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0 Image Characteristics Projection GeometryThe following slides describe ImageCharacteristics and Projection Geometry.Both of these areas influence how diagnostica radiograph will be.In navigating through the slides, you should clickon the left mouse button when you see themouse holding an x-ray tubehead or you aredone reading a slide. Hitting “Enter” or “PageDown” will also work. To go back to the previousslide, hit “backspace” or “page up”.
Film DensityFilm density represents the degree of darkeningof an exposed x-ray film. White areas (e.g.,metallic restorations) have no density and blackareas (air spaces) have maximum density. Theareas in between these two extremes (toothstructure, bone) are represented by variousshades of gray.
Film DensityRadiolucent: refers to high film density, whichappears in a range from dark gray to black. Softtissue, air spaces, and pulp tissue, all of whichhave low object density, appear as radiolucentareas on a film (see next slide).Radiopaque: refers to area with low film density,which appear in a range from light gray to whiteon the film. (The “white” areas of the film areactually clear, but appear white when the lightfrom a viewbox passes through the film).Structures with high object density, such asenamel, bone and metallic restorations willappear radiopaque (see next slide).
Radiolucent RadiopaqueSoft tissue Cement baseAir space EnamelPulp tissue AmalgamMental foramen Bone
The overall density of the film affects thediagnostic value of the film. Only the centerfilm below has the proper density. The one onthe left is too light (low density) and the filmon the right is too dark (high density); both ofthese films are non-diagnostic.
Film Density influenced by:Patient size: the larger the patient’s head, themore x-rays that are needed to produce anideal film densityExposure factors (mA, kVp, exposure time).Some patients require a change in exposurefactors (increase for large adult, decrease forchild) to maintain proper film density. Anunnecessary increase in any of these factorsresults in an increase in film density.
Film Density influenced by:Object density: determined by type ofmaterial (metal, tooth structure,composite, etc.) and by amount ofmaterial. Metallic restorations havehigher object density than toothstructure. Film density decreases (filmgets lighter) when object densityincreases, assuming no changes aremade in the exposure factors.In the film at right, the post and corein each tooth has a high objectdensity, resulting in low film density.
Film Density influenced by:Film fog: This is an increased film densityresulting from causes other than exposure to theprimary x-ray beam. This includes scatterradiation, improper safelighting, improper filmstorage, and using expired film. All of thesethings will cause extra silver halide crystals onthe film to be converted to black metallic silver,resulting in an overall increase in the film densityand making the film less diagnostic. fog
ContrastContrast refers to the difference in filmdensities between various regions on aradiograph. Structures with different objectdensities produce images with different filmdensities.
High ContrastHigh contrast implies that there is a pronouncedchange from the light to the dark areas of thefilm. There are fewer shades of gray, thepredominant densities being either very light orvery dark. High contrast is also known as shortscale contrast.Theoretically, high contrast is best for cariesdetection, the radiolucent carious lesionshowing up distinctly against the surroundingradiopaque enamel.
Low ContrastWith low contrast, there are many shades ofgray seen on the film, with less pronouncedchanges from light to dark. This is also knownas long scale contrast.Low contrast is best for periapical orperiodontal evaluation. Slight changes causedby bone loss will be more evident, showing upas a darker gray than the surrounding area.
Contrast influenced by:Subject Contrast: In order tosee an image on the film, theobjects being radiographedmust have different objectdensities. If everything had thesame object density, the filmwould be blank. In the film atright, the teeth, restorations,bone, air spaces, etc., all havedifferent object densities,allowing us to see them on thefilm.
Contrast influenced by:kVp: kVp controls the energy(penetrating ability) of the x-rays. The higher the kVp, themore easily the x-rays passthrough objects in their path,resulting in many shades ofgray (low contrast). At lowerkVp settings, it is harder forx-rays to pass throughobjects with higher object 40 50 60 70 80 90 100densities, resulting in a kVp settingshigher contrast (short scale).
0 Contrast influenced by:Film contrast: this is incorporated into the film bythe manufacturer. In general, high film contrast(green curve below) requires very preciseexposure of the film; if it is too high or too low, thefilm will be too dark or too light, resulting in a non-diagnostic film. With low film contrast (purplecurve) the film will be diagnostic over a broaderrange of film exposure. Density Exposure of film
0 Contrast influenced by:Film fog: as discussed under density, film fogmakes the whole film darker. This makes itharder to see the density differences (contrast),making the film less diagnostic. fog Fogged film
LatitudeThe latitude of a film represents the range ofexposures that will produce diagnosticallyacceptable densities on a film. A wide latitudefilm will more readily image both hard and softtissues on a film.As the latitude of a film increases, the contrast ofthe film decreases. High Contrast Density Wide Latitude Log Relative Exposure
SpeedThe speed of a film represents the amount ofradiation required to produce a radiograph ofacceptable density. The higher the speed, theless radiation needed to properly expose the film.Higher speed films have larger silver halidecrystals; the larger crystals cover more area andare more likely to interact with the x-rays.F-speed film (Insight) has the highest speed ofintraoral films. An F-speed film requires 60% lessradiation than a D-speed film.
Projection GeometryProjection geometry pertains to the source of thex-ray beam and the relationship between the x-raybeam, the structures being radiographed and theposition of the x-ray film. In order to achieve theoptimal radiograph, the following situations needto be considered:1. The radiation source should be as small as possible2. The source-tooth distance should be large3. The tooth-film distance should be small4. The tooth and film should be parallel5. The x-ray beam should be perpendicular to tooth/film
Radiation source as small as possible 0The sharpness (detail) of images seen on aradiograph is influenced by the size of the focalspot (area in the target where x-rays are produced).The smaller the focal spot (target, source), thesharper the image of the teeth will be.During x-ray production, a lot of heat is generated.If the target is too small, it will overheat and burnup. In order to get a small focal spot, whilemaintaining an adequately large target to withstandheat buildup , the line focus principle is used.
Line Focus Principle 0 Target (Anode) Cathode Apparent (effective) focal spot size Actual focal spot size PIDThe target is at an angle (not perpendicular) to the electronbeam from the filament (see above). Because of this angle,the x-rays that exit through the PID “appear” to come froma smaller focal spot (see next slide). Even though theactual focal spot (target) size is larger (to withstand heatbuildup), the smaller size of the apparent focal spotprovides the sharper image needed for a proper diagnosis.
Line Focus Principle 0Actual focal spot size The target is at an angle to(looking perpendicular the electron beam. If youto the target surface; see looked up through the PID atprevious slide); the this angled target, it wouldlength is indicated by “appear” to be smaller, asthe white dotted lines seen above. Click to rotatebelow. target and see altered size (indicated by yellow dotted lines below left). Looking up at target PID through open end of PID
Source-tooth distance large 0The “source” refers to where the x-rays are produced,which is the target of the x-ray tube. This source, ortarget, is also referred to as the focal spot. Moving thesource farther away from the teeth results in a sharperimage that is less magnified. (Sharpness andmagnification will be discussed later). Source (target)
The most common way to increase the source-toothdistance is to increase the length of the PID. However, bydoing this, the exposure time is increased dramatically, asseen below. This increase in exposure time increases thechances of patient movement and this needs to beconsidered in deciding how long a PID you will use. 8” Exposure time = 4 impulses 12” Exposure time = 9 impulses 16” Exposure time = 16 impulses
0 Tooth-film distance small paralleling bisectingTo achieve the sharpest image with the leastmagnification, the film should be as close to the teeth aspossible. In general, the film can be placed closer to theteeth using the bisecting angle technique (with fingerretention) than with the paralleling technique. However,there will be more distortion of the image with thebisecting technique.
Teeth and film parallel X-ray beam perpendicular to teeth/filmHaving the teeth and film parallel to each other isaccomplished using the paralleling technique. If the filmand teeth are parallel, then the x-ray beam can bedirected perpendicular to both the long axis of the teethand the long axis of the film. This relationship will keepdistortion of the image to a minimum.
SharpnessThe sharpness of an image is a measure ofhow well the details (boundaries/edges) ofan object are reproduced on a radiograph.The sharper the image, the easier it is tomake a diagnosis concerning subtlechanges in bone or tooth structure. Thesharpness of an image is dependent on thesize of the penumbra.
PenumbraThe area on the film that representsthe image of a tooth is called theumbra, or complete shadow. Thearea around the umbra is called thepenumbra or partial shadow. Thepenumbra is the zone ofunsharpness along the edge of theimage; the larger it is, the lesssharp the image will be. Thediagram at right shows how the Umbrapenumbra is formed. X-rays fromeither extreme of the target, andfrom many points in between, pass Penumbrathrough the edge of the object andcontribute to the penumbra.
Decrease focal spot size, increase sharpnessThe larger the target, the wider the area available fromwhich x-rays can be generated. As seen in the diagrambelow, x-rays from opposite ends of the larger target (atright) pass through the edge of the tooth and create alarger penumbra around the image of the tooth on thefilm. Target (source) Tooth Umbra Penumbra
Increase source-tooth distance, increase sharpnessCompare the penumbras Ain the diagrams at right.When the target is closer Bto the tooth, as in B, thepenumbra is larger. If thetarget is moved fartherfrom the tooth (A), thepenumbra surroundingthe tooth image issmaller, creating asharper image. The filmdistance from the tooth tothe film is unchanged. Target (source) Umbra Tooth Penumbra
0Decrease tooth-film distance, increase sharpnessAs x-rays coming fromopposite ends of the targetpass through the edge of thetooth they continue in astraight line, diverging fromeach other. The farther the filmis from the tooth, the more thex-rays diverge, creating a widerpenumbra. This decreases thesharpness of the image. Whenthe film is moved closer to the filmtooth ( ), the penumbra issmaller, creating a sharperimage. Target (source) Umbra Teeth Penumbra
Intensifying screens decrease sharpness 0Extraoral films use intensifying screens which containspecial phosphor crystals that produce light whenstruck by x-rays ( ). This light in turn exposes thefilm. Notice how the light spreads out as it leaves thephosphor crystal. This results in a less sharp image.Compare the periapical film and the same area on apanoramic film. The periapical image is much sharper.film panoramic periapical
Patient motion decreases sharpnessIf the patient moves during the exposure of a film, theimages will be blurred, or unsharp, as seen below.
0 MagnificationMagnification is an increase in the size of an object. In radiology, it is caused by the divergence (spreading out) of the x-ray beam as it moves away from the target (in the x-ray tube) where the x-rays are produced.The amount of magnification can be reduced by: 1. Increasing the distance from the target to the teeth (source-object distance). 2. Decrease the distance from the teeth to the film (object-film distance).(See next two slides)
Magnification 0Increase source-object distance, decrease magnification The closertarget is moved the teeth, the more the x- When the the target is to farther from the teeth (from rays spread the diagram pass by the x-ray beam does 8” to 16” in out as they below), the teeth, resulting in increased magnification and the magnification is not spread out as much (see diagram below). decreased. Target 16” Target 8”
Magnification 0Decrease object-film distance, decrease magnification When the film is placed farther to thethe tooth, as closer from tooth as seen diagram below, the x-ray beam spreads out in thebelow, the x-ray beam does not spread out as much increases magnification. more andand magnification is decreased. Target 16”
Distortion 0Distortion is a change in the shape 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 is the long axis of the film at an angle to the long axis of the teeth.2. The alignment of the x-ray beam (the angle the x- ray beam forms with both the film and the teeth). Is the beam perpendicular to both the teeth and the film (paralleling) or is it at an angle to both the teeth and film (bisecting angle and occlusal techniques).
Distortion 0In the paralleling technique, the long axis of the filmand the long axis of the tooth are parallel. The x-raybeam is directed perpendicular to both the long axisof the tooth and the long axis of the x-ray film. As aresult, distortion is minimized or eliminated. In theradiograph of the maxillary first molar, below, theshape and relationship of the buccal and palatalroots are accurately imaged.
Distortion 0In the bisecting angle and occlusal techniques there isan angle between the teeth and film, dependent on thepatient’s oral anatomy, which influences filmplacement, and the technique used. (Occlusaltechnique requires a larger angle between the film andteeth, approaching 90 degrees). The bisecting angleradiograph of the maxillary molar, below, shows thedistortion of the relationship between the buccal andpalatal roots.
0 This slide compares the distortion resulting from paralleling, bisecting angle, and occlusal techniques. The variation in tooth-film relationship in the different techniques requires a change in the angle of the x-ray beam. In the diagram below, the ring around the cervical portion of the tooth is distorted in its relationship to the tooth in the bisecting angle technique; in the occlusal technique, the distortion is even more severe.paralleling bisecting occlusal angle paralleling bisecting occlusal angle
0 Ideal RadiographIn the ideal radiograph, the image is the samesize as the object, has the same shape and hasa sharp outline with good density and contrast.Because the film must always be at somedistance from the object, with bone and softtissue in between, the object will always bemagnified to some degree. Though magnified,the image of the object will usually have thesame shape as the object when using theparalleling technique. The sharpness, densityand contrast are maximized by using a longerPID and proper exposure factors.
0The mandibular molar periapical film comesclosest to satisfying the properties of an idealradiograph (either paralleling or bisecting). Thefilm is closer to the teeth in this location than inany other part of the mouth and the film isusually parallel with the teeth.
0This concludes the section on ImageCharacteristics and Projection Geometry.Additional self-study modules are availableat: http://dent.osu.edu/radiology/resources.htmIf you have any questions, you may e-mailme at firstname.lastname@example.org.Robert M. Jaynes, DDS, MSDirector, Radiology GroupCollege of DentistryOhio State University