1. 0
Image Characteristics
Projection Geometry
The following slides describe Image
Characteristics and Projection Geometry.
Both of these areas influence how diagnostic
a radiograph will be.
In navigating through the slides, you should click
on the left mouse button when you see the
mouse holding an x-ray tubehead or you are
done reading a slide. Hitting “Enter” or “Page
Down” will also work. To go back to the previous
slide, hit “backspace” or “page up”.

2. Image Characteristics
Image characteristics include
density, contrast, speed, and
latitude.

3. Film Density
Film density represents the degree of darkening
of an exposed x-ray film. White areas (e.g.,
metallic restorations) have no density and black
areas (air spaces) have maximum density. The
areas in between these two extremes (tooth
structure, bone) are represented by various
shades of gray.

4. Film Density
Radiolucent: refers to high film density, which
appears in a range from dark gray to black. Soft
tissue, air spaces, and pulp tissue, all of which
have low object density, appear as radiolucent
areas on a film (see next slide).
Radiopaque: refers to area with low film density,
which appear in a range from light gray to white
on the film. (The “white” areas of the film are
actually clear, but appear white when the light
from a viewbox passes through the film).
Structures with high object density, such as
enamel, bone and metallic restorations will
appear radiopaque (see next slide).

5. Radiolucent Radiopaque
Soft tissue Cement base
Air space Enamel
Pulp tissue Amalgam
Mental foramen Bone

6. The overall density of the film affects the
diagnostic value of the film. Only the center
film below has the proper density. The one on
the left is too light (low density) and the film
on the right is too dark (high density); both of
these films are non-diagnostic.

7. Film Density influenced by:
Patient size: the larger the patient’s head, the
more x-rays that are needed to produce an
ideal film density
Exposure factors (mA, kVp, exposure time).
Some patients require a change in exposure
factors (increase for large adult, decrease for
child) to maintain proper film density. An
unnecessary increase in any of these factors
results in an increase in film density.

8. Film Density influenced by:
Object density: determined by type of
material (metal, tooth structure,
composite, etc.) and by amount of
material. Metallic restorations have
higher object density than tooth
structure. Film density decreases (film
gets lighter) when object density
increases, assuming no changes are
made in the exposure factors.
In the film at right, the post and core
in each tooth has a high object
density, resulting in low film density.

9. Film Density influenced by:
Film fog: This is an increased film density
resulting from causes other than exposure to the
primary x-ray beam. This includes scatter
radiation, improper safelighting, improper film
storage, and using expired film. All of these
things will cause extra silver halide crystals on
the film to be converted to black metallic silver,
resulting in an overall increase in the film density
and making the film less diagnostic.
fog

10. Contrast
Contrast refers to the difference in film
densities between various regions on a
radiograph. Structures with different object
densities produce images with different film
densities.

11. High Contrast
High contrast implies that there is a pronounced
change from the light to the dark areas of the
film. There are fewer shades of gray, the
predominant densities being either very light or
very dark. High contrast is also known as short
scale contrast.
Theoretically, high contrast is best for caries
detection, the radiolucent carious lesion
showing up distinctly against the surrounding
radiopaque enamel.

12. Low Contrast
With low contrast, there are many shades of
gray seen on the film, with less pronounced
changes from light to dark. This is also known
as long scale contrast.
Low contrast is best for periapical or
periodontal evaluation. Slight changes caused
by bone loss will be more evident, showing up
as a darker gray than the surrounding area.

13. Contrast influenced by:
Subject Contrast: In order to
see an image on the film, the
objects being radiographed
must have different object
densities. If everything had the
same object density, the film
would be blank. In the film at
right, the teeth, restorations,
bone, air spaces, etc., all have
different object densities,
allowing us to see them on the
film.

14. Contrast influenced by:
kVp: kVp controls the energy
(penetrating ability) of the x-
rays. The higher the kVp, the
more easily the x-rays pass
through objects in their path,
resulting in many shades of
gray (low contrast). At lower
kVp settings, it is harder for
x-rays to pass through
objects with higher object
40 50 60 70 80 90 100
densities, resulting in a
kVp settings
higher contrast (short scale).

15. 0
Contrast influenced by:
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

16. 0
Contrast influenced by:
Film fog: as discussed under density, film fog
makes the whole film darker. This makes it
harder to see the density differences (contrast),
making the film less diagnostic.
fog
Fogged film

17. Latitude
The latitude of a film represents the range of
exposures that will produce diagnostically
acceptable densities on a film. A wide latitude
film will more readily image both hard and soft
tissues on a film.
As the latitude of a film increases, the contrast of
the film decreases.
High Contrast
Density
Wide Latitude
Log Relative Exposure

18. Speed
The speed of a film represents the amount of
radiation required to produce a radiograph of
acceptable density. The higher the speed, the
less radiation needed to properly expose the film.
Higher speed films have larger silver halide
crystals; the larger crystals cover more area and
are more likely to interact with the x-rays.
F-speed film (Insight) has the highest speed of
intraoral films. An F-speed film requires 60% less
radiation than a D-speed film.

19. Projection Geometry
Projection geometry pertains to the source of the
x-ray beam and the relationship between the x-ray
beam, the structures being radiographed and the
position of the x-ray film. In order to achieve the
optimal radiograph, the following situations need
to be considered:
1. The radiation source should be as small as possible
2. The source-tooth distance should be large
3. The tooth-film distance should be small
4. The tooth and film should be parallel
5. The x-ray beam should be perpendicular to tooth/film

20. Radiation source as small as possible 0
The sharpness (detail) of images seen on a
radiograph is influenced by the size of the focal
spot (area in the target where x-rays are produced).
The smaller the focal spot (target, source), the
sharper 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 burn
up. In order to get a small focal spot, while
maintaining an adequately large target to withstand
heat buildup , the line focus principle is used.

21. Line Focus Principle 0
Target
(Anode)
Cathode
Apparent (effective)
focal spot size
Actual focal
spot size
PID
The target is at an angle (not perpendicular) to the electron
beam from the filament (see above). Because of this angle,
the x-rays that exit through the PID “appear” to come from
a smaller focal spot (see next slide). Even though the
actual focal spot (target) size is larger (to withstand heat
buildup), the smaller size of the apparent focal spot
provides the sharper image needed for a proper diagnosis.

22. Line Focus Principle
0
Actual focal spot size The target is at an angle to
(looking perpendicular the electron beam. If you
to the target surface; see looked up through the PID at
previous slide); the this angled target, it would
length is indicated by “appear” to be smaller, as
the white dotted lines seen above. Click to rotate
below. 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 0
The “source” refers to where the x-rays are produced,
which is the target of the x-ray tube. This source, or
target, is also referred to as the focal spot. Moving the
source farther away from the teeth results in a sharper
image that is less magnified. (Sharpness and
magnification will be discussed later).
Source
(target)

24. The most common way to increase the source-tooth
distance is to increase the length of the PID. However, by
doing this, the exposure time is increased dramatically, as
seen below. This increase in exposure time increases the
chances of patient movement and this needs to be
considered 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
bisecting
To achieve the sharpest image with the least
magnification, the film should be as close to the teeth as
possible. In general, the film can be placed closer to the
teeth using the bisecting angle technique (with finger
retention) than with the paralleling technique. However,
there will be more distortion of the image with the
bisecting technique.

26. Teeth and film parallel
X-ray beam perpendicular to teeth/film
Having the teeth and film parallel to each other is
accomplished using the paralleling technique. If the film
and teeth are parallel, then the x-ray beam can be
directed perpendicular to both the long axis of the teeth
and the long axis of the film. This relationship will keep
distortion of the image to a minimum.

27. Sharpness
The sharpness of an image is a measure of
how well the details (boundaries/edges) of
an object are reproduced on a radiograph.
The sharper the image, the easier it is to
make a diagnosis concerning subtle
changes in bone or tooth structure. The
sharpness of an image is dependent on the
size of the penumbra.

28. Penumbra
The area on the film that represents
the image of a tooth is called the
umbra, or complete shadow. The
area around the umbra is called the
penumbra or partial shadow. The
penumbra is the zone of
unsharpness along the edge of the
image; the larger it is, the less
sharp the image will be. The
diagram at right shows how the Umbra
penumbra is formed. X-rays from
either extreme of the target, and
from many points in between, pass Penumbra
through the edge of the object and
contribute to the penumbra.

29. Sharpness is determined by:
1. Focal spot size
2. Source–object (teeth) distance
3. Object (teeth)-film distance
4. Intensifying screens
5. Patient motion

30. Decrease focal spot size, increase sharpness
The larger the target, the wider the area available from
which x-rays can be generated. 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

31. Increase source-tooth distance, increase sharpness
Compare the penumbras A
in the diagrams at right.
When the target is closer
B
to the tooth, as in B, the
penumbra is larger. If the
target is moved farther
from the tooth (A), the
penumbra surrounding
the tooth image is
smaller, creating a
sharper image. The film
distance from the tooth to
the film is unchanged. Target (source) Umbra
Tooth Penumbra

32. 0
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
penumbra. This decreases the
sharpness of the image. When
the film is moved closer to the film
tooth ( ), the penumbra is
smaller, creating a sharper
image. Target (source) Umbra
Teeth Penumbra

33. Intensifying screens decrease sharpness 0
Extraoral films use intensifying screens which contain
special phosphor crystals that produce light when
struck by 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

34. Patient motion decreases sharpness
If the patient moves during the exposure of a film, the
images will be blurred, or unsharp, as seen below.

35. 0
Magnification
Magnification 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
0
Increase 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
0
Decrease 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 0
Distortion 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 0
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 the long axis of the x-ray film. As a
result, distortion is minimized or eliminated. In the
radiograph of the maxillary first molar, below, the
shape and relationship of the buccal and palatal
roots are accurately imaged.

40. Distortion 0
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). The bisecting angle
radiograph of the maxillary molar, below, shows the
distortion of the relationship between the buccal and
palatal 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 Radiograph
In the ideal radiograph, the image is the same
size as the object, has the same shape and has
a sharp outline with good density and contrast.
Because the film must always be at some
distance from the object, with bone and soft
tissue in between, the object will always be
magnified to some degree. Though magnified,
the image of the object will usually have the
same shape as the object when using the
paralleling technique. The sharpness, density
and contrast are maximized by using a longer
PID and proper exposure factors.

43. 0
The mandibular molar periapical film comes
closest to satisfying the properties of an ideal
radiograph (either paralleling or bisecting). The
film is closer to the teeth in this location than in
any other part of the mouth and the film is
usually parallel with the teeth.

44. 0
This concludes the section on Image
Characteristics and Projection Geometry.
Additional self-study modules are available
at: http://dent.osu.edu/radiology/resources.htm
If you have any questions, you may e-mail
me at jaynes.1@osu.edu.
Robert M. Jaynes, DDS, MS
Director, Radiology Group
College of Dentistry
Ohio State University