Radiographic Quality

1
Radiographic Quality

• Chapter 5

2
Radiographic quality

• Refers to how easily details can be perceived on
  a radiograph.
• Need to obtain as much diagnostic information as
  possible about the internal structures of the
  patient.

3
Quality depends on

• Radiographic Density
• Contrast
• Geometric Factors that affect detail

4
Radiographic Density

• Defined as the degree of blackness or darkness on
  a radiograph.
• Black areas on a developed radiograph are
  produced by deposits of metallic silver in the
  film emulsion that result from exposure to x-rays
  and their subsequent processing.

5
Density continued .

• X-rays make radiographic film black
• Degree of blackness on a radiograph depends on
  the amount of x-rays reaching the film.
• Density is influenced by the quantity and quality
  of the x-ray beam, as well as the type and
  thickness of the tissue under examination.

6
Factors Affecting Radiographic Density.

• Greater radiographic density may be produced by
  increasing
• Total of x-rays that reach the film
• The penetrating power of the x-rays
• The developing time
• The temperature of the developer

7
Lets Review

• mAs- number of x-rays leaving the x-ray tube in a
  set period of time.
• When kVp increases, the penetrating power
  increases as well. This means more x-rays will
  reach the film causing a darker radiograph.

8
Other Density Influences

• Thickness and type of tissue being radiographed.
  
• Increase in thickness, means that the patient can
  absorb more x-rays which will results in a
  lighter image.
• Large animals absorb more x-rays, so less x-rays
  reach the film, so the lighter the image. To
  compensate, we increase the kVp.

9
Type of Tissue

• Type of tissue affects density as well. Higher
  density tissues will cause less x-rays to reach
  film, therefore have lighter areas on the film at
  that place.

10
Gray film, lacks good density
11
(No Transcript)
12
Contrast

• Defined as visible difference between two
  adjacent radiographic densities
• Is divided into
• Radiographic Contrast
• Subject Contrast

13
Radiographic Contrast

• Density between two adjacent areas on a
  radiograph.
• Many black and white areas means you have
  contrast.
• If a radiograph has many grays and a small
  density difference between 2 adjacent areas, then
  it has low contrast.
• Need right amount of contrast. Not desirable to
  have to high or too low of contrast. Need it to
  be just right. Want grays, blacks and whites so
  eye can easily see detail.

14
(No Transcript)
15
(No Transcript)
16
Contrast is influenced by

• kVp level
• Subject contrast
• Scatter radiation
• Film type
• Film fog

17
Subject Contrast

• Defined as the difference in density and mass
  between two adjacent anatomic structures
• Depends on
• Thickness of the anatomic part.
• Density of the anatomic part.

18
Contrast Continued

• Bone will have more white on the film than soft
  tissue.
• Bone -gt high contrast -gt low kVp
• Soft tissue -gt low contrast -gt high kVp

19
Exposure factors

• Poor contrast is due to inappropriate exposure
  factors.
• mAs- affects contrast when too little or too much
  is used.
• Primarily affects density so should not affect
  contrast if proper kVp is used.
• If mAs is insufficient, contrast is reduced
  because overall density of the radiograph is
  reduced.
• If quantity of x-rays reaching the film is too
  low, film will be pale.
• If too much mAs is applied, overall film will be
  blacker but less effect on contrast.

20
How should mAs be adjusted?
21
Kilovoltage

• Affects both contrast and density
• Increase in kVp Increase in penetrating power.
• Increase in kVp shorter wavelengths are
  produced.
• As penetrating power increases, so does scatter
  radiation. This can alter radiographic contrast.

22
kVp

• If too low, will have gray and white appearance
  and image will be imperceptible.
• Will have low density because unable to reach
  film.
• Causes difficulty in distinguishing anatomic
  organs

23
(No Transcript)
24
Scatter Radiation

• Radiation that does not form an image and is
  scattered in all directions.
• Contrast is decreased because inappropriate areas
  of the film are being exposed.

25
Sources of Scatter Radiation

• From the Patient
• Table
• Film Tray

26
Backscatter

• Backscatter- radiation arising from sources
  behind the image plane that are scattered back to
  the image.
• Avoid backscatter by limiting the size of the
  x-ray beam so that the field does not exceed the
  image receptor.
• Cassettes contain lead- foil backing to prevent
  backscatter from reaching the film.

27
Grids

• When thick body parts are being radiographed, you
  want to minimize scatter radiation, you do this
  by using a grid.
• Grid- device placed between the patient and the
  radiographic film designed to absorb non-image
  forming x-rays.
• Composed of alternating strips of lead and spacer
  material.

28
Grids Continued

• Spacer material usually consists of fiber,
  alumininum, or plastic because they have low
  x-ray absorption
• Grids may be
• Placed on top of the cassette.
• Built into the cassette.
• Placed under the table between the patient and
  the cassette.

29
Grid Pattern

• Linear Grid- most table top machines are equipped
  with a linear grid- Lead strips parallel in their
  longitudinal axis.
• Crossed Grid- 2 superimposed linear grids absorbs
  maximum amount of scatter radiation

30
Grids continued

• Generally grids now are a part of the cassette or
  the x-ray table.
• Potter Bucky Diaphragm- usually found in large
  animal situations.
• Grids must be adequately cared for, can not be
  dropped because will result in permanent damage.

31
Radiographic Detail and Definition

• Terms used to describe image sharpness, clarity,
  distinctness, and perceptibility.
• Lack of detail factors
• 1. Geometric unsharpness- loss of detail due to
  geometric distortion
• Large focal spot size
• Decreased SID
• Motion
• Screens and films

32

• 2. Geometric Distortion and Magnification-
  variation in size and shape of anatomic
  structures due to their position in relation to
  the x-ray source and film.
• Important to keep areas being radiographed
  parallel to image receptor to avoid distortion.
• Distortions include Magnification, elongation,
  foreshortening.

33
(No Transcript)