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”.
2. Image CharacteristicsImage characteristics includedensity, contrast, speed, andlatitude.
3. 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.
4. 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
5. Radiolucent RadiopaqueSoft tissue Cement baseAir space EnamelPulp tissue
AmalgamMental foramen Bone
6. 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.
7. 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
8. 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.
9. 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
10. ContrastContrast refers to the difference in filmdensities between various regions on
aradiograph. Structures with different objectdensities produce images with different filmdensities.
11. 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
12. 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.
13. 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.
14. 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
15. 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
16. 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
17. 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
18. 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.
19. 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
20. 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.
21. 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.
22. 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
23. 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)
24. 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
25. 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
26. 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.
27. 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.
28. 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.
29. Sharpness is determined by:1. Focal spot size2. Source–object (teeth) distance3. Object
(teeth)-film distance4. Intensifying screens5. Patient motion
30. 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
31. 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
32. 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
33. 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
34. Patient motion decreases sharpnessIf the patient moves during the exposure of a film,
theimages will be blurred, or unsharp, as seen below.
35. 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)
36. 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”
37. 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”
38. 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).
39. 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
40. 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.
41. 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
42. 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.
43. 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.