produces a radiograph that has Low Contrast (more shades of gray).  A 70-kVp setting produces a radiograph with High Contrast (fewer shades of gray). 40 50 60 70 80 90 100 1- Subject Contrast: IIs the features of the subject that affect radiographic contrast. It is affected largely by: a) The subject's thickness. b) Density: In order to see an image on the film, the subject being radiographed must have different subject densities. c) By increasing or decreasing the kilovoltage. SUBJECT CONTRAST IN RELATION TO IMAGE CONTRAST  Dense structures (thickness, density and composition) as bone  absorb rays more than soft tissues, cavities or canals.  Increasing KV > 90 with low subject contrast  long grayscale image.  Decreasing KV < 60 with high subject contrast  short gray scale mage.  Step Wedge lead bar aluminum Step Wedge Pattern  White No white Many gray Few gray shades shades Black Black Short gray scale Long gray scale High Contrast Low Contrast  2- Film Contrast:  Is the capacity of radiographic films to display differences in subject contrast.  A high-contrast film shows areas of small difference in subject contrast more than does a low-contrast film.  The film contrast can be affected by: A) The hidden qualities of the film: The quality of the film are under the control of the film manufacturer and cannot be changed by the dental radiographer. B) Film processing: is under the control of the dental radiographer. Development time or the temperature of the developer solution affects the contrast of a dental radiograph. An increase in development time or developer temperature results in a film with increased contrast. The recommended Increased developer time developing time and or temperature temperature. C) Improper handling of film: such as storage at too high a temperature, exposure to excessively bright safelights, or light leaks in the darkroom, also degrades film contrast. D) Fogging on an x-ray film: results in increased film Density. Film contrast is reduced by the addition of this undesirable density. Common causes of film fog: a) Improper safe lighting b) Development of film at an excessive temperature or for a prolonged period c) Exhausted processing chemicals d) Storage of film at too high a temperature e) Scatter radiation f) Using expired film  Film Fog:  Fogging on an x-ray film: results in increased film density. Film contrast is reduced by the addition of this undesirable density.  Film fog can be reduced by proper film processing and storage. Film contrast:  This is incorporated into the film by the manufacturer.  In general, high film contrast (green curve below) requires very precise exposure of the film; if it is too high or too low, the film will be too dark or too light, resulting in a non-diagnostic film.  With low film contrast (purple curve) the film will be diagnostic over a broader range of film exposure. Density Exposure of film The quality, or average energy, of the x-ray beam is increased with an increase in kVp or an increase in filtration. The quantity, or number of x-rays, is increased with an increase in mA, exposure time and kVp setting. Quality Quantity kVp (primarily) mA no change Time no change Filtration Collimation no change Distance Intensity per cross-sectional area Important Relationship kVp and Scatter The amount and energy of scatter radiation exiting the patient depends, in part, on the kVp selected. Examinations using higher kVp produce a greater proportion of higher- energy scattered x-rays compared with examinations using low kVp. Volume of Tissue Irradiated and Scatter The volume of tissue irradiated is affected by both the part thickness and the x-ray beam field size. Therefore, the greater the volume of tissue irradiated, because of either or both factors, the greater the amount of scatter radiation produced. Beam Restriction and Patient Dose As beam restriction or collimation increases, field size decreases, and patient dose decreases. As beam restriction or collimation decreases, field size increases, and patient dose increases. Collimation and Scatter Radiation As collimation increases, the field size decreases, and the quantity of scatter radiation decreases; as collimation decreases, the field size increases, and the quantity of scatter radiation increases. Collimation and Radiographic Contrast As collimation increases, the quantity of scatter radiation decreases, and radiographic contrast increases; as collimation decreases, the quantity of scatter radiation increases, and radiographic contrast decreases. Over-collimation → ↓↓ size of the beam → ↓↓ x-ray photons → ↓↓density. Under-collimation → ↑↑ size of the beam → ↑↑ x-ray photons → ↑↑ density. It will also ↑↑ the scattered radiation → decreased patient safety and contrast.  Over-filtration → removal of excess rays → ↓↓ x-ray photons → ↓↓density. N.B.: image contrast will not be directly affected if the kVp is not changed.  Under-filtration → beam of both long and short wave length → bad contrast and more scattering. N.B.: beam quantity will not be affected if the mA is not changed.  Therefore; IMAGE CHARACTERISTICS are : I-Visual II-Geometric Sharpness Density (Definition) Size Contrast Shape Geometric Characteristics 1. Sharpness & Resolution. 2. Magnification. 3. Distortion. 33 1- Sharpness & Resolution  Sharpness: It is the ability of a radiograph to reproduce the boundaries / edges of an objects.  Resolution: It is the ability to reveal small objects that are close together. Both are interdependent, influenced by the same geometric variables. 34 Sharpness and Resolution It usually is measured by radiographing an object made up of a series of thin lead strips with alternating radiolucent spaces of the same thickness. The groups of lines and spaces are arranged in the test target in order of increasing numbers of lines and spaces per millimeter. Typically, direct exposure films used for intraoral radiography can delineate 20 lp/mm (line pairs per millimeter) or more. Screen film combinations for panoramic and cephalometric radiography resolve approximately 5 lp/mm.  Penumbra  Umbra: it is the area on the film that represents the image of a tooth.  Penumbra: it is the area around the umbra. So, The penumbra is the zone of Umbra unsharpness along the edge of the image; the larger it is, the less sharp the image will be. Penumbra How The Penumbra Is Formed? X-rays is emitted in divergent way from either extreme of the target, and from many points in between, pass through the edge of the object and give a share in the penumbra. Penumbra Umbra 37 Sharpness affected by: A. Focal spot size. B. Source – object (teeth) distance. C. Object (teeth) - film distance. D. Intensifying screens. E. Motion. 38  A- Decrease Focal Spot Size, Increase Sharpness  As seen in the diagram below, x-rays from opposite ends of the larger target (at right) pass through the edge of the tooth and create a larger penumbra around the image of the tooth on the film. Target (source) Tooth Umbra Penumbra During purchasing an X-ray Unit ,we must put in mind that one of the components of the machine (called the focal spot ) affects the sharpness of the image (sharpness is not a visual character but is a geometric character). Focal spot size The smaller the focal spot (target), the shaper the image (teeth) will be. Sharpness measures how well the details (boundaries/edges) of an object are reproduced on a radiograph Focal spot size B- Increase Source-tooth Distance, Increase Sharpness  When the target is closer to the tooth, as in (B), the A penumbra is larger.  If the target is moved away B from the tooth (A), the penumbra surrounding the tooth image is smaller, creating a sharper image.  N.B, The distance from the tooth to the film is Film unchanged. Target (source) Umbra Tooth Penumbra  C- Decrease Tooth-film Distance, Increase Sharpness  As x-rays coming from opposite ends of the target pass through the edge of the tooth they continue in a straight line, diverging from each other.  The farther the film is from the tooth, the more the x-rays diverge, creating a wider Film penumbra. This decreases the sharpness of the image. Target (source) Umbra Teeth Penumbra  D- Intensifying Screens Decrease Sharpness  Extra oral films use intensifying screens which contain special phosphor crystals that produce light when exposed to x-rays.  This light in turn exposes the film. Notice how the light spreads out as it leaves the phosphor crystal, this results in a less sharp image.  Compare the Periapical film and the same area on a panoramic film. The Periapical image is much sharper. Film Panoramic Periapical E- Patient Motion Decreases Sharpness  If the patient moves during the exposure of a film, the images will be blurred, or unsharp, as seen below.  2-Magnification  In radiology, magnification is an increase in the size of an object.  It is caused by the divergence of the x-ray beam as it moves away from the x-ray tube.  The amount of magnification can be reduced by:  Increasing the distance from the target to the teeth (Source-object Distance).  Decrease the distance from the teeth to the film (Object-film Distance). 46  Increase Source-object Distance, Decrease Magnification  The closer the source is to the teeth, the more the x-rays spread out as they pass by the teeth, resulting in increased magnification & vice versa. Target 16” Target 8” Source-object Distance Large  Decrease Object-film Distance, Decrease Magnification  When the film is placed closer to the tooth as seen below, the x-ray beam does not spread out as much and magnification is decreased & vice versa. Target 16” Increase Source-object Distance, Decrease Magnification Target 16” Target 8” Decrease Object-film Distance, Decrease Magnification Target 16”  3- Distortion  Distortion is unequal magnification i.e. a change in the shape & size of an object or the relationship of that object with surrounding objects.  It is affected by: 1) The film-teeth relationship (angle between the film and teeth):  Are they parallel with each other.  Or the long axis of the film at an angle to the long axis of the teeth. 2) The alignment of the x-ray beam:  Is the beam perpendicular to both the teeth and the film (Paralleling Technique).  Or is it at an angle to both the teeth and film (Bisecting Angle & Occlusal Techniques). In the paralleling technique:  The long axis of the film and the long axis of the tooth are parallel.  The x-ray beam is directed perpendicular to both the long axis of the tooth and x-ray film. As a result, distortion is minimized or eliminated. In the bisecting angle and Occlusal techniques:  There is an angle between the teeth and film, dependent on the patient’s oral anatomy, which influences film placement, and the technique used.  Occlusal technique requires a larger angle between the film and teeth, approaching 90 degrees).  Geometric Effect of influencing Influencing factors characteristics factors  focal spot size =  sharpness Sharpness Focal spot size focal spot size =  sharpness  Crystal size =  sharpness Film composition  Crystal size =  sharpness  movement =  sharpness Movement  movement =  sharpness  TFD =  magnification Magnification Target-film (TFD) distance  TFD =  magnification  OFD = magnification Object-film alignment  OFD =  magnification Object and film parallel =  distortion Distorsion Object-film alignment Object and film not =  distortion parallel Beam perpendicular to =  distortion x-ray beam alignment object and film Beam not perpendicular =  distortion to object and film  Ideal Radiograph 1. Image same size as object. 2. Image same shape as object. 3. Image has good detail. 4. Image has good density and contrast. Mandibular molar periapical film comes closest to satisfying properties of an ideal radiograph (either paralleling or bisecting). Questions